
Date: Wed, 10 Apr 91 13:12:20 MDT
From: thorson@typhoon.atmos.colostate.edu (Bill Thorson)


 
 
 
 
__________=====
__________=====
__________=====    NSDSSO
 
 
 
                  Flexible  Image  Transport  System  (FITS)
 
 
 
                                December, 1990
 
 
 
                                Draft Standard
 
 
 
                                NSDSSO 100-0.1
 
 
 
                        NASA Science Data Systems Standards Office
                        Code 933
                        NASA Goddard Space Flight Center
                        Greenbelt MD 20771

 
 
 
    The NASA Science Data Systems Standards Office (NSDSSO) has been
established to serve the space science communities in evolving cost effective,
interoperable, data systems. The NSDSSO performs a number of functions designed
to facilitate the recognition, development, adoption, and use of standards by
the space science communities.
    Approval of an NSDSSO Standard requires verification by the NSDSSO that the
following requirements have been met: consensus by the technical panel; proper
adjudication of the comments received from the targeted space and Earth
science community; and conformance to the accreditation process.
    An NSDSSO standard represents the consensus of the technical panel convened
by the NASA Science Data Systems Standards Office (NSDSSO) of the National
Space Science Data Center (NSSDC) of the National Aeronautics and Space
Administration (NASA). Consensus is established when the NSDSSO Accreditation
Panel determines that substantial agreement has been reached by the Technical
Panel. However consensus does not necessarily imply that all members were in
full agreement with every item in the standard. NSDSSO standards are not
binding as published, however they may serve as a basis for mandatory standards
when adopted by NASA or other organizations.
    An NSDSSO standard may be revised at any time depending on developments in
the areas covered by the standard. Also, within five years from the date of its
issuance, this standard will be reviewed by the NSDSSO to determine whether it
should 1) remain in effect without change, 2) be changed to reflect the impact
of new technologies or new requirements, or 3) be retired or cancelled.
    The Technical Panel which developed this standard consisted of the
following members:
 
 
 
      Robert J. Hanisch, Chair           Space Telescope Science Institute
      Barry M. Schlesinger, Secretary    ST Systems Corporation
      Lee E. Brotzman                    ST Systems Corporation
      Edward Kemper                      ST Systems Corporation
      Peter Teuben                       University of Maryland
      Michael E. Van Steenberg           NASA Goddard Space Flight Center
      Wayne H. Warren Jr.                ST Systems Corporation
      Richard A. White                   NASA Goddard Space Flight Center
 
    This standard is published and maintained by the NSDSSO. Send comments and
orders for NSDSSO documents to:
 
              NSDSSO, Code 933, NASA Goddard Space Flight Center
              Greenbelt MD 20771
              Internet:  NSDSSO@nssdca.gsfc.nasa.gov
              SPAN: nssdca::NSDSSO
              301-286-3575

 
CONTENTS_____________________________________________________________________i
 
 
Contents
 
 
Introduction                                                                v
 
1    Overview                                                               1
     1.1    Purpose  ...................................................    1
     1.2    Scope  .....................................................    1
     1.3    Applicability   ............................................    1
     1.4    Organization and Recommendations   .........................    2
 
2    References                                                             3
 
3    Definitions, Acronyms, and Symbols                                     5
 
4    Data Set Organization                                                  9
     4.1    Overall Structure  .........................................    9
     4.2    FITS Element Structure  ....................................    9
     4.3    Primary Header and Data  ...................................    9
            4.3.1     Primary Header    ................................   10
            4.3.2     Primary Data  ....................................   10
     4.4    Extensions  ................................................   10
            4.4.1     Requirements for Conforming Extensions   .........   10
            4.4.2     Standard Extensions  .............................   11
            4.4.3     Order of Extensions  .............................   11
     4.5    Special Records  ...........................................   11
 
5    Headers                                                               13
     5.1    Card Images    .............................................   13
            5.1.1     Syntax   .........................................   13
            5.1.2     Components    ....................................   13
     5.2    Keywords   .................................................   14
            5.2.1     Mandatory Keywords   .............................   14
            5.2.2     Other Reserved Keywords  .........................   18
            5.2.3     Additional Keywords  .............................   23
    5.3     Value/Comment   ............................................   24
            5.3.1     Character String   ...............................   24
            5.3.2     Logical Variable   ...............................   24
            5.3.3     Integer  .........................................   24
            5.3.4     Real Floating Point Number   .....................   24
            5.3.5     Complex Floating Point Number  ...................   25
 
6   Data Representation                                                    27
    6.1     Characters  ................................................   27
    6.2     Integers   .................................................   27
            6.2.1     Eight-bit    .....................................   27
            6.2.2     Sixteen-bit  .....................................   27
            6.2.3     Thirty-two-bit   .................................   27
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ii_____________________________________________________________________CONTENTS
 
 
 
    6.3     IEEE-754 Floating Point   ..................................   28
            6.3.1     Thirty-two-bit Floating Point  ...................   28
            6.3.2     Sixty-four-bit Floating Point  ...................   29
 
7   Data Records                                                           31
    7.1     Primary Array   ............................................   31
    7.2     ASCII Tables Extension  ....................................   31
            7.2.1     Storage   ........................................   31
            7.2.2     Fields   .........................................   33
            7.2.3     Entries  .........................................   33
 
8   Restrictions on Changes                                                35
 
Appendixes
 
A   Random Groups Structure                                                37
    A.1     Keywords   .................................................   37
            A.1.1     Mandatory Keywords   .............................   37
            A.1.2     Reserved Keywords  ...............................   39
    A.2     Data Storage   .............................................   40
    A.3     Data Representation   ......................................   40
B   A Binary Table Extension                                               41
    B.1     Identification   ...........................................   41
    B.2     Header   ...................................................   41
            B.2.1     Required Keywords  ...............................   41
            B.2.2     Reserved Keywords   ..............................   44
     B.3    Storage   ..................................................   44
     B.4    Format   ...................................................   44
     B.5    Data Types   ...............................................   45
            B.5.1     Logical   ........................................   45
            B.5.2     Bit Arrays   .....................................   45
            B.5.3     ASCII Characters    ..............................   45
            B.5.4     Eight-bit Integers   .............................   45
            B.5.5     Sixteen-bit Integers   ...........................   45
            B.5.6     Thirty-two-bit Integers   ........................   45
            B.5.7     Thirty-two-bit Floating Point   ..................   46
            B.5.8     Sixty-four-bit Floating Point   ..................   46
 
C    Implementation on Physical Media                                      47
     C.1    Block Size   ...............................................   47
            C.1.1     Magnetic Tape   ..................................   47
            C.1.2     Other Media   ....................................   47
     C.2    Physical Properties of Media   .............................   48
     C.3    Labeling   .................................................   48
            C.3.1     Tape   ...........................................   48
 
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List_of_Tables______________________________________________________________iii
 
 
 
            C.3.2     Other Media   ....................................   48
     C.4    FITS File Boundaries   .....................................   48
            C.4.1     Magnetic Tape   ..................................   48
            C.4.2     Other Media   ....................................   48
     C.5    Multiple Physical Volumes   ................................   48
 
D    Summary of Keywords                                                   49
 
E    NSDSSO Publications                                                   51
 
 
List  of  Tables
 
     5.1    Principal mandatory keywords   .............................   14
     5.2    Interpretation of BITPIX value   ...........................   15
     5.3    Mandatory keywords in conforming extensions   ..............   16
     5.4    Mandatory keywords in ASCII tables extensions   ............   17
     5.5    Valid TFORMn format values in TABLE extensions   ...........   18
 
 
     6.1    Content of 32-bit floating point bit positions   ...........   28
     6.2    Content of 64-bit floating point bit positions   ...........   29
 
     B.1    Required keywords in the interim binary table extension   ..   42
     B.2    Valid TFORMn format values in the interim binary table
            extension     ..............................................   43
 
     D.1    Mandatory FITS keywords   ..................................   49
     D.2    Reserved FITS keywords   ...................................   50
     D.3    General Reserved FITS keywords   ...........................   50
 
 
     E.1    NSDSSO Publications   ......................................   51
 
 
 
List  of  Figures
 
 
    7.1     Array data storage sequence   ..............................   32
 
 
 
 
 
 
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iv_____________________________________________________________________CONTENTS
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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_____________________________________________________________________________v
 
 
 
Introduction
 
 
 
The Flexible Image Transport System (FITS) evolved out of the recognition that
a standard format was needed for transferring astronomical data from one
installation to another. The original form, or Basic FITS, was designed for the
transfer of images and consisted of a binary array, usually multidimensional,
preceded by an ASCII text header with information describing the structure and
contents of the array. The FITS concept was later expanded to accommodate more
complex data structures. For example, a new structure for image transfer called
random groups was defined in which the data consists of a series of arrays,
with each array accompanied by a set of associated parameters. These structures
were formally endorsed by the International Astronomical Union (IAU) in
1982[1]. Provisions for data structures other than simple arrays or groups were
later made. These structures appear in extensions, consisting of an ASCII
header followed by the data structure it describes. One particular extension
structure, ASCII Tables, was formally approved by the IAU in 1988. At the same
time, a set of rules governing all extensions was endorsed, to ensure that FITS
readers encountering an unfamiliar extension would be able to identify it,
interpret it, and pass on to the next.
    FITS was originally designed and defined for 9-track half-inch magnetic
tape. However, as improvements in technology have brought forward other data
storage and data distribution media, it has generally been agreed that the FITS
structure is to be understood as a logical structure, and not defined in terms
of the physical characteristics of any particular data storage medium or media.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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vi_____________________________________________________________________CONTENTS
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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______________________________________________________________________________1
 
 
 
Section   1
 
 
 
Overview
 
 
 
1.1        Purpose
 
 
This standard formally defines the FITS format for data structuring and
exchange. It is intended as a formal codification of the FITS structure that
has been endorsed by the IAU for transfer of astronomical data, as described in
four published papers [2,3,4,5] and in a formal agreement [6] endorsed by the
IAU FITS Working Group under Commission 5. Minor ambiguities and
inconsistencies in FITS as described in the original papers are eliminated.
 
 
 
1.2        Scope
 
 
This standard specifies the structure and content of FITS data sets. It
specifies the content and organization of the header and data for all standard
FITS formats: Basic FITS, the ASCII tables extension, and the random groups
structure. It also specifies minimum structural requirements for new extensions
and general principles governing the creation of new extensions. For headers,
it specifies the proper syntax for card images and defines required and
reserved keywords. For data, it specifies character and value representations.
It defines the general rules to which new extensions are required to conform.
 
 
 
1.3        Applicability
 
 
All spacecraft projects and astrophysics data archives under the management of
the Astrophysics Division of the National Aeronautics and Space Administration
are required to make processed data available to users in the format defined by
this standard, unless the Astrophysics Division specifically determines
otherwise. The IAU has recommended that all astronomical computer facilities
support FITS for the interchange of binary data.  This standard may also be
used
 
 
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2_________________________________________________________SECTION_1.___OVERVIEW
 
 
 
used to define the format for data transport in other disciplines, as may be
determined by the appropriate authorities.
 
 
 
1.4        Organization  and  Recommendations
 
 
Following the definitions in Section 3, this document describes the overall
organization of a FITS data set and the rules for creating new FITS extensions
in Section 4. Subsequent sections describe the contents of FITS files. Section
5 describes headers, including the required keywords and the conditions under
which each is required; also, reserved keywords are specified. Section 6
describes how data are represented, and Section 7 describes data structures.
Each section contains information relevant to all FITS formats. Throughout the
document, deprecation of structures or syntax is noted where relevant. Data
sets containing such constructs are valid FITS, but these constructs should not
be used in new data sets; the old data sets using them remain standard because
of the principle that no FITS structure, once valid, shall ever become invalid.
    Although it is part of the FITS standard, the random groups structure has
not been widely used. New formats under development will replace it, and use of
it is discouraged. Consequently, it is described in Appendix A rather than in
the main body of the standard. One new format, 3D binary tables, is described
for informational purposes in Appendix B, which is not part of the formal
standard.
    While FITS is designed as a logical format, the astronomical community has
developed recommendations for its expression on various physical media. The
accepted recommendations are provided in Appendix C as a guide to FITS
practices, but they are not part of this standard. Appendix D gives a tabular
summary of the FITS keywords.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Section   2
 
 
References
 
 
    1.  IAU Information Bulletin No. 49, p. 14, 1983.
 
 
    2.  Wells, D. C., Greisen, E. W., and Harten, R. H. 1981, "FITS: A Flexible
        Image Transport System," Astron. Astrophys. Suppl., 44, 363-370.
 
 
    3.  Greisen, E. W. and Harten, R. H. 1981, "An Extension of FITS for Small
        Arrays of Data," Astron. Astrophys. Suppl., 44, 371-374.
 
 
    4.  Grosbol, P., Harten, R. H., Greisen, E. W., and Wells, D. C. 1988,
        "Generalized Extensions and Blocking Factors for FITS," Astron.
        Astrophys. Suppl., 73, 359-364.
 
 
    5.  Harten, R. H., Grosbol, P., Greisen, E. W., and Wells, D. C. 1988, "The
        FITS Tables Extension," Astron. Astrophys. Suppl., 73, 365-372.
 
 
    6.  Wells, D. C. and Grosbol, P. 1990, "Floating Point Agreement for FITS"
        (available from the authors at National Radio Astronomy Observatory,
        Edgemont Road Charlottesville, Virginia 22903 USA or European Southern
        Observatory Karl-Schwarzschild Strasse 2, D-8046 Garching bei Munchen,
        FRG.)
 
 
    7.  "American National Standard for Information Processing: Programming
        Language FORTRAN," ANSI X3.9 - 1978 (ISO 1539). Published by American
        National Standards Institute, Inc., New York, 1978.
 
 
    8.  "American National Standard for Information Processing: Code for
        Information Interchange," ANSI X3.4 - 1977 (ISO 646). Published by
        American National Standards Institute, Inc., New York, 1977.
 
 
    9.  "American National Standard - IEEE Standard for Binary Floating Point
        Arithmetic". ANSI/IEEE 754-1985, Published by American National
        Standards Institute, Inc., New York, 1985.
 
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4_______________________________________________________SECTION_2.___REFERENCES
 
 
 
  10.   "American National Standard for Information Processing: Unrecorded
        Magnetic Tape," ANSI X3.40 - 1976, Published by American National
        Standards Institute, Inc., New York, 1976.
 
 
  11.   "American National Standard for Information Processing: Magnetic Tape
        Labels and File Structure," ANSI X3.27 - 1978, Published by American
        National Standards Institute, Inc., New York, 1978.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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______________________________________________________________________________5
 
 
Section   3
 
 
Definitions, Acronyms, and Symbols
 
~ (tilda)     Used to designate an ASCII blank.
 
 
Array          A sequence of data values, of zero or more dimensions.
 
 
Array value    An element of an array.
 
 
ASCII          Abbreviation of American National Standard Code for Information
               Interchange.
 
 
Basic FITS     The FITS structure consisting of the primary header and primary
               data containing a single array.
 
 
Bit            A single binary digit.
 
 
Byte           A string of eight bits treated as a single entity.
 
 
Card image     A sequence of 80 bytes with values between hexadecimal 20 and 7E
               inclusive, treated as a logical record.
 
 
Conforming extension    An extension whose keywords and structure adhere to the
               requirements for conforming extensions defined in this standard.
 
 
Deprecate      To express earnest disapproval of (permitted but discouraged).
 
 
FITS Element   One of the components of a FITS data set: the primary HDU, an
               extension, or the special records collectively.
 
 
Entry          A single value in a table.
 
 
Extension      A FITS HDU following the primary HDU in a FITS file.
 
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6_______________________________SECTION_3.___DEFINITIONS,_ACRONYMS,_AND_SYMBOLS
 
 
Field          A set of zero or more table entries collectively described by a
               single format.
 
 
File           A sequence of one or more records terminated by an end-of-file
               indicator.
 
 
FITS           Abbreviation of Flexible Image Transport System.
 
 
FITS file      A file with a structure that matches the specifications in this
               document.
 
 
FITS logical record      A record of 23040 bits, corresponding to 2880 8-bit
               bytes within a FITS file.
 
 
Floating point      A number stored internally as a mantissa and exponent,
               whose ASCII representation contains an explicit decimal point
               and may include a power-of-ten exponent.
 
 
Group Format   One of two possible data organization formats for the primary
               HDU.
 
 
Header         A series of card images organized within one or more FITS
               Logical Records which describes structures and/or data which
               follow it in the FITS file.
 
 
Header and Data Unit (HDU)    A data structure consisting of a Header and the
               Data structure the Header describes.  Note that an HDU may
               consist entirely of a header with no data records.
 
 
IAU            Abbreviation of International Astronomical Union.
 
 
IEEE           Abbreviation of Institute of Electrical and Electronic
               Engineers.
 
 
IEEE NaN       Abbreviation of IEEE Not-a-Number value.
 
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IEEE special values      (-0, +/- infinity).
 
 
Keyword        The first eight bytes of a header card image.
 
 
Mandatory keyword   A keyword which must be used in all FITS data sets or a
               keyword required in conjunction with particular FITS structures.
 
 
Matrix         A data array of two or more dimensions.
 
 
NSDSSO         Abbreviation of NASA Science Data Systems Standards Office.
 
 
Physical value      The value in physical units represented by a member of an
               array and possibly derived from the array value using a linear
               transformation.
 
 
Picture element     A single location within an image array.
 
 
Pixel          Abbreviation of "picture element".
 
 
Primary array       The data array contained in the Primary HDU.
 
 
Primary header      The first header in a FITS file, containing information on
               the overall structure of the file as well as on the primary
               array.
 
 
Random Group        A primary HDU data structure, which consists of a sequence
               of individual values followed by a data array.
 
 
Random Group parameters       The sequence of individual values that precede
               the data array in a Random Group.
 
 
Record         A sequence of bits treated as a single logical entity.
 
 
 
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8_______________________________SECTION_3.___DEFINITIONS,_ACRONYMS,_AND_SYMBOLS
 
 
Reference pixel     The pixel along a given coordinate axis at which a value
               and increment are defined.
 
 
Reserved keyword    An optional keyword that may be used only in the manner
               defined in this standard.
 
 
Special Records     A series of 23040-bit (2880 8-bit byte) records, at the end
               of the FITS file, whose internal structure does not otherwise
               conform to that for the primary HDU or to that specified for a
               conforming extension in this standard.
 
 
Standard Extension       A conforming extension whose header and data content
               are specified explicitly in this standard.
 
 
Valid value    A member of a data array or table corresponding to an actual
               physical quantity.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Section   4
 
 
Data   Set   Organization
 
 
4.1        Overall  Structure
 
 
A FITS file shall be composed of the following FITS elements, in the order
listed:
 
 
     o  Primary HDU
 
 
     o  Extensions (optional)
 
 
     o  Special records (optional)
 
 
     Each FITS element shall consist of an integral number of FITS logical
records. The primary HDU shall start with the first record of the FITS file.
The first record of each subsequent FITS element shall be the record
immediately following the last record of the preceding FITS element. The size
of a FITS logical record shall be 23040 bits, corresponding to 2880 8-bit
bytes.
 
 
4.2        FITS  Element  Structure
 
 
The primary HDU and every extension shall consist of an integral number of
header records consisting of ASCII text, which may be followed by an integral
number of data records. The first record of data shall be the record
immediately following the last record of the header.
 
 
4.3        Primary  Header  and  Data
 
 
The first component of a FITS data set shall be the primary header. The primary
header may, but need not be, followed by primary data. The presence or absence
of primary data shall be indicated by the value of the NAXIS keyword in the
primary header (Section 5.2.1.1).
 
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10___________________________________________SECTION_4.___DATA_SET_ORGANIZATION
 
 
4.3.1       Primary  Header
 
 
The header of a primary HDU shall consist of a series of card images in ASCII
code. All header records shall consist of 36 card images. Card images without
information shall be filled with ASCII blanks (hexadecimal 20).
 
 
4.3.2       Primary  Data
 
 
The primary data, if present, may be either in Basic FITS array format or
random groups format. In Basic FITS format, the primary data shall consist of a
single data array of 1-999 dimensions. If the data array does not fill the
final record, the remaining storage locations shall be filled with zero values
with the same data representation as the values in the array. Random groups
format, which this document deprecates, is described in Appendix A.
 
 
4.4        Extensions
 
 
All extensions, whether or not described in this standard, shall fulfill the
following requirements to be in conformance with this FITS standard.
 
 
4.4.1       Requirements  for  Conforming  Extensions
 
 
4.4.1.1       Identity
 
 
Each extension shall have a unique name, specified in the header according to
the syntax codified in Section 5.2.1.2. To preclude conflict, extension names
must be registered with the NSDSSO or IAU Commission 5 who will coordinate name
allocations.
 
 
4.4.1.2       Size Specification
 
 
The total number of bits in the data of each extension shall be specified in
the header for that extension, in the manner prescribed in Section 5.2.1.2.
 
 
 
 
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4.5.___SPECIAL_RECORDS_______________________________________________________11
 
 
4.4.1.3       Compatibility with Existing FITS Structures
 
 
No extension shall be constructed that invalidates existing FITS data sets.
 
 
4.4.2       Standard  Extensions
 
 
A standard extension shall be a conforming extension whose structure and
content are completely specified in this standard. Only one FITS format shall
be approved for each type of data organization. Each standard extension shall
have a unique name.
 
 
4.4.3       Order  of  Extensions
 
 
Standard extensions and other conforming extensions may appear in any order in
a FITS data set.
 
 
4.5        Special  Records
 
 
The first 8 bytes of special records must not contain the string "XTENSION" or
the string "SIMPLE  ". The records must have the standard FITS 23040-bit record
length. The contents of special records are not otherwise specified by this
standard.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Section   5
 
 
Headers
 
 
5.1        Card  Images
 
 
5.1.1       Syntax
 
 
Header card images shall consist of a keyword, an optional value, and an
optional comment. If a value is present, column 9 shall contain the symbol "=",
column 10 shall be blank, and columns 11-80 shall be as specified in the
remainder of Section 5.2. If no value is present, column 9 may contain any
ASCII text except the symbol "=", and columns 10-80 may contain any ASCII text.
 
 
5.1.2       Components
 
 
5.1.2.1       Keyword (bytes 1-8)
 
 
The keyword shall be a left justified, 8-character, blank filled, ASCII string
with no embedded blanks. All digits (hexadecimal 30 to 39, "0123456789") and
upper case Latin alphabetic characters (hexadecimal 41 to 5A, "ABCDEFG HIJKLMN
OPQRST UVWXYZ") are permitted; no lower case characters shall be used. The
underscore (hexadecimal 60, "_") and hyphen (hexadecimal 2D, "-") are also
permitted. No other characters are permitted. If the keyword itself requires
less than eight characters it must be filled with ASCII blanks (hexadecimal
20). The value and comments field on a card image may be restricted for
particular keywords.
 
 
5.1.2.2       Value Indicator (bytes 9-10)
 
 
This field shall contain an ASCII "= " for keywords with an associated value
field. If there is no associated value field, byte 9 may not contain "="
(hexadecimal 3D); otherwise, this field may contain any ASCII character text.
 
 
 
 
 
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14_________________________________________________________SECTION_5.___HEADERS
 
 
5.1.2.3       Value/Comment
 
 
This field, when used, shall contain the value, if any, of the keyword followed
by optional comments. Separation of the value and comments by a slash
(hexadecimal 2F, "/"), and a space between the value and the slash are strongly
recommended. The value shall be the ASCII representation of a string or
constant, structured as specified in Section 5.3. The comment may contain any
printable 7-bit ASCII characters (hexadecimal 20 through 7E).
 
 
5.2        Keywords
 
 
5.2.1       Mandatory  Keywords
 
 
Mandatory keywords are required as described in the remainder of this
subsection. They may be used only as described in this standard.
 
 
5.2.1.1       Principal
 
 
Principal Mandatory keywords other than SIMPLE are required in all FITS
headers. The SIMPLE keyword is required in all primary headers and may not
appear elsewhere. The card images of any primary header must contain the
keywords shown in Table 5.1 in the order given.
 
                       1      SIMPLE
                       2      BITPIX
                       3      NAXIS
                       4      NAXISn, n = 1, . . ., NAXIS
                              .
                              .
                              (other keywords)
                              .
                              .
                      last    END
 
               Table 5.1:  Principal mandatory keywords.
 
SIMPLE Keyword     The value field shall contain a logical constant with the
value T if the file conforms to this standard. This keyword is mandatory for
the primary header and must not appear as the first keyword in a conforming
extension header.
 
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5.2.___KEYWORDS______________________________________________________________15
 
 
BITPIX Keyword     The value field shall contain an integer. The absolute value
is used in computing the sizes of data structures. It shall specify the number
of bits that represent a data value (see Table 5.2).
 
               __Value____________Data_Represented______________
 
                     8    Character or unsigned binary integer
                    16    16-bit twos complement binary integer
                    32    32-bit twos complement binary integer
                   -32    IEEE single precision floating point
                   -64    IEEE double precision floating point
 
                  Table 5.2:  Interpretation of BITPIX value.
 
 
NAXIS Keyword      The value field shall contain an integer no greater than
999, representing the number of axes in an ordinary data array. A value of zero
signifies that no data follow the header.
 
 
NAXISn  Keywords   The value field shall contain an integer, representing the
number of positions along axis n of an ordinary data array. If NAXIS is equal
to 0, there should not be any NAXISn keywords.
 
 
END Keyword    This keyword has no associated value. Columns 9-80 shall be
blank.
 
 
5.2.1.2       Conforming Extensions
 
 
The use of FITS extensions requires the use of a single additional keyword in
the primary header.
 
 
EXTEND Keyword     If the FITS data set contains extensions, a card image with
the keyword EXTEND and the value field containing the logical value T must
appear in the primary header immediately after the last NAXISn card image, or,
if NAXIS=0, the NAXIS card image. The presence of this keyword with the value T
in the primary header does not require that extensions be present.
     The card images of any extension header must use the keywords defined in
Table 5.3 in the order specified. This organization is required for any
conforming extension, whether or not further specified in this standard.
     The total number of bits in the extension data array exclusive of the fill
described in Section 4.3.2 must be given by the following expression:
 
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16_________________________________________________________SECTION_5.___HEADERS
 
                            1      XTENSION
                            2      BITPIX
                            3      NAXIS
                            4      NAXISn, n = 1, . . ., NAXIS
                                   .
                                   .
                                   (other keywords, including . . .)
                                   PCOUNT
                                   GCOUNT
                                   .
                                   .
                         last      END
 
           Table 5.3:  Mandatory keywords in conforming extensions.
 
                    NBITS = |BITPIX| x GCOUNT  x
                            (PCOUNT + NAXIS1 x NAXIS2 x ... x NAXISm);
 
where NBITS is the number of bits excluding fill, m is the value of NAXIS, and
BITPIX, GCOUNT, PCOUNT, and the NAXISn represent the values associated with
those keywords.
 
 
XTENSION  Keyword  The value field shall contain a character string giving the
name of the extension type. This keyword is mandatory for an extension header
and must not appear in the primary header. For an extension that is not a
standard extension, the type name must not be the same as that of a standard
extension. NSDSSO, in collaboration with IAU Commission 5, may specify
additional names that must be used only to identify specific types of
extensions.
 
 
PCOUNT  Keyword    The value field shall contain an integer that shall be used
in any way appropriate to define the data structure, consistent with the
equation for NBITS. Usage is analogous to that of the PCOUNT keyword for the
Random Groups structure (Section A.1.1), which represents the number of
parameters in a parameter-array group.
 
 
GCOUNT  Keyword    The value field shall contain an integer that shall be used
in any way appropriate to define the data structure, consistent with the
equation for NBITS. Usage is analogous to that of the GCOUNT keyword for the
Random Groups structure (Section A.1.1), which represents the number of
parameter-array groups.
 
 
END Keyword    This keyword has no associated value.  Columns 9-80 shall be
blank.
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5.2.___KEYWORDS______________________________________________________________17
 
 
5.2.1.3       ASCII Tables Extension Keywords
 
 
The card images in the header of an ASCII Tables Extension must use the
keywords defined in Table 5.4 in the order specified.
 
 
           1      XTENSION
           2      BITPIX
           3      NAXIS
           4      NAXIS1
           5      NAXIS2
           6      PCOUNT
           7      GCOUNT
           8      TFIELDS
                  ..
                  .
                  (other keywords, which must include . . .)
                  TBCOLn, n=1,2,. . .,k where k is the value of TFIELDS
                  TFORMn, n=1,2,. . .,k where k is the value of TFIELDS
                  ..
                  .
         last     END
 
          Table 5.4:  Mandatory keywords in ASCII tables extensions.
 
 
XTENSION Keyword   The value field shall contain the character string
'TABLE~~~'.
 
 
BITPIX Keyword     The value field shall contain the integer 8, denoting that
the array contains 8-bit printable ASCII characters with the eighth bit, the
parity bit, set to zero (i.e. hexadecimal codes 20 through 7E).
 
 
NAXIS Keyword      The value field shall contain the integer 2, denoting that
the included data array is two-dimensional: rows and columns.
 
 
NAXIS1  Keyword    The value field shall contain an integer, representing the
width of the table in characters.
 
 
NAXIS2  Keyword    The value field shall contain an integer of value 1 or
greater, representing the number of rows in the table.
 
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18_________________________________________________________SECTION_5.___HEADERS
 
 
PCOUNT  Keyword    The value field shall contain the integer 0; no data values
precede the table.
 
 
GCOUNT Keyword     The value field shall contain the integer 1; the data
records contain a single table.
 
 
TFIELDS  Keyword   The value field shall contain an integer representing the
number of fields in each row. The maximum permissible value is 999.
 
 
TBCOLn  Keywords   The value field shall contain an integer specifying the
column in which field n starts.
 
 
TFORMn  Keywords   The value field shall contain a character string describing
the FORTRAN-77 [7] format in which field n is coded. The formats in Table 5.5
may be used for encoding.
 
 
         _Field_Value_________Data_Type_____________________________
 
               Aw        Character
               Iw        Integer
               Fw.d      Real
               Ew.d      Single precision real, exponential notation
               Dw.d      Double precision real, exponential notation
 
          Table 5.5:  Valid TFORMn format values in TABLE extensions.
 
     Repetition of a format from one field to the next must be indicated by
using separate pairs of TBCOLn and TFORMn keywords for each field; format
repetition may not be indicated by prefixing the format by a number.
 
 
END Keyword    This keyword has no associated value.  Columns 9-80 shall be
blank.
 
 
5.2.2       Other  Reserved  Keywords
 
 
These keywords are optional but may be used only as defined in this standard.
These keywords apply to any FITS structure except where specifically further
restricted.
 
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5.2.___KEYWORDS______________________________________________________________19
 
5.2.2.1     Keywords Describing the History or Physical Construction of the HDU
 
 
DATE  Keyword      The value field shall contain a character string giving the
date on which the HDU was created, in the form DD/MM/YY, where DD shall be the
day of the month, MM the month number, with January given by 01 and December by
12, and YY the last two digits of the year. For all data sets created after the
adoption of this standard, the date shall be that specified by local civil time
at the location where the data set was created. Copying of a FITS file does not
require changing any of the keyword values in the file's HDUs.
 
 
ORIGIN  Keyword    The value field shall contain a character string identifying
the organization creating the data set.
 
 
BLOCKED  Keyword   This keyword may be used only in the primary header. It must
appear immediately after the EXTEND card image, or if this card image is not
present, after the last NAXISn card image or the NAXIS card image with value
zero. Its presence with the required logical value of T advises that the
physical block size of the FITS data set on which it appears may be an integral
multiple of the logical record length. Physical block size and logical record
length may be equal even if this keyword is present or unequal if it is absent.
It is reserved primarily to prevent its use with other meanings. The issuance
of this standard deprecates the BLOCKED keyword.
 
 
5.2.2.2       Keywords Describing Observations
 
DATE-OBS Keyword   The value field shall contain a character string giving the
day on which the observations represented by the array were made, in Universal
Time in the form form DD/MM/YY, where DD shall be the day of the month, MM the
month number, with January given by 01 and December by 12, and YY the last two
digits of the year.
 
 
TELESCOP  Keyword  The value field shall contain a character string identifying
the telescope used to acquire the data contained in the array.
 
 
INSTRUME  Keyword  The value field shall contain a character string identifying
the instrument used to acquire the data contained in the array.
 
 
OBSERVER Keyword   The value field shall contain a character string identifying
the individual who acquired the data associated with the header. This keyword
is appropriate when the data describe the results of observations.
 
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20_________________________________________________________SECTION_5.___HEADERS
 
 
OBJECT Keyword     The value field shall contain a character string giving the
name of the object observed.
 
 
EQUINOX Keyword    The value field shall contain a character string giving the
equinox for the celestial coordinate system in which positions given in either
the header or data are expressed.
 
 
EPOCH  Keyword     The value field shall contain a character string giving the
equinox for the celestial coordinate system in which positions given in either
the header or data are expressed. This document deprecates the use of the EPOCH
keyword and thus it shall not be used in data sets created after the adoption
of this standard; rather, the EQUINOX keyword shall be used.
 
 
5.2.2.3       Bibliographic Keywords
 
 
AUTHOR  Keyword    The value field shall contain a character string identifying
the individual who compiled the information in the data associated with the
header. This keyword is appropriate when the data originate in a published
paper or are a compilation of results from many sources.
 
 
REFERENC Keyword   The value field shall contain a character string citing a
reference where the data associated with the header are published.
 
 
5.2.2.4       Commentary Keywords
 
 
COMMENT  Keyword   This keyword shall have no associated value; columns 9-80
may contain any ASCII text, except that column 9 may not contain "=". Any
number of COMMENT records may appear in a header.
 
 
HISTORY  Keyword   This keyword shall have no associated value; columns 9-80
may contain any ASCII text, except that column 9 may not contain "=". The text
should contain a history of steps and procedures associated with the processing
of the associated data. Any number of HISTORY records may appear in a header.
 
 
Keyword Field is Blank   Columns 1-8 are blank. Columns 9-80 may contain any
ASCII text except that column 9 may not contain "=", as for a keyword with no
associated value. Any number of blank records may appear in a header.
 
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5.2.___KEYWORDS______________________________________________________________21
 
5.2.2.5       Array Keywords
 
These keywords are used to describe the contents of a simple array, either
alone, or in a series of random groups. They are optional, but if they appear
in the header describing an array, they must be used as defined in this section
of this standard. They shall not be used in headers describing other structures
unless the meaning is the same as that for a primary array.
 
 
BSCALE  Keyword    This keyword shall be used, along with the BZERO keyword,
when the array pixel values are not the true physical values, to transform the
primary array values to the true physical values they represent, using the
linear transformation equation defined below. The value field shall contain a
floating point number representing the coefficient of the linear term in the
scaling equation, the ratio of physical value to array value at zero offset.
 
 
BZERO  Keyword     This keyword shall be used, along with the BSCALE keyword,
when the array pixel values are not the true physical values, to transform the
primary array values to the true values. The value field shall contain a
floating point number representing the physical value corresponding to an array
value of zero. The transformation equation is as follows:
 
                 physical value = BZERO + BSCALE x array value
 
 
BUNIT  Keyword     The value field shall contain a character string, describing
the ultimate physical units in which the quantities of the array are expressed.
 
 
BLANK  Keyword     This keyword shall be used only in headers with positive
values of BITPIX (i.e. in arrays with integer data). Columns 1-8 contain the
string, "BLANK~~~" (blanks in columns 6-8). The value field shall contain an
integer that specifies the representation of array members whose data values
are undefined.
 
 
CTYPEn Keywords    The value field shall contain a character string, giving the
physical coordinate represented by axis n.
 
CRPIXn  Keywords   The value field shall contain a floating point number,
identifying the location of a reference point along axis n, in units of the
axis index. The reference point need not coincide with the center of a pixel
(i.e. need not be an integer), nor lie within the actual data array.
 
CRVALn  Keywords   The value field shall contain a floating point number,
giving the value of the coordinate specified by the CTYPEn keyword at the
reference pixel CRPIXn.
 
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22_________________________________________________________SECTION_5.___HEADERS
 
 
CDELTn  Keywords   The value field shall contain a floating point number,
giving the partial derivative of the coordinate specified by the CTYPEn
keywords with respect to the pixel index, evaluated at the reference pixel,
CRPIXn, in units of the coordinate specified by the CTYPEn keyword.
 
CROTAn  Keywords   This keyword is used to indicate a rotation from a standard
coordinate system described by the CTYPEn to a different coordinate system in
which the values in the array are actually expressed. Rules for such rotations
are not further specified in this standard; the rotation should be explained in
comments. The value field shall contain a floating point number, giving the
rotation angle in degrees between axis n and the direction implied by the
coordinate system defined by CTYPEn.
 
 
DATAMAX  Keyword   The value field shall always contain a floating point
number, regardless of the value of BITPIX. This number shall give the maximum
valid physical value represented in the array.
 
 
DATAMIN  Keyword   The value field shall always contain a floating point
number, regardless of the value of BITPIX. This number shall give the minimum
valid physical value represented in the array.
 
 
5.2.2.6       Extension Keywords
 
These keywords are used to describe an extension.
 
 
EXTNAME  Keyword   The value field shall contain a character string, to be used
to distinguish among different extensions with the same value of XTENSION in a
FITS data set.
 
 
EXTVER  Keyword    The value field shall contain an integer, to be used to
distinguish among different extensions in a FITS data set with the same values
for XTENSION and EXTNAME. The values need not start with 1 for the first
extension with a particular value of EXTNAME and need not be in sequence for
subsequent values. If the EXTVER keyword is absent, the data set should be
treated as if the value were 1.
 
EXTLEVEL Keyword   The value field shall contain an integer, specifying the
level in a hierarchy of extension levels of the extension header containing it.
The value shall be 1 for the highest level; levels with a higher value of this
keyword shall be subordinate to levels with a lower value. If the EXTLEVEL
keyword is absent, the data set should be treated as if the value were 1.
 
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5.2.___KEYWORDS______________________________________________________________23
 
 
5.2.2.7       ASCII Tables Keywords
 
In addition to the mandatory keywords defined in section 5.2.1, these keywords
may be used to describe the structure of an ASCII Tables data array. They are
optional, but if they appear within an ASCII tables extension header, they must
be used as defined in this section of this standard.
 
 
TSCALn Keywords    This keyword shall be used, along with the TZEROn keyword,
when the quantity in field n does not represent a true physical quantity. The
value field shall contain a floating point number representing the coefficient
of the linear term in the linear transformation equation below, which must be
used to compute the true physical value of the field. The default value for
this keyword is 1.0. This keyword may not be used for A-format fields.
 
 
TZEROn Keywords    This keyword shall be used, along with the TSCALn keyword,
when the quantity in field n does not represent a true physical quantity. The
value field shall contain a floating point number representing the zero point
for the true physical value of field n. The default value for this keyword is
0.0. This keyword may not be used for A-format fields.
     The transformation equation used to compute a true physical value from the
quantity in field n is
 
         physical value = TZEROn + TSCALn x field value
 
TNULLn  Keywords   The value field shall contain the character string that
represents an undefined value for field n. The character value should be in the
format given by the TFORMn keyword for field n.
 
TTYPEn  Keywords   The value field shall contain a character string, giving the
name of field n. It is recommended that only upper case letters, digits and
underscore (hexadecimal code 5F) be used in the name. The use of identical
names for different fields should be avoided.
 
TUNITn  Keywords   The value field shall contain a character string describing
the ultimate physical units in which the quantity in field n is expressed.
 
 
5.2.3       Additional  Keywords
 
 
5.2.3.1       Requirements
 
New keywords may be devised in addition to those described in this standard, so
long as they are consistent with the generalized rules for keywords and do not
 
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24_________________________________________________________SECTION_5.___HEADERS
 
 
conflict with mandatory or reserved keywords.
 
 
5.2.3.2       Restrictions
 
No keyword in the primary header shall specify the presence of a specific
extension in a FITS file; only the EXTEND keyword described in Section 5.2.1.2
shall be used to indicate the possible presence of extensions. No keyword in
either the primary or extension header shall explicitly specify the physical
block size, other than the BLOCKED keyword of Section 5.2.2.1
 
5.3        Value/Comment
 
The structure of the value field shall be determined by the type of the
variable.
 
5.3.1       Character  String
 
If the value is a character string, column 11 shall contain a single quote
(hexadecimal code 27, "'"); the string shall follow, starting in column 12,
followed by a closing single quote (also hexadecimal code 27) that should not
occur before column 20 and must occur in or before column 80. Proper
interpretation of the data set should not require decoding any more than the
first eight characters of a character string. The Character string can only be
composed of the printable ASCII characters (hexadecimal codes 20 through 7E).
 
 
5.3.2       Logical  Variable
 
If the value is a logical constant it shall appear as a T or F in column 30.
 
 
5.3.3       Integer
 
If the value is an integer, the ASCII representation shall appear right
justified in columns 11-30. For a complex integer, the imaginary part shall be
right justified in columns 31-50.
 
 
5.3.4       Real  Floating  Point  Number
 
If the value is a real floating point number, the ASCII representation shall
appear in columns 11-30. Letters in the exponential form shall be upper case.
The value shall be right justified, and the decimal point must appear. Note:
The full precision of 64-bit values can not be expressed as a single value
following the rules of this standard.
 
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5.3.___VALUE/COMMENT_________________________________________________________25
 
 
5.3.5       Complex  Floating  Point  Number
 
 
5.3.5.1       Real Part
 
 
If the value is a complex floating point number, the ASCII representation of
the real part shall appear in the same manner as a real floating point number
(see above).
 
 
5.3.5.2       Imaginary Part
 
 
The ASCII representation of the imaginary part of a complex floating point
number shall appear in columns 31 - 50. Letters in the exponential form shall
be upper case. The value shall be right justified, and the decimal point must
appear. Note: The full precision of 64-bit values can not be expressed as a
single value following the rules of this standard.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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26_________________________________________________________SECTION_5.___HEADERS
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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_____________________________________________________________________________27
 
 
Section   6
 
 
Data   Representation
 
 
Primary and extension data shall appear in one of the formats described in this
section.
 
 
6.1        Characters
 
 
Each character shall be represented by one byte. A character shall be written
in 7-bit ASCII [8], right justified in the byte. The high-order parity bit
shall be zero.
 
 
6.2        Integers
 
 
6.2.1       Eight-bit
 
 
Eight-bit integers shall be unsigned binary integers, stored in one byte.
 
 
6.2.2       Sixteen-bit
 
 
Sixteen-bit integers shall be twos-complement signed binary integers, contained
in two bytes. The sign bit shall be the leftmost bit of the leftmost byte.
Significance shall decrease from left to right; the least significant bit shall
be the rightmost bit of the rightmost byte.
 
 
6.2.3       Thirty-two-bit
 
 
Thirty-two bit integers shall be twos-complement signed binary integers,
contained in four bytes. The sign bit shall be the leftmost bit of the leftmost
byte. Significance shall decrease from left to right; the least significant bit
shall be the rightmost bit of the rightmost byte.
 
 
 
 
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28_____________________________________________SECTION_6.___DATA_REPRESENTATION
 
 
6.3        IEEE-754  Floating  Point
 
 
Transmission of 32- and 64-bit floating point data within the FITS format will
use the ANSI/IEEE-754 standard [9]. BITPIX = -32 and BITPIX = -64 signify 32-
and 64-bit IEEE floating point numbers; the absolute value of BITPIX is used
for computing the sizes of data structures. The full IEEE set of number forms
is allowed for FITS interchange, including all special values (e.g., the
"Not-a-Number" cases). The order of the bytes will be sign and exponent first,
followed by the mantissa bytes in order of decreasing significance. The BLANK
keyword should not be used when BITPIX = -32 or -64.
 
 
6.3.1       Thirty-two-bit  Floating  Point
 
 
6.3.1.1       Interpretation
 
 
Lowest numbered (leftmost) bits are most significant. The IEEE NaN
(Not-a-Number) values shall be used to represent undefined values. All IEEE
special values are recognized.
 
                    sign    (exponent - 127)
        value = (-1)     x 2                 x mantissa
 
 
6.3.1.2       Structure
 
 
Table 6.1 describes the bit structure of 32-bit floating point values.
 
 
                        Bit Positions        Content
                     __(left_to_right)_____________________
 
                              1                sign
                            2 - 9            exponent
                           10 - 32           mantissa
 
          Table 6.1:  Content of 32-bit floating point bit positions.
 
 
 
 
 
 
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6.3.___IEEE-754_FLOATING_POINT_______________________________________________29
 
 
6.3.2       Sixty-four-bit  Floating  Point
 
 
6.3.2.1       Interpretation
 
 
Lowest numbered (leftmost) bits are most significant. The IEEE NaN
(Not-a-Number) values shall be used to represent undefined values. All IEEE
special values are recognized.
 
                           sign    (exponent - 1023)
               value = (-1)     x 2                  x mantissa
 
 
6.3.2.2       Structure
 
 
Table 6.2 describes the bit structure of 64-bit floating point values.
 
 
                        Bit Positions         Content
                     __(left_to_right)_____________________
 
                              1                sign
                            2 - 12           exponent
                           13 - 64           mantissa
 
          Table 6.2:  Content of 64-bit floating point bit positions.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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30_____________________________________________SECTION_6.___DATA_REPRESENTATION
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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_____________________________________________________________________________31
 
 
Section   7
 
 
Data   Records
 
 
7.1        Primary  Array
 
 
The data values shall be stored as a byte stream, with no embedded fill or
blank space. The first value shall be stored in the first position of the first
array record. The first value of each subsequent row of the array shall be
stored in the position immediately following the last value of the previous
row, i.e., the data values are stored as a byte stream, with no embedded fill
or blank space. A primary array, when present, shall consist of an array of 1
to 999 dimensions. Arrays of more than one dimension shall be stored in a
sequence such that the index along axis 1 varies most rapidly, that along axis
2 next most rapidly, and those along subsequent axes progressively less
rapidly, with that along axis n varying least rapidly; i. e., the elements of
an array A(x1 ; x2 ; . . .; xm ) shall be in the order shown in Figure 7.1,
where m is the value of NAXIS. The remainder of the last data record, following
the last element of the array shall be filled as prescribed in Section 4.3.2.
 
 
7.2        ASCII  Tables  Extension
 
 
Data shall appear as an ASCII Tables extension if the primary header of the
FITS file has the keyword EXTEND set to T and the first keyword of that
extension header has XTENSION=~'TABLE~~~'.
 
 
7.2.1       Storage
 
 
The data shall be stored as a two-dimensional character array. The row length
and the number of rows shall be specified by keywords in the associated header
records. The number of characters in a row and the number of rows in the table
shall determine the size of the character array. Every row in the array shall
have the same number of characters. The first character of the first row shall
be at the start of the record immediately following the last header record.
The first character of subsequent rows shall follow immediately the character
at the end of the previous row, independent of the record structure. The
positions in the last data record after the last character of the last row of
the data array shall be filled with ASCII blanks.
 
 
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32____________________________________________________SECTION_7.___DATA_RECORDS
 
 
 
 
 
 
 
 
 
 
                              A(1, 1, . . ., 1),
                              A(2, 1, . . ., 1),
                                       .
                                       .
                                       .,
                            A(NAXIS1, 1, . . ., 1),
                              A(1, 2, . . ., 1),
                              A(2, 2, . . ., 1),
                                       .
                                       .
                                       .,
                            A(NAXIS1, 2, . . ., 1),
                                       .
                                       .
                                       .,
                         A(1, NAXIS2, . . ., NAXISm),
                                       .
                                       .
                                       .,
                       A(NAXIS1, NAXIS2, . . ., NAXISm)
 
 
Figure 7.1: Arrays of more than one dimension shall be stored in a sequence
such that the index along axis 1 varies most rapidly and those along subsequent
axes progressively less rapidly. Except for the location of the first element,
array structure is independent of record structure.
 
 
 
 
 
 
 
 
 
 
 
 
 
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7.2.___ASCII_TABLES_EXTENSION________________________________________________33
 
 
7.2.2       Fields
 
 
Each row in the array shall consist of a sequence of fields, with one entry in
each field. For every field, the format of the stored information, location in
the row of the beginning of the field and (optionally) the field name, shall be
specified in keywords of the associated header records. A separate format
keyword must be provided for each field. The location and format of fields
shall be the same for every row.
 
 
7.2.3       Entries
 
 
All data in an ASCII tables extension record shall be 7-bit printable ASCII
characters with the eighth (parity) bit set to zero (hexadecimal codes 20
through 7E). The only possible formats shall be FORTRAN I, A, F, E, or D. If
values of -0 and +0 must be distinguished, then the sign character should be
stored in a separate field in character format. TNULL keywords may be used to
specify a character string that represents an undefined value in each field.
The characters representing an undefined value may differ from field to field
but must be the same within a field.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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34____________________________________________________SECTION_7.___DATA_RECORDS
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Section   8
 
 
Restrictions   on   Changes
 
 
Any structure that is a valid FITS structure shall remain a valid FITS
structure at all future times. Use of certain valid FITS structures may be
deprecated by this or future FITS standard documents.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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36_________________________________________SECTION_8.___RESTRICTIONS_ON_CHANGES
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Appendix   A
 
 
Random   Groups   Structure
 
 
(This Appendix is a part of the NSDSSO FITS Standard)
     Although it is part of the FITS standard, the random groups structure for
the primary data has been used almost exclusively for applications in radio
interferometry; outside this field, few FITS readers can read data in random
groups format. The evolving binary tables format will eventually be able to
accommodate the structure described by random groups. While existing FITS data
sets use the format, and it is therefore included in this standard, its use for
future applications is deprecated by this document.
 
 
A.1         Keywords
 
 
A.1.1        Mandatory  Keywords
 
 
If the primary data are in Random Groups format, the card images of the primary
header must use the following keywords in the order given here.
 
 
    1.  SIMPLE
 
    2.  BITPIX
 
    3.  NAXIS
 
    3.  + n ( n=1, . . ., value of NAXIS ). NAXISn
          .
          .
          .
          (other keywords, including)
 
        GROUPS
 
        PCOUNT
 
        GCOUNT
          .
          .
          .
 last   END
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38________________________________________APPENDIX_A.___RANDOM_GROUPS_STRUCTURE
 
 
A.1.1.1        SIMPLE Keyword
 
 
The card image containing this keyword is structured in the same way as for a
simple array (Section 5.2.1).
 
 
A.1.1.2        BITPIX Keyword
 
 
The card image containing this keyword is structured in the same way as for a
simple array (Section 5.2.1).
 
 
A.1.1.3        NAXIS Keyword
 
 
The value field shall contain an integer ranging from 1 to 999, representing
one more than the number of axes in each data array.
 
 
A.1.1.4        NAXIS1 Keyword
 
 
The value field shall contain the integer 0, as a signature of random groups
format.
 
 
A.1.1.5        NAXISn Keywords (n=2, . . ., value of NAXIS)
 
 
The value field shall contain an integer, representing the number of positions
along axis n-1 of each ordinary data array.
 
 
A.1.1.6        GROUPS Keyword
 
 
The value field shall contain the logical constant T. The value T associated
with this keyword implies that the primary array is in random groups format.
 
 
A.1.1.7        PCOUNT Keyword
 
 
The value field shall contain an integer equal to the number of parameters
preceding each group.
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A.2.___DATA_STORAGE__________________________________________________________39
 
 
A.1.1.8        GCOUNT Keyword
 
 
The value field shall contain an integer equal to the number of random groups
in the data array.
 
 
A.1.1.9        END Keyword
 
 
The card image containing this keyword is structured in the same way as for a
simple array (Section 5.2.1).
 
 
A.1.2        Reserved  Keywords
 
 
A.1.2.1        PTYPEn Keywords
 
 
The value field shall contain a character string, describing the physical
quantity or coordinate represented by the value for parameter n, as derived
from the quantity stored in the FITS data set and the PSCALn and PZEROn
keywords. If the PTYPEn keywords for more than one value of n have the same
associated value, then the data value for the physical quantity or coordinate
so defined is to be obtained by adding the derived data values of the
corresponding parameters.
 
 
A.1.2.2        PSCALn Keywords
 
 
This keyword shall be used, along with the PZEROn keyword, when the data set
parameter values are not the true physical values, to transform the data set
parameter values to the true physical values they represent, using the linear
transformation equation below with the PZEROn keyword. The value field shall
contain a floating point number representing the coefficient of the linear term
in the scaling equation, the scaling factor between true values and data set
parameter values at zero offset.
 
 
A.1.2.3 PZEROn Keywords
 
 
This keyword shall be used, along with the PSCALn keyword, when the data set
param- eter values are not the true physical values, to transform data set
parameter values to the true values. The value field shall contain a floating
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40________________________________________APPENDIX_A.___RANDOM_GROUPS_STRUCTURE
 
 
point number, representing the true value corresponding to a data set parameter
value of zero. The transformation equation is as follows:
 
               physical value = PZEROn + PSCALn x data set value
 
 
A.2         Data  Storage
 
 
Values shall be stored as a series of groups. The number of groups shall be
specified by the GCOUNT keyword in the associated header record. Each group
shall consist of the number of parameters specified by the PCOUNT keyword
followed by an array. The first parameter of the first group shall appear in
the first location of the first data record. The first element of each array
shall immediately follow the last parameter associated with that group. The
first parameter of any subsequent group shall immediately follow the last
member of the array of the previous group. The arrays shall be organized
internally in the same way as an ordinary primary array.
 
 
A.3         Data  Representation
 
 
Permissible data representations are the same as those for an ordinary primary
array. Parameters and members of associated data arrays shall have the same
representation. Should more precision be required for an associated parameter
than for a member of a data array, the parameter shall be divided into two
addends, represented by the same value for the PTYPEn keyword. The value shall
be the sum of the physical values, which may have been obtained from the array
values using the PSCALn and PZEROn keywords.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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_____________________________________________________________________________41
 
 
Appendix   B
 
 
A   Binary   Table   Extension
 
 
(This Appendix is not part of the NSDSSO FITS Standard but is included for
informational purposes only.)
     The new "3D"[1] binary tables format is under consideration by the
regional and international FITS committees. Its present form is expected to be
a subset of the eventual structure that is adopted; some structures and
capabilities remain to be added. It is in use at a number of installations, has
been implemented under AIPS and exchanged between AIPS and MIDAS, but much of
the existing FITS-reading software yet cannot decode the format. Because it is
becoming widely used, and because it illustrates an application of the rules
for conforming extensions, it is described in this Appendix.
 
 
B.1         Identification
 
 
The interim identification of a binary table is indicated if the primary header
of the FITS file has the keyword EXTEND set to T and the first keyword of the
associated extension header has XTENSION= 'A3DTABLE'.
 
 
B.2         Header
 
 
The header of a binary table extension shall conform to the requirements for
FITS headers specified in Section 5.
 
 
B.2.1        Required  Keywords
 
 
The card images in the header of a binary table extension use the following
keywords in the order given in Table B.1.
 
 
 
 
 
_________________________________________________
[1] The term "3D" derives from the capacity to store vectors as fields in
binary tables. Thus, the table effectively has depth.
 
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42_______________________________________APPENDIX_B.___A_BINARY_TABLE_EXTENSION
 
 
                             1      XTENSION
                             2      BITPIX
                             3      NAXIS
                             4      NAXIS1
                             5      NAXIS2
                             6      PCOUNT
                             7      GCOUNT
                             8      TFIELDS
                                    .
                                    .
                                    (other keywords, which must include)
                                    TFORMn, n = 1, 2, . . ., TFIELDS
                                    .
                                    .
                            last    END
 
     Table B.1:  Required keywords in the interim binary table extension.
 
 
B.2.1.1       XTENSION Keyword
 
The value field shall contain the character string 'A3DTABLE'.
 
 
B.2.1.2       BITPIX Keyword
 
The value field shall contain the integer 8, indicating that row size is given
in 8-bit bytes.
 
 
B.2.1.3       NAXIS Keyword
 
The value field shall contain the integer 2, indicating two axes, rows and
columns.
 
 
B.2.1.4       NAXIS1 Keyword
 
The value field shall contain an integer, representing the width of the table
in 8-bit bytes.
 
 
B.2.1.5       NAXIS2 Keyword
 
The value field shall contain an integer of value 1 or greater, representing
the number of rows in the table.
 
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B.2.___HEADER________________________________________________________________43
 
 
B.2.1.6        PCOUNT Keyword
 
The value field shall contain the integer 0; no data values precede the table.
 
 
B.2.1.7        GCOUNT Keyword
 
The value field shall contain the integer 1; the data records contain a single
table.
 
 
B.2.1.8        TFIELDS Keyword
 
The value field contains an integer representing the number of fields in each
row.
 
 
B.2.1.9        TFORMn Keywords
 
The value field contains a character string describing the data type of field
n. The following values are permitted, each representing the data type
indicated:
 
               _Value_field________Data_Type_____________________
 
                 rL                Logical
                 rX                Bit array
                 rA                ASCII characters
                 rI                16-bit integer
                 rJ                32-bit integer
                 rE                32-bit floating point
                 rD                64-bit floating point
 
 Table B.2:  Valid TFORMn format values in the interim binary table extension.
 
 
     The lower case r preceding the upper case type indicator is the repeat
count, describing how many times the data type appears within field n. A
repeat count of zero indicates that the entry defined in the header is omitted
in the table.
 
 
B.2.1.10         END Keyword
 
This keyword has no associated value.  Columns 9-80 shall be blank.
 
 
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44_______________________________________APPENDIX_B.___A_BINARY_TABLE_EXTENSION
 
 
B.2.2        Reserved  Keywords
 
These keywords are optional but are used in A 3-D Tables extensions only in the
manner defined in this document.
 
 
B.2.2.1       TTYPEn Keyword
 
The value field contains a character string representing the label or heading
for field n.
 
 
B.2.2.2       TUNITn Keywords
 
The value field contains a character string representing the physical units of
the quantities contained in field n.
 
 
B.2.2.3       TNULL Keyword.
 
The value field contains an integer representing the value used in integer
fields to represent an undefined value. The IEEE NaN (Not-a-Number) is used
for the same purpose in floating point fields.
 
 
B.3         Storage
 
Data are stored as a two dimensional array. The number of bytes in a row and
the number of rows are specified by the NAXIS1 and NAXIS2 keywords in the
associated extension header. Every row in the array must have the same number
of bytes. The number of bytes in a row and the number of rows in the table
determines the size of the array. The first byte of the first row is at the
start of the record immediately following the last header record. The first
byte of every subsequent row immediately follows the last byte of the row
preceding it.
 
 
B.4         Format
 
Each row in a binary table extension array consists of a sequence of fields,
each of which consists of zero or more entries. The location, length and format
of each field are the same for every row. The field sequence is specified by
the TFORMn keywords of the associated header. All entries in a field must be of
the same data type. For each field, the number of entries, data type, and
(optionally) the field name, are specified in keywords of the associated header
records. A separate format keyword must be provided for each field.
 
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B.5.___DATA_TYPES____________________________________________________________45
 
 
B.5         Data  Types
 
Each field in the table contains a binary representation of information in one
of the seven following forms:
 
 
B.5.1        Logical
 
A logical value consists of an 8-bit ASCII T for true or F for false.
 
 
B.5.2        Bit  Arrays
 
A bit array consists of an integral number of bytes with trailing bits zero. A
bit array shall start in the most significant bit of the first byte; bits will
follow in order of decreasing significance
 
 
B.5.3        ASCII  Characters
 
Each character shall be represented by one byte. A character shall be written
in 7-bit ASCII [8], right justified in the byte. The high-order parity bit
shall be zero.
 
 
B.5.4        Eight-bit  Integers
 
Eight-bit integers shall be unsigned binary integers, stored in one byte.
 
 
B.5.5        Sixteen-bit  Integers
 
Sixteen-bit integers shall be twos-complement signed binary integers, contained
in two bytes. The sign bit shall be the leftmost bit of the leftmost byte.
Significance shall decrease from left to right; the least significant bit shall
be the rightmost bit of the rightmost byte. The TNULL keyword is used if needed
to specify the symbol corresponding to an undefined value.
 
 
B.5.6        Thirty-two-bit  Integers
 
Thirty-two bit integers shall be twos-complement signed binary integers,
contained in four bytes. The sign bit shall be the leftmost bit of the leftmost
byte. Significance shall decrease from left to right; the least significant bit
shall be the rightmost bit of the rightmost byte. The TNULL keyword is used if
needed to specify the symbol corresponding to an undefined value.
 
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46_______________________________________APPENDIX_B.___A_BINARY_TABLE_EXTENSION
 
 
B.5.7        Thirty-two-bit  Floating  Point
 
 
B.5.7.1        Interpretation
 
Lowest numbered (leftmost) bits are most significant. The IEEE NaN
(Not-a-Number) values shall be used to represent undefined values. All IEEE
special values are recognized.
 
                      sign    (exponent - 127)
          value = (-1)     x 2                 x mantissa
 
 
B.5.7.2       Structure
 
 
Table 6.1 describes the bit structure of 32-bit floating point values.
 
 
B.5.8       Sixty-four-bit  Floating  Point
 
 
B.5.8.1       Interpretation
 
 
Lowest numbered (leftmost) bits are most significant. The IEEE NaN
(Not-a-Number) values shall be used to represent undefined values. All IEEE
special values are recognized.
 
 
                     sign    (exponent - 1023)
         value = (-1)     x 2                  x mantissa
 
 
B.5.8.2       Structure
 
 
Table 6.2 describes the bit structure of 64-bit floating point values.
 
 
 
 
 
 
 
 
 
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_____________________________________________________________________________47
 
 
Appendix   C
 
 
Implementation   on   Physical   Media
 
 
(This Appendix is not part of the NSDSSO FITS Standard, but is included as a
guide to recommended practices)
 
 
C.1         Block  Size
 
 
The block size (physical record length) for transport of data should, where
possible, equal the logical record length or an integer blocking factor times
this record length. Standard values of the blocking factor may be specified for
each medium; if not otherwise specified, the expected value is unity.
 
 
C.1.1        Magnetic  Tape
 
 
For nine-track magnetic tapes conforming to the ANSI X3.40-1983 specifications
[10], there should be from one to 10 logical records per physical block. The
BLOCKED keyword (section 5.2.2.1) may be used to warn that there may be more
than one logical record per physical block. The last physical block of a FITS
file should be truncated to the minimum number of FITS logical records required
to hold the remaining data, in accordance with ANSI X3.27-1978 specifications
[11]. With the issuance of this standard the BLOCKED keyword is deprecated by
this document.
 
 
C.1.2        Other  Media
 
 
For media where the physical block size cannot be equal to or an integral
multiple of the FITS logical record length of 23040-bits (2880 8-bit bytes),
records should be written over multiple blocks. Conventions regarding the
relation between physical block size and logical record length of FITS data
sets have not been otherwise established for other media.
 
 
 
 
 
 
 
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48_______________________________APPENDIX_C.___IMPLEMENTATION_ON_PHYSICAL_MEDIA
 
 
C.2         Physical  Properties  of  Media
 
The arrangement of digital bits and other physical properties of any medium
should be in conformance with the ANSI standard for that medium as specified in
the relevant document.
 
 
C.3         Labeling
 
C.3.1       Tape
 
Tapes may be either ANSI standard labeled or unlabeled. Unlabeled tapes are
preferred.
 
 
C.3.2       Other  Media
 
Conventions regarding labels for FITS data sets have not been established for
other media.
 
 
C.4         FITS  File  Boundaries
 
C.4.1       Magnetic  Tape
 
Individual FITS files are terminated by a tape-mark.
 
 
C.4.2       Other  Media
 
For media where the physical record size cannot be equal to or an integral
multiple of the standard FITS logical record length, a record of fewer than
23040 bits (2880 8-bit bytes) immediately following the end of the primary
header, data, or an extension should be treated as an end-of-file. Otherwise,
individual FITS data sets should be terminated by a delimiter appropriate to
the medium, analogous to the tape end-of-file mark. If more than one FITS data
set appears on a physical structure, the appropriate end-of-file indicator
should immediately precede the start of the primary headers of all data sets
after the first. FITS data sets should start at the start of a file.
 
 
C.5         Multiple  Physical  Volumes
 
Storage of a single FITS file on more than one tape or on multiple units of any
other medium is not supported.
 
 
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_____________________________________________________________________________49
 
 
Appendix   D
 
 
Summary   of   Keywords
 
 
(This Appendix is not part of the NSDSSO FITS Standard, but is included for
convenient reference).
 
 
 Principal    Conforming    ASCII Table     Random Groups   Binary Table
 ___HDU________Extension______Extension_________Extension_____Extension_____
 
  SIMPLE      XTENSION      XTENSION [1]    SIMPLE           XTENSION [2]
  BITPIX      BITPIX        BITPIX  =  8    BITPIX           BITPIX  =  8
  NAXIS       NAXIS         NAXIS  =  2     NAXIS            NAXIS  =  2
  NAXISn      NAXISn        NAXIS1          NAXIS1  =  0     NAXIS1
  EXTEND      PCOUNT        NAXIS2          NAXISn           NAXIS2
  END         GCOUNT        PCOUNT  =  0    GROUPS           PCOUNT  =  0
              END           GCOUNT  =  1    PCOUNT           GCOUNT  =  1
                            TFIELDS         GCOUNT           TFIELDS
                            TBCOLn          END              TFORMn
                            TFORMn                           END
 ___________________________END______________________________________________
 
 
[1] XTENSION= 'TABLE   ' for the ASCII Table extension.
[2] XTENSION= 'A3DTABLE' for the interim binary table extension.
 
 
Table D.1: Mandatory FITS keywords for the structures described in this
document.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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50____________________________________________APPENDIX_D.___SUMMARY_OF_KEYWORDS
 
 
   Principal HDU      Conforming    ASCII Table   Random Groups  Binary Table
 _General____Array____Extension_____Extension_____Extension______Extension___
 
 DATE       BSCALE    EXTNAME       TSCALn        PTYPEn         TTYPEn
 ORIGIN     BZERO     EXTVER        TZEROn        PSCALn         TUNITn
 BLOCKED    BUNIT     EXTLEVEL      TNULLn        PZEROn         TNULL
 AUTHOR     BLANK                   TTYPEn
 REFERENC   CTYPEn                  TUNITn
 COMMENT    CRPIXn
 HISTORY    CROTAn
            CRVALn
 DATE-OBS   CDELTn
 TELESCOP   DATAMAX
 INSTRUME   DATAMIN
 OBSERVER
 OBJECT
 EQUINOX
 EPOCH_______________________________________________________________________
 
 
Table D.2: Reserved FITS keywords for the structures described in this
document. Note that the EPOCH and BLOCKED keywords are deprecated by this
document.
 
 
   __Production___Bibliographic___Commentary___Observation_____Array___
 
     DATE         AUTHOR          COMMENT      DATE-OBS      BSCALE
     ORIGIN       REFERENC        HISTORY      TELESCOP      BZERO
     BLOCKED                                   INSTRUME      BUNIT
                                               OBSERVER      BLANK
                                               OBJECT        CTYPEn
                                               EQUINOX       CRPIXn
                                               EPOCH         CROTAn
                                                             CRVALn
                                                             CDELTn
                                                             DATAMAX
   __________________________________________________________DATAMIN___
 
 
Table D.3: General Reserved FITS keywords described in this document. Note that
the EPOCH and BLOCKED keywords are deprecated by this document.
 
 
 
 
 
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_____________________________________________________________________________51
 
 
Appendix   E
 
 
NSDSSO   Publications
 
 
   _____Document__________Title______________Date___________Status_____
 
   NSDSSO 100-0.1     FITS Standard     December, 1990   Draft Standard
 
   ____________________________________________________________________
 
 
 
                        Table E.1:  NSDSSO Publications
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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