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 contains only 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
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
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