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[fitsbits] question about WCS keywords for pixel list



 
 
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Old September 4th 09, 04:12 PM posted to sci.astro.fits
Phil Hodge
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Default [fitsbits] question about WCS keywords for pixel list

According to Table 2 in WCS Paper I, all the keywords for the pixel list
representation of an image include the table column number as part of
the keyword name. This description agrees with the example in Table 10
of WCS Paper II. It seems to me, however, that several of these
keywords should include the world coordinate axis number rather than the
column number. I checked the "unofficial errata" for Papers I and II
but didn't find any mention of this.

Consider the case of a time-tag table containing columns TIME, X, and Y,
which are respectively the time when a photon was detected and the pixel
coordinates in a 2-D image where it was detected. To use a specific
example, suppose the world coordinates are right ascension and
declination. The reference pixel keywords are TCRPX2 and TCRPX3, giving
the column numbers of the X and Y columns. But shouldn't the keywords
for the world coordinates at the reference pixel be TCRVL1 and TCRVL2
rather than TCRVL2 and TCRVL3 (and similarly for several other
keywords)? The WCS papers say the latter keywords are the ones to use,
but it would be more consistent with the keywords for an image in the
primary array if the former keywords were used.

Phil
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