7.5.3.1: Zero Size FITS Image

FITS requires that all files begin with a PHDU containing an image. In the case of FITS files whose primary purpose is to contain tables it is common to begin the file with an image of zero size. The HDU for such an image need only contain the required keywords. A schematic representation of such a FITS header follows:

SIMPLE  =                    T / file does conform to FITS standard
BITPIX  =                    8 / number of bits per data pixel
NAXIS   =                    0 / number of data axes
EXTEND  =                    T / FITS dataset may contain extensions
END

The header is padded out with ASCII blanks such that it consists of a total of 2880 bytes (36 card images of 80 characters). There are no 2880-byte blocks of image data; the next HDU follows immediately.

In practice it may be advantageous to insert duplicates of various header cards from the group members. Such duplicate cards serve as human-readable indexing information.

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Steve Allen <sla@ucolick.org>

$Date: 1996/03/13 21:29:29 $