[fitsbits] Is this legal FITS?
Tom McGlynn
tom.mcglynn at nasa.gov
Tue Jul 25 08:54:47 EDT 2017
From the Draft V4 standard (though this likely hasn't changed):
The NAXISn keywords must be present for
all values n = 1, ..., NAXIS, in increasing order of n, and
for no other values of n.
So having NAXIS=2 along with an NAXIS3 keyword unequivocally breaks the
standard. What various
libraries doe reading it is presumably undefined.
Tom
Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS INC] wrote:
> Folks:
>
> I was handed a couple of FITS files that our IDL software was having
> trouble reading. I was told that these FITS files were produced by
> DS9. Looking inside the files, I find the following keywords:
>
> SIMPLE = T / Fits standard
> BITPIX = -32 / number of bits per data pixel
> NAXIS = 2 / number of data axes
> NAXIS1 = 360 / length of data axis 1
> NAXIS2 = 180 / length of data axis 2
> NAXIS3 = 0 / length of data axis
>
> Our software was choking on the NAXIS3 = 0 entry. We tried several
> IDL routines written by different people over the years, but all of
> them had problems. My question is, is it even legal to have an NAXIS3
> keyword if NAXIS=2? The documentation states:
>
>> If NAXIS is equal to 0, there
>> shall not be any NAXISn keywords.
>
> To my mind, that infers that there should never be any NAXISn keywords
> for values of n greater than NAXIS.
>
> Also, according to the documentation,
>
>> A value of zero for any of
>> the NAXISn signifies that no data follow the header in the HDU
>> (however, the random groups structure described in Sect. 6 has
>> NAXIS1 = 0, but will have data following the header if the other
>> NAXISn keywords are non-zero).
>
> However, these files did contain image data after the header. One can
> argue that the NAXIS3 keyword is meaningless, but the fact that it
> shows up in the header with a value of 0 does make it potentially look
> like there shouldn't be a data part to the HDU.
>
> I was able to render these files readable in our software by changing
> the value of NAXIS3 to 1 instead of 0. I can modify at least one IDL
> reader to work with these files without modification, but I do have
> doubts about these files being properly formatted.
>
> Bill Thompson
>
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