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Re: [fitsbits] WAVE-TAB implementation

Subject: Re: [fitsbits] WAVE-TAB implementation
From: Mark Calabretta
Date: Thu, 08 Sep 2005 19:07:57 +1000
Newsgroups: sci.astro.fits
On Tue 2005/09/06 11:06:30 MST, Tom Jarrett wrote
in a message to: fitsbits@xxxxxxxxxxxxxxxxx
and copied to: 

>is correct.  I would greatly appreciate it if someone out there has a look
>at one of our cubes, checking out the FITS header and the
>CTYPE3  = 'WAVE-TAB'           /Wavelength
>definition.  Please let me know if you find a problem or potential
>problem with what we have done.  Here is the sample, it is a spectral map

It's basically correct but there are a few things you need to fix:

1) According to Paper I, if any CDi_ja cards are given then those not
   given all default to zero.  Therefore CD3_3 == 0 and your matrix is
   singular.

2) In general, the coordinate array is of dimensionality M+1 with axis
   lengths (M, K1, K2,...) which are to be recorded in a TDIMn card.

   Although your table is 1-dimensional (M == 1), the coordinate array
   is formally considered to be 2-dimensional though with a degenerate
   first axis, and you should write the TDIMn card.

3) CRPIX3 and CRVAL3 are both omitted from the header which is bad form.

Also, note that -TAB does not allow extrapolation; although it could be
accomodated for non-degenerate 1-D tables with default indexing (perhaps
it should be) it can't easily be generalized to higher dimensions or
non-default indexing.

Hence, your -TAB only defines wavelengths for pixel coords between 1.0
and NAXIS3.  If you want to compute wavelengths at pixel coordinate 0.5
or NAXIS3+0.5 you'll need to extend the coordinate array slightly and
adjust CRPIX3 and/or CRVAL3 accordingly.  

Mark Calabretta
ATNF


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