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Subsections

8.2.1 Mandatory Keywords

  The XTENSION keyword is required to be the first keyword of all image extensions. The  card images in the header of an image extension must use the keywords defined in Table 8.3 in the order specified. No other keywords may intervene between the XTENSION and GCOUNT keywords.


 
Table 8.3: Mandatory keywords in image extensions.
1 XTENSION
2 BITPIX
3 NAXIS
4 NAXISn, n = 1, ..., NAXIS
5 PCOUNT
6 GCOUNT
  .
.
.
  (other keywords ...)
  .
.
.
last END

XTENSION Keyword

The  value field shall contain the character  string value text 'IMAGE'.

BITPIX Keyword

The value field shall contain an integer. The  absolute value is used in computing the sizes of data structures. It shall specify the number of bits that represent a data value. The only valid values of BITPIX are given in Table 5.2.

NAXIS Keyword

The value field shall contain a non-negative integer no greater than 999, representing the number of axes in the associated data  array. A value of zero signifies that no data follow the header in the image  extension.

NAXISn Keywords

  The value field of this indexed keyword shall contain a non-negative integer, representing the number of elements along axis n of a data array. The NAXISn must be present for all values n = 1, ..., NAXIS, and for no other values of n. A value of zero for any of the NAXISn signifies that no data follow the header in the  image extension. If NAXIS is equal to 0, there should not be any NAXISn   keywords.

PCOUNT Keyword

The value field shall contain the integer 0. 

GCOUNT Keyword

The value field shall contain the integer 1; each  image extension contains a single array.

END Keyword

This keyword has no associated value. Columns 9-80  shall be filled with ASCII blanks.


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Next: 8.2.2 Units Up: 8.2 Image Extension Previous: 8.2 Image Extension

5/13/1999