8.3.2 Other Reserved Keywords

  In  addition to the mandatory keywords defined in §8.3.1, these keywords may be used to describe the structure of a binary table data array. They are optional, but if they appear within a binary table extension header, they must be used as defined in this section of this standard.

TTYPEn Keywords

The value field for this indexed keyword shall contain a  character string, giving the name of field n. It is recommended that only letters, digits, and underscore (hexadecimal code 5F, ``_'') be used in the name. String comparisons with the values of TTYPEn keywords should not be case sensitive. The use of identical names for different fields should be avoided.

TUNITn Keywords

The value field shall contain a character string  describing the physical units  in which the quantity in field n, after any application  of TSCALn and TZEROn, is  expressed. Units must follow the prescriptions in §5.3.

TNULLn Keywords

The value field for this indexed keyword shall contain the integer  that represents  an undefined value for field n of data type B, I, or J. The keyword may not be used if field n is of any other data type.

TSCALn Keywords

This indexed keyword shall be used, along with the TZEROn keyword, when the quantity in field n does not  represent a true physical quantity. It may not be used if the format of field n is A, L, or X. The interpretation for fields of type P is not defined. A proposed interpretation is described in Appendix B.1. For fields with all other data types, the value field shall contain a floating point number representing the coefficient of the linear term in Eq. 8.1, which is used to compute the true physical value  of the field, or, in the case of the complex data types C and M, of the real part of the field, with the imaginary part of the scaling factor set to zero. The default value for this keyword is 1.0.

TZEROn Keywords

This indexed keyword shall be used, along with the TSCALn keyword, when the quantity in field n does not  represent a true physical quantity. It may not be used if the format of field n is A, L, or X. The interpretation for fields of type P is not defined. A proposed interpretation is described in Appendix B.1. For fields with all other data types, the value field shall contain a floating point number representing the true physical value corresponding to a value of zero in field n of the FITS file, or, in the case of the complex data types C and M, in the real part of the field, with the imaginary part set to zero. The default value for this keyword is 0.0. Equation 8.1 is used to compute  a true physical value from the quantity in field n.

TDISPn Keywords

The value field of this indexed keyword shall contain a  character string describing the format recommended for the display of the contents of field n. If the table value has been scaled, the physical value, derived using Eq. 8.1, 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 a an unsigned integer. Arrays of type A may be terminated with a zero byte. Only the format codes in Table 8.6, discussed in §8.3.4, 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. The meaning of this keyword is not defined for fields of type P in this standard but may be defined in conventions using such fields.

 

Field Value Data Type

Aw Character

Lw Logical

Iw.m Integer

Bw.m Binary, integers only

Ow.m Octal, integers only

Zw.m Hexadecimal, integers only

Fw.d Single precision real

Ew.dEe Single precision real, exponential notation

ENw.d Engineering; E format with exponent multiple of 3

ESw.d Scientific; same as EN but nonzero leading digit if not zero

Gw.dEe General; appears as F if significance not lost, else E.

Dw.dEe Double precision real, exponential notation

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 shall be the product  of the values of NAXIS1 and  NAXIS2. This keyword shall not be used if the value of PCOUNT is zero. A proposed application of this keyword is presented in Appendix B.1.

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 multidimensional array, providing the number of dimensions and the length along each axis. The form of the value is not further specified by this standard. A proposed convention is described in Appendix B.2.