7.2. THE ASCII TABLE EXTENSION

51

number representing the coefficient of the linear term in the scaling equation. The

default value for this keyword is 1.0. This keyword must not be used for A-format

fields.

The transformation equation used to compute a true physical value from the quantity

in field n is

physical_value = TZEROn + TSCALn × field_value.

(7.1)

where field_value is the value that is actually stored in that table field in the FITS

file

TZEROn Keywords  This indexed keyword shall be used, along with the TSCALn key-

word, to linearly scale the values in the table field n to transform them into the physical

values that they represent using Eq. 7.1. The value field shall contain a floating-point

number representing the physical value corresponding to an array value of zero. The

default value for this keyword is 0.0. This keyword must not be used for A-format fields.

TNULLn Keywords  The value field for this indexed keyword shall contain the character

string that represents an undefined value for field n. The string is implicitly space filled

to the width of 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 of the contents of field n. This keyword overrides the default display format given

by the TFORMn keyword. 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. Only the format codes in Table 7.4, 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.

7.2.3

Data Sequence

The table is constructed from a two-dimensional array of ASCII characters. The row

length and the number of rows shall be those specified, respectively, by the NAXIS1 and

NAXIS2 keywords of the associated header. The number of characters in a row and the

number of rows in the table shall determine the size of the character array. Every row

in the array shall have the same number of characters. The first character of the first

row shall be at the start of the data block immediately following the last header block.

The first character of subsequent rows shall follow immediately the character at the end

FITS Standard