[AstroPy] TAN--SIP -> TAN PV WCS projection conversion

[Guang Yang]

Hi Octavi,


The astrometry.net group has some related codes. ( http://astrometry.net/doc/readme.html#fits-utilities ) You may try wcs-resample in them, But I'm not sure it works for your case. 


Are you trying to correct astrometry of an image so that it matches well with another? If so, we can discuss this, since that's what I did last year.


Thanks,
Guang


-----Original email-----
From: "Octavi Fors" <octavi at live.unc.edu>
Sent Time: Jul 25, 2014 7:47:11 AM
To: astropy at scipy.org
Cc:
Subject: [AstroPy] TAN--SIP -> TAN PV WCS projection conversion


Dear all,


I've been struggling with this for two weeks ago, but no luck so far.



I'm getting the astrometry plate solution of images from a 40ºx40º FoV camera-lens CCD with Astrometry.net 0.49, TAN--SIP projection, and *5th* order SIP distortion polynomials.


If I overplot with ds9 the Tycho2 stars in the field in J2000 coordinates (Analysis->Catalogs->Optical->Tycho2 menu), the green circles match the positions of all the stars in the FoV.



*However*, after running SExtractor 2.19.5 over the plate solved image, while the pixel coordinates (XWIN_IMAGE,YWIN_IMAGE) *do* match the stars, the precessed celestial (ALPHAWIN_J2000,DELTAWIN_J2000) ones *do not* (they show offsets of several arcmins).



I've asked SExtractor developers if such software does support TAN--SIP WCS convertion. Still waiting, but my guess is it does not.


So a TAN--SIP->TAN-PV projection converter is what I'm looking for.

Caltech PTF team (see SPIE paper) made these SIP<->PV converters to 4th order (not to 5th order, which is what I need). For license issues, they can only release the binaries here , but not the source code (which would allow me to extend the conversion formulas to 5th order).


Is there anybody in astropy community who is planning to implement a method in WCS class for converting from SIP to PV projection?
I would be more than happy to provide testing images.


Thanks in advance,



Octavi.


-- 

Octavi Fors
Postdoctoral Research Associate
Department of Physics and Astronomy
The University of North Carolina at Chapel Hill
CB #3255, #157 Phillips Hall
Chapel Hill, NC 27599


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