[AstroPy] Inverting SIP distortions

[Jacob Vanderplas]

Hi,
yes, I should have specified, it's a standard header type:

>>> print wcs.wcs.ctype
['RA---TAN-SIP', 'DEC--TAN-SIP']

  Jake


On Thu, Oct 31, 2013 at 6:02 PM, Paul Kuin <npkuin at gmail.com> wrote:
>
> I suppose the WCS you have has the right type, e.g., like:
>
>        fh[int(ext)].header['CTYPE1']='RA---TAN-SIP'
>        fh[int(ext)].header['CTYPE2']='DEC--TAN-SIP'
>
> If so, I happen to use, to go from pixle position to ra,dec,
>
> world = wS.all_pix2world([bore],0)[0]
>
> did not invert, but it seems I'm using different function/method
> whatever-its-called.  Any help?
>
> Paul
>
>
> On Fri, Nov 1, 2013 at 12:08 AM, Jacob Vanderplas
> <jakevdp at cs.washington.edu> wrote:
>>
>> Hello,
>> I'm working on a problem that requires the inversion of SIP
>> corrections: that is, I have points in world coordinates that I want
>> to project onto an image plane.
>>
>> My FITS header contains both the (A_p_q, B_p_q) coefficients for the
>> forward transformation, as well as the (AP_p_q, BP_p_q) required for
>> the backward transformation, according to the Shupe et al 2005
>> reference.
>>
>> The astropy WCS object has methods sip_foc2pix() and sip_pix2foc(),
>> which appeared to be what I needed, but these don't seem to actually
>> invert each other. That is:
>>
>> >>> from astropy.wcs import WCS
>> >>> wcs = WCS(header)
>> >>> Xpix = [[1, 1]]
>> >>> Xfoc = wcs.sip_pix2foc(Xpix, 0)
>> >>> print wcs.sip_foc2pix(Xfoc, 0)
>> [[ 1.2522118   1.74039233]]
>>
>> I would have expected this to recover the input pixels, but it's
>> clearly not the case.  Is there any functionality in astropy that will
>> utilize the available AP/BP coefficients in my header to perform the
>> inverse SIP distortion?  Is there anything I'm doing wrong or missing
>> here?
>> Thanks,
>>    Jake
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>
>
>
>
> --
>
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