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

Fri Jul 25 14:25:55 EDT 2014

Hi Octavi,

Got it, but sorry that I don't know any tool can achieve this. 

Good luck,
Guang

-----Original email-----
From: "Octavi Fors" <octavi at live.unc.edu>
Sent Time: Jul 26, 2014 2:19:21 AM
To: "Astronomical Python mailing list" <astropy at scipy.org>
Cc:
Subject: Re: [AstroPy] TAN--SIP -> TAN PV WCS projection conversion

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

_______________________________________________
AstroPy mailing list
AstroPy at scipy.org
http://mail.scipy.org/mailman/listinfo/astropy

-- 

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

-- 

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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