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[fitsbits] Interpretation of repeat count in binary tables
Paddy,
In the example you cite below, the binary table contains 12288 rows of data, and each row contains 3 vectors, containing 1024 single precision floating point numbers leash. There is no explicit relationship between the vectors in different rows of the table. Structurally, the table begins with the 1024 element vector in the first column and first row, followed by the vectors in the 2nd and 3rd columns in the first row. These are followed by the 3 vectors in the 2nd row, then the 3rd row, and finally ending with the 3 vectors in the 12288th row. The TDIMn keyword, if present in this case, would specify the dimensionality of each individual 1024 element vector in that column. For example, TDIM1 = '(16,64)', would mean that each vector in column 1 should be interpreted as a 16 x 64 2D array. If there is no TDIMn keyword, then the vector would be interpreted as a 1024 element 1D array. Bill Pence Paddy Leahy wrote: 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 2D array, or as a 12582912element 1D 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. William Pence NASA/GSFC Code 662 HEASARC +13012864599 (voice) Greenbelt MD 20771 +13012861684 (fax) 
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