[AstroPy] Inverting SIP distortions

[Paul Kuin]

I think this should be part of the fits verification, not any thing else.
This should be easy to do as an additional check.

I'll send a note to the fitsio people as well.

Paul


On Fri, Nov 1, 2013 at 5:43 PM, Perry Greenfield <stsci.perry at gmail.com>wrote:

> At the very least, there should be a easy-to-use test of the supplied
> inverse to see how accurately it round trips, and a means of specifically
> using an iterative inverse based on the forward transform, even if an
> inverse has been provided.
>
> Perry
>
> On Nov 1, 2013, at 1:40 PM, David Berry wrote:
>
> > I've seen several SIP headers where the inverse transformation given by
> the AP* BP* headers is a poor inverse of the forward transformation given
> by the A* B* headers. In fact this issue was discussed recently on the
> astropy list (subject "Running wcs_pix2world for all image pixels") in the
> context of a header presented by Maik Riechert, which showed a similar
> problem. From that case I came to the conclusion that astropy ignroes the
> AP/BP headers and creates its own inverse transformation based on the
> forward transformation - maybe I was wrong.
> >
> > For what it's worth, pyast does in fact do this - so it should always
> provide a good inverse for a SIP header.
> >
> > David
> >
> >
> > On 1 November 2013 11:56, Michael Droettboom <mdroe at stsci.edu> wrote:
> > On 10/31/2013 08:08 PM, Jacob Vanderplas 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?
> > I would have expected the above to work.  Are we sure the AP* and BP*
> > coeffs are a true inverse of the A* and B* ones?  You can send me the
> > FITS file directly (off-list) and I'd be happy to have a look.
> >
> > Mike
> >
> >
> > --
> >                     _
> > |\/|o _|_  _. _ | | \.__  __|__|_|_  _  _ ._ _
> > |  ||(_| |(_|(/_| |_/|(_)(/_|_ |_|_)(_)(_)| | |
> >
> > http://www.droettboom.com
> >
> > _______________________________________________
> > AstroPy mailing list
> > AstroPy at scipy.org
> > http://mail.scipy.org/mailman/listinfo/astropy
> >
> > _______________________________________________
> > AstroPy mailing list
> > AstroPy at scipy.org
> > http://mail.scipy.org/mailman/listinfo/astropy
>
> _______________________________________________
> AstroPy mailing list
> AstroPy at scipy.org
> http://mail.scipy.org/mailman/listinfo/astropy
>



-- 

* * * * * * * * http://www.mssl.ucl.ac.uk/~npmk/ * * * *
Dr. N.P.M. Kuin      (n.kuin at ucl.ac.uk)
phone +44-(0)1483 (prefix) -204927 (work)
mobile +44(0)7806985366  skype ID: npkuin
Mullard Space Science Laboratory  – University College London  –
Holmbury St Mary – Dorking – Surrey RH5 6NT–  U.K.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/astropy/attachments/20131101/0cd7c784/attachment.html>