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These keywords are optional in a binary table extension but may be used
only with the meanings specified below. They may appear in any order
between the TFIELDS and END keywords. Note that some of
these keywords are the same as those for an ASCII table extension. The
reason is that they are used for a binary table in a way analogous to
their use for an ASCII table. However, the allowed values and their
meaning may differ somewhat from those for an ASCII table, as the rows
of an ASCII table are composed of characters and those of a binary table
are composed of bytes.
- TTYPE (character) has a value giving the label or
heading for field . While the rules do not further prescribe the
value, following the recommendations for the TTYPE keyword of
ASCII tables is a good practice: using letters, digits, and underscore
but not hyphen; also, string comparisons involving the values should
be case insensitive. HEASARC has made this practice one of their
internal standards (section 5.6.1.1).
- TUNIT (character) has a value giving the physical units of
field . The rules should follow the prescriptions given in
section 3.1.1.4.
- TSCAL (floating) has a value providing a scale factor
for use in converting stored table values for field to physical
values. The default value is 1.
- TZERO (floating) has a value providing the offset for
field . The default value is 0.
As is the case for arrays, care should be taken to avoid overflows when
scaling floating point numbers.
For L, X, and A format fields, the TSCAL and
TZERO keywords have no meaning and should not be used. The
meaning has not been formally defined for P format fields, but the
general understanding is that the scaling should apply to the heap data
pointed to by the array descriptors.
- TNULL (integer) has the value that signifies an
undefined value for the integer data types B, I, and J. It should not
be used if the value of the corresponding TFORM specifies any
other data type. Null values for other data types are discussed in
section 3.6.3.
- TDISP (character) has a value giving the Fortran 90
format recommended for display of the contents of field . (If
Fortran 90 formats are not available to the software printing a table,
FORTRAN-77 formats may be used instead.) All entries in a single field
are displayed with the same format. If the field data are scaled, the
physical values, derived by applying the scaling transformation, are
displayed. For bit and byte arrays, each byte is considered to be an
unsigned integer for purposes of display. Characters and logical values
may be null (zero byte) terminated. The following formats are
allowed:
- Lw
- Logical
- Aw
- Character
- Iw.m
- Integer
- Bw.m
- Binary, integers only
- Ow.m
- Octal, integers only
- Zw.m
- Hexadecimal, integers only,
- Fw.d
- Single precision real, no exponent
- Ew.dEe
- Single precision real, exponential notation
- ENw.d
- Engineering format - single precision real, exponential
notation with exponent a multiple of 3
- ESw.d
- Scientific format - single precision real, exponential
notation with exponent a multiple of 3, nonzero leading digit (unless
value is zero)
- Gw.dEe
- General - appears as F format if significance will not be
lost; otherwise appears as E
- Dw.dEe
- Double precision real, exponential notation
In these formats, is the number
of characters in the displayed values, is the minimum number of
digits (leading zeroes may be required), is the number of digits
following the decimal point, and is the number of digits in the
exponent of an exponential form. Usage of this keyword in some ways
parallels that of the TFORM keyword of ASCII tables, in that
it provides a formatted value for the number. However, the format given
by the TFORM keyword in an ASCII table describes the format
of the number in the FITS file, but the format given by the
TDISP keyword of a binary table is different from that of the
number in the file.
The following keywords are reserved for proposed binary table
conventions:
- TDIM (character) is used by the multidimensional array
convention (section 5.2.3). For that convention, it has a value
giving the number of dimensions of field in the table, when those
dimensions are two or more. The value is of form '(i,j,k,...)',
where ,,,...are the dimensions of the array stored in
field . This size must be consistent with the repeat count specified
by the value of TFORM.
- THEAP (integer) is used by the variable length array
convention (section 5.2.1). For that convention, its value
gives the location of the start of the heap used to store variable
length arrays. The value is equal to the number of bytes of extension
data preceding the start of the heap, including the main table and any
gap between the main table and the heap.
All keywords reserved under the generalized extensions agreement
(section 3.3.2) apply to binary tables.
Next: Binary Table Extension Data
Up: Binary Tables
Previous: Required Keywords for Binary
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William Pence
2004-01-07