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

[Perry Greenfield]

So you are saying you want an equivalent WCS (equivalent in the sense the x,y-->ra,dec mapping is the same, but the underlying representation is different as FITS keywords go)? But not to actually resample the image?

Perry

On Jul 25, 2014, at 2:19 PM, Octavi Fors wrote:

> Hi Guang,
> 
> I don't want to work-around this on the SExtractor catalog level (via wcs-xy2rd in astrometry.net, xy2sky in WCSTools, or w.all_pix2world(x,y,1) in astropy, which I actually have working) since the astrometrically solved images must be shared with other users. These, as most of us, use SExtractor as photometry extractor engine. 
> So, in my opinion, it is quite inefficient that they have to use the same catalog-based work-around solution instead of straight-forward using an already projection converted image (as David Shupe did in PTF).
> 
> Hope it helps,
> 
> Octavi.
> 
> 
> 
> On Fri, Jul 25, 2014 at 10:18 AM, Guang Yang <yg1991 at mail.ustc.edu.cn> wrote:
> Hi Octavi,
> 
> So, if my understanding is right, you want sextractor to detect the sources and transfrom their (x, y) to (ra, dec). Well, I think it's unnecessary to rely on sextractor to do the transformation. I mean, you can output the source position directly in (x, y) and transform the (x, y) to (ra, dec) via other tools, like, wcs-xy2rd in astrometry.net tools. Does that make sense?
> 
> Thanks,
> Guang
> 
> 
> -----Original email-----
> From: "Octavi Fors" <octavi at live.unc.edu>
> Sent Time: Jul 25, 2014 9:43:49 PM
> To: "Astronomical Python mailing list" <astropy at scipy.org>
> Cc: 
> Subject: Re: [AstroPy] TAN--SIP -> TAN PV WCS projection conversion
> 
> 
> Hi Guang,
> 
> I understand that wcs-resample requires a second (target) image file with the WCS you want to apply to the input image, which has different WCS projection.
> 
> In my case, I only have one image with TAN--SIP projection (which SExtractor doesn't seem to understand), and my aim is to get an output image with the TAN PV_i_j projection (which is of one the projections SExtractor understands).
> 
> Thanks,
> 
> Octavi.
> 
> 
> 
> On Thu, Jul 24, 2014 at 10:33 PM, Guang Yang <yg1991 at mail.ustc.edu.cn> wrote:
> 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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> 
> 
> 
> -- 
> 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
> Office: (919) 962-3606
> Fax:    (919) 962-0480
> 
> 
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> 
> 
> 
> -- 
> 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
> Office: (919) 962-3606
> Fax:    (919) 962-0480
> _______________________________________________
> AstroPy mailing list
> AstroPy at scipy.org
> http://mail.scipy.org/mailman/listinfo/astropy