7.3. BINARY TABLE EXTENSION

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setting TZEROn = 32768 and TSCALn = 1. If it is desired to use elements that have an

unsigned value (i.e., the physical value) equal to 0 to represent undefined elements in

the field, then the TNULLn keyword must be set to the value -32768 because that is the

actual value stored in the FITS file for those elements in the field.

TDISPn Keywords  The value field of this indexed keyword shall contain a character

string describing the format recommended for displaying an ASCII text representation

of the contents of field n. If the table value has been scaled, the physical value, derived

using Eq. 7.1, shall be displayed. All elements in a field shall be displayed with a single,

repeated format. For purposes of display, each byte of bit (type X) and byte (type B)

arrays is treated as an unsigned integer. Arrays of type A may be terminated with a zero

byte. Only the format codes in Table 7.8, interpreted as Fortran [17] output formats,

and discussed in more detail in §7.3.4, are permitted for encoding. The format codes

must be specified in upper case. If the Bw.m, Ow.m, and Zw.m formats are not readily

available to the reader, the Iw.m display format may be used instead, and if the ENw.d

and ESw.d formats are not available, Ew.d may be used. In the case of fields of type

P or Q, the TDISPn value applies to the data array pointed to by the array descriptor

(§7.3.5), not the values in the array descriptor itself.

THEAP Keyword  The value field of this keyword shall contain an integer providing

the separation, in bytes, between the start of the main data table and the start of a

supplemental data area called the heap. The default value, which is also the minimum

allowed value, shall be the product of the values of NAXIS1 and NAXIS2. This keyword

shall not be used if the value of PCOUNT is zero. The use of this keyword is described in

in §7.3.5.

TDIMn Keywords  The value field of this indexed keyword shall contain a character

string describing how to interpret the contents of field n as a multi-dimensional array

with a format of '(l,m,n...)' where l, m, n, . . . are the dimensions of the array. The

data are ordered such that the array index of the first dimension given (l) is the most

rapidly varying and that of the last dimension given is the least rapidly varying. The

total number of elements in the array equals the product of the dimensions specified

in the TDIMn keyword. The size must be less than or equal to the repeat count on the

TFORMn keyword, or, in the case of columns that have a 'P' or 'Q' TFORMn data type,

less than or equal to the array length specified in the variable-length array descriptor

(see §7.3.5). In the special case where the variable-length array descriptor has a size of

zero, then the TDIMn keyword is not applicable. If the number of elements in the array

implied by the TDIMn is less than the allocated size of the array in the FITS file, then

the unused trailing elements should be interpreted as containing undefined fill values.

A character string is represented in a binary table by a one-dimensional character

array, as described under `Character' in the list of data types in §7.3.3.1. For example,

FITS Standard