E Differences from IAU-endorsed Publications

  (This Appendix is not part of the NOST FITS Standard but is included for informational purposes only.)

Note: In  this discussion, the term the FITS papers refers to [1], [2], [4], [5], [9], and [10] collectively, the term Floating Point Agreement (FPA) refers  to [8], the term  Blocking Agreement refers to [11]; and the term DATExxxx Agreement refers to the redefinition of the value format for date keywords approved by the IAUFWG in 1997.

1.

§3 -- Definitions, Acronyms, and Symbols

Array value

-- This  precise definition is not used in the original FITS papers.

ASCII text

-- This  permissible subset of the ASCII character set, used in many contexts, is not precisely defined in the FITS papers.

Basic FITS

-- This definition  includes the possibility of floating point data arrays, while the terminology in the FITS papers refers to FITS as described in [1], where only integer arrays were possible.

Conforming Extension

-- This terminology is not used in the  FITS papers. 

Deprecate

-- The concept of deprecation does  not appear in the FITS papers.

FITS structure

-- This  terminology is not used in the FITS papers in the precise way that it is in this standard.

Fraction

-- This  terminology and the distinction between fraction and mantissa do  not appear in the Floating Point Agreement.

Header and Data Unit

-- This terminology is not used in the FITS papers.

Indexed keyword

-- This terminology is not  used in the original FITS papers.

Physical value

-- This  precise definition is not used in the original FITS papers.

Reference point

-- This term replaces the reference pixel of the  FITS papers. The new terminology is consistent with the fact that the array need not represent a digital image and that the reference point (or pixel) need not lie within the array.

Repeat count

--This terminology is not used in the FITS papers.

Reserved keyword

-- The FITS papers  describe optional keywords but do not say explicitly that they are reserved.

Standard Extension

-- This  precise  definition is new. The term standard extension is used in some contexts in the FITS papers to refer to what this standard defines as a standard extension and in others to refer to what this standard defines  as  conforming extension.

2.

§4.3.2 Primary Data Array
Fill  format -- This  specification is new. The FITS papers and the FPA do not precisely specify the format  of data fill for the primary data array.

3.

§4.4.1.1 Identity (of conforming extensions)
The FITS papers specify that creators of new  extension types  should check with the FITS standards committee. This standard identifies the committee specifically, introduces the role of the FITS Support Office  as its  agent, and mandates registration.

4.

§4.6 Physical Blocking
This material is based entirely on the  Blocking Agreement. Material in the early FITS papers [1,4] specifying the expression of FITS on specific physical media is not part of this standard.

5.

§4.6.1 Bitstream Devices
The Blocking Agreement specifies that this rule applies to FITS files written to logical file systems. This standard applies the rule to all bitstream devices, not only logical file systems.

6.

§4.6.2.1 Fixed Block
The Blocking Agreement specifies that this rule applies to FITS files written to optical disks, (accessed as a sequential set of records), QIC format 1/4-inch cartridge tapes and Local Area networks. This standard extends the rule to other fixed block length sequential media.

7.

§4.6.2.2 Variable Block
The Blocking Agreement specifies that this rule applies to FITS files written to 1/2-inch 9 track tapes, DDS/DAT 4mm cartridge tapes and 8mm cartridge tape (Exabyte). This standard extends the rule to all variable block length sequential media and eliminates references to specific products.

8.

§5.1.2.1 Keyword (as header component)
The specification of permissible keyword characters is new. The FITS papers do not precisely define the permissible characters for keywords.

9.

§5.1.2.2 Value Indicator (bytes 9-10)
The FITS papers do not specifically address the permissibility of null values. This standard states explicitly that they are permitted.

10.

§5.1.2.3 Value/Comment (bytes 11-80)
In the FITS papers, the slash between the value and comment is optional. This standard requires the slash, consistent with the prescription of FORTRAN-77 list-directed input.

11.

§5.2 Value, including its subsections
The FITS papers specify that the value field is to be written following the rules of ANSI FORTRAN-77  list-directed  input, with some restrictions. This standard explicitly describes the format of the value field. The FITS papers permit the value field to contain an array of values. This standard specifies that there shall be only one value in the value field. The FITS papers require the fixed format for the most essential parameters. This standard identifies those parameters with the values of the mandatory  keywords.

12.

§5.2.1 Character String
The standard explicitly describes how single quotes are to be coded into keyword values, a rule only implied by the FORTRAN-77 list-directed  read  requirements of the FITS papers.

The standard states that in general, character-valued keywords can have lengths up to the maximum 68 character length.

13.

§5.2.3 Integer
The standard explicitly notes that the fixed format for complex integers    does not  conform to the rules for  ANSI FORTRAN list-directed read. 

14.

§5.2.4 Real Floating Point Number
The standard explicitly notes that the full precision of 64-bit values cannot be expressed as a single value using the fixed  format.

15.

§5.2.5 Complex Integer Number
The standard does not support the fixed format for complex integers defined in the FITS papers but is consistent with FORTRAN-77 list-directed read  as required in the FITS papers for free format. Because the fixed format of the FITS papers did not conform to the rules for FORTRAN-77 list-directed I/O, consistency with both was impossible. There are no known FITS files that use the fixed format for complex integers that was defined in the FITS papers.

16.

§5.2.6 Complex Floating Point Number
The standard does not support the fixed format for complex floating point numbers   defined in the FITS papers but is consistent with FORTRAN-77 list-directed read  as required in the FITS papers for free format. Because the fixed format of the FITS papers did not conform to the rules for FORTRAN-77 list-directed I/O, consistency with both was impossible. There are no known FITS files that use the fixed format for complex floating point numbers that was defined in the FITS papers.

17.

§5.3 Units
The FITS papers recommend the use of SI units  and identify certain other units standard in astronomy. This standard codifies the recommendation and makes it more specific by referring to the IAU Style  Manual [7], while explicitly recommending degrees for angular measure and requiring degrees for celestial coordinates.

18.

§5.4.1.1 Principal (mandatory keywords)

(a)

SIMPLE keyword -- The explicit prohibition against the appearance of the SIMPLE keyword in extensions does not appear in the FITS papers.

(b)

NAXIS keyword -- The  requirement that the NAXIS keyword  may not be negative is not explicitly specified in the FITS papers.

(c)

NAXISn keyword -- The requirement that the NAXISn keyword may not be negative  is not explicitly specified in the FITS papers.

19.

§5.4.1.2 Conforming Extensions

(a)

N_bits -- The requirement  that  N_bits may not be negative  is not explicitly specified in the FITS papers.

(b)

XTENSION keyword -- That this keyword  may not appear in the primary header is only implied by the FITS papers; the prohibition is explicit in this standard. The FITS papers name a FITS standards committee as the keeper of the list of accepted extension  type names. This standard specifically identifies the committee and introduces the role of the FITS Support Office  as its agent.

20.

§5.4.2 Other Reserved Keywords
That  the optional keywords defined in the FITS papers are to be reserved for both the primary HDUs and all extensions with the meanings and usage defined in those papers, as in the standard, is not explicitly stated in all of them, although some keywords are explicitly reserved in the papers describing the image and binary table extensions.

21.

§5.4.2.1 Keywords Describing the History or Physical Construction of the HDU

(a)

DATE Keyword -- The notation for four-digit year number is YYYY rather than the CCYY of the ``DATExxxx Agreement''. The recommendation for use of Universal Time in the superseded format with a two-digit year is not in the FITS papers.

(b)

BLOCKED keyword -- The FITS papers require  the BLOCKED keyword to appear in the first record of the primary header  even though it cannot when the value of NAXIS exceeds  the values described in the text. They do not address this contradiction. This standard deprecates the BLOCKED keyword.

22.

§5.4.2.2 Keywords Describing Observations

(a)

DATE-OBS Keyword -- The recommendation  for use of Universal  Time in the superseded format with a two-digit year is not in the FITS papers.

(b)

EQUINOX and EPOCH keywords -- This standard  replaces the  EPOCH keyword with the more appropriately named EQUINOX keyword and deprecates  the EPOCH name.

23.

§5.4.2.4 Commentary keywords
Keyword field is blank -- Reference [1] contains the text ``BLANK'' to represent a blank keyword field. The standard clarifies the intention. 

24.

§5.4.2.5 Array keywords

(a)

BUNIT Keyword -- The FITS papers recommend the use of SI  units, degrees as the appropriate unit for  angles, and identify  other units standard in astronomy. This standard specically applies the recommendations of §5.3 to the BUNIT keyword.

(b)

CTYPEn, CRVALn, CDELTn, and CROTAn Keywords  -- This  standard  extends  the recommendations on units  to coordinate axes, explicitly requiring decimal degrees for coordinates.

(c)

CRPIXn Keywords -- This standard explicitly notes the ambiguity  in the location of the index number relative to an image pixel.

(d)

CDELTn Keywords -- The definition in the standard differs from that in the FITS papers in that it provides for the case where  the spacing between index points varies over the grid. For the case of constant spacing, it is identical to the specification in the FITS papers.

(e)

DATAMAX and DATAMIN Keywords -- The standard clarifies that the value refers to the physical value  represented by the  array , after any  scaling, not the array value  before scaling. The standard also notes that special values  are not to be considered when determining the values of DATAMAX and DATAMIN, an issue not specifically addressed by the FITS papers or the FPA.

25.

§7 Random Groups  Structure
The standard deprecates  the Random Groups structure.

26.

§7.1.2 Reserved Keywords (random groups)
That  the optional keywords defined in the FITS papers are to be reserved with the meanings and usage defined in those papers, as in the standard, is not explicitly stated in them.

27.

§7.1.2.2 PSCALn Keywords -- The default value is explicitly specified in the standard, whereas in the FITS papers it is assumed by analogy  with the  BSCALE keyword.

28.

§7.1.2.3 PZEROn Keywords -- The default value is explicitly specified in the standard, whereas in the FITS papers it is assumed by analogy  with the  BZERO keyword.

29.

§8.1 ASCII Table Extension
The name ASCII table is given to the  ``tables'' extension discussed  in the FITS papers to distinguish it from the binary table extension.

30.

§8.1.1 Mandatory Keywords (ASCII table)

(a)

NAXIS1 keyword -- The  requirement that the NAXIS1 keyword may not be negative in an ASCII table header is not explicitly specified in  the FITS papers.

(b)

NAXIS2 keyword -- The requirement that the NAXIS2 keyword may not be negative in an ASCII table header is not explicitly specified in  the FITS papers.

(c)

TFIELDS keyword -- The requirement that the TFIELDS keyword may not be negative is not explicitly specified in  the FITS papers.

(d)

TFORMn keyword -- The requirement that format codes must be specified in upper case is implied but not explicitly specified in  the FITS papers.

31.

§8.1.2 Other Reserved Keywords (ASCII table)
That  the optional keywords defined in the FITS papers are to be reserved with the meanings and usage defined in those papers, as in the standard, is not explicitly stated in them.

(a)

TUNITn Keywords -- The FITS papers  do not explicitly recommend the use of any particular units  for this keyword, although the reference to the BUNIT keyword may be considered an implicit extension of  the recommendation for that keyword. This standard makes the recommendation more specific for the TUNITn keyword by requiring conformance to the prescriptions in §5.3.

(b)

TSCALn Keywords -- The prohibition against use in A-format fields is stronger than the statement in the FITS papers that the keyword  ``is not relevant''.

(c)

TZEROn Keywords -- The prohibition against use in A-format fields is stronger than the statement in the FITS papers that the keyword  ``is not relevant''.

32.

§8.3.2 Other Reserved Keywords (Binary Table)
The EXTNAME, EXTVER, EXTLEVEL, AUTHOR, and REFERENC keywords explicitly reserved for binary tables in the defining paper are reserved in the standard under the general prescription of §5.4.2.

(a)

TUNITn Keywords -- The FITS papers  do not explicitly recommend the use of any particular units  for this keyword. This standard makes the recommendation more specific for the TUNITn keyword by requiring conformance to the prescriptions of §5.3.

(b)

TDISPn Keywords -- The version   of the BINTABLE paper upon which the FITS committees voted stated incorrectly that the values used to display bit and byte arrays should be considered signed. This standard follows the text in the published BINTABLE paper, which specifies that these values should be unsigned. The BINTABLE paper does not specify how a TDISPn value for a field of type P is interpreted; this standard explicitly mandates no interpretation but allows conventions to provide interpretations. The requirement that format codes must be specified in upper case is implied but not explicitly specified in the BINTABLE paper.

(c)

THEAP Keywords -- The FITS papers  state only that the keyword is reserved for use in the convention described in in Appendix B.1. This standard makes the more specific statement that this keyword is used to provide the separation, in bytes, between the start of the main data table and the start of a supplemental data area called the  heap and identifies the default value.

(d)

TDIMn Keywords -- The FITS papers state  only that the keyword is reserved for use in the convention described in Appendix B.2. This standard makes the more specific statement that the contents of the value field contain a character string describing how to interpret the contents of a field as a multidimensional array.

33.

§8.3.4 Data Display
The BINTABLE paper suggests that the format for display suggested by the TDISPn should be understood as a Fortran-90 format or, where Fortran-90 is unavailable, a FORTRAN-77 format. This standard explicitly describes the formats. The statement in the standard concerning differences between E and D format codes, which notes that the latter implies greater precision in the internal datum, does not appear in the BINTABLE paper.

34.

§9 Restrictions on Changes
The FITS papers do not provide for the concept of deprecation.

35.

Appendix C Implementation on Physical Media
Material in the FITS papers specifying the expression of FITS on specific physical media is not part of this standard; what is provided in the appendix is purely as a guide to recommended practices.