[fitsbits] Interpretation of repeat count in binary tables

[Paddy Leahy]

Is the repeat count in TFORM descriptors provided purely for efficiency 
reasons, or does it have some implications for the logical structure of 
the data? Specifically, suppose we have

NAXIS1  =                12288 / width of table in bytes
NAXIS2  =                12288 / number of rows in table
...
TFIELDS = 3
TFORM1 = '1024E'
TFORM2 = '1024E'
TFORM3 = '1024E'

Should the three columns each be interpreted as a 1024 x 12288 element 
2-D array, or as a 12582912-element 1-D array? The first implies that if 
TFORMn = 'rTa', the default value of TDIMn = '(r)'. The second implies 
that if TDIMn is not explicitly specified then the repeat count is just 
provided for (rather spurious) efficiency reasons. Some FITS writers 
certainly use it as such.

This question arises out of pure ignorance, but I have failed to find an 
answer in either the original binary table paper (Cotton et al 1995) or
the current (2.0) or draft (3.0) FITS standards. In fact, none of these 
documents give any hint as to what the repeat count is for (unless TDIMn
is explicitly present, or it is a character field).

regards,
 		Paddy

======================================================
Dr J. P. Leahy, Jodrell Bank Centre for Astrophysics,
Current address: Osservatorio Astronomico di Trieste
INAF, Via GB Tiepolo 11, 34143 Trieste, Italy
Tel.+39 040 3199160 Fax +39 040 309418