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Better error for SCAMP WCS or support for TPV #3559
Description
I've run into files which use the SCAMP PV?_? convention for distortion coefficients (not standard WCS). This was first raised in #299 and a partial fix was made in #1278. The fix was only partial because it only ignores PV?_? if SIP coefficients are also present, but for files with pure SCAMP distortions, users still get:
InvalidTransformError: ERROR 6 in wcsset() at line 1726 of file cextern/wcslib/C/wcs.c:
PV1_5 : Unrecognized coordinate transformation parameter.
I wonder whether it would be worth at the very least detecting this kind of situation and giving a more explicit error message and possible mitigation strategies. @mdboom indicated in #1278 that he had no time to work on implementing support for the SCAMP distortion terms, but it doesn't mean someone else can't do it - if we could support this deviation from WCS it would make life easier for users, even if it's not 100% standard-compliant.
Activity
[-]Better error for SCAMP WCS[/-][+]Better error for SCAMP WCS or support for TPV[/+]astrofrog commentedon Mar 1, 2015
@stargaser - since you've already worked on this, do you think it would be easy to convert the PVi_j formalism to SIP on-the-fly based on your existing work in https://github.com/stargaser/sip_tpv?
astrofrog commentedon Mar 1, 2015
An even better solution would be if support for TPV could be added to WCSLib. @mdboom - do you think that is likely/possible? The standard is described here: http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1363103
stargaser commentedon Mar 1, 2015
@astrofrog - yes, it's possible to convert TPV to SIP on-the-fly in an exact way, providing no radial terms are used in TPV (as is the case for SCAMP-generated TPV polynomials).
Sadly, both SIP and TPV are in violation of the FITS WCS standards (since both originate from a time before the standards were completed). I think that @mdboom has put in a workaround in WCSLIB to handle SIP's violation (CTYPEn that are longer than 8 characters). Something similar would need to be done for TPV's use of PV keywords that are reserved in the FITS WCS Paper II.
ofors commentedon Mar 1, 2015
@astrofrog and @mdboom - there is an additional motivation for astropy.wcs supporting TPV convention, specially for very wide FoV images (where at least 5th order distortion polynomials are required).
SIP images with (A_i_j, B_i_j, AP_i_j, BP_i_j distortion keywords) work fine with astropy.wcs. However, the (ra,dec) coordinates which SExtractor derives from them are not correct, but they are a few arcmin offset in a not systematic fashion (maybe because of the wcslib SIP violation?).
I worked around by converting SIP images to TAN ones with 5th order distortion polynomials (PV_i_j, not PVi_j), using @stargaser 's SIP->TAN exact converter at https://github.com/stargaser/sip_tpv/tree/modularize . In contrary to SIP ones, SExtractor provided correct (ra,dec) coordinates (I matched them to Tycho2) for these converted TAN+PV_i_j images.
I know this is a SExtractor issue, but it is a widely used detection tool, I thought it was worth mentioning this.
ofors commentedon Mar 1, 2015
Oops, let me amend my last message. they're PVi_j, not PV_i_j keywords.
mdboom commentedon Mar 2, 2015
I have a branch for TPV support here (https://github.com/mdboom/astropy/tree/wcs/tpv), which does require changes to wcslib. (No guarantees it is 100% correct yet, but it at least shows the way). I don't want to maintain patches against wcslib, so it would be helpful if someone who needs this functionality could make a case for inclusion in wcslib to Mark Calabretta.
ofors commentedon Mar 3, 2015
Thanks so much @mdboom .
I'll do that: I'll contact Mark Calabretta to request TPV support on wcslib side.
mdboom commentedon Apr 7, 2015
#3669 includes wcslib 5.1 which adds TPV support.
astrofrog commentedon Jun 25, 2015
@mdboom - is this addressed by #3878?
mdboom commentedon Jun 25, 2015
Yes, it is!
ofors commentedon Jun 25, 2015
Thanks a lot @mdboom !
astrofrog commentedon Jun 25, 2015
@mdboom - it might be worth advertising this on astropy-dev since a number of people have in the past asked for support for this if I recall.