Next: F Summary of Keywords
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(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)
- Nbits
-- The requirement
that
Nbits
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.
Next: F Summary of Keywords
Up: Definition of the Flexible
Previous: D Suggested Time Scale
5/13/1999