[AstroPy] Rebin FITS images and preserve or recalculate WCS?

Tue Aug 4 11:42:59 EDT 2020

Hi Eric,

I posted a very similar question to the Python users in Astronomy Facebook
group a few weeks ago (July 7). You might look up that discussion. There
were some interesting suggestions, though I wouldn't say any definitive
answers.

Regards,
Jon

On Tue, Aug 4, 2020 at 9:18 AM <astropy-request at python.org> wrote:

> Today's Topics:
>
>    1. Re: Rebin FITS images and preserve or recalculate WCS?
>       (Eric Jensen)
>    2. Re: Rebin FITS images and preserve or recalculate WCS?
>       (Eric Jensen)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 4 Aug 2020 09:16:29 -0400
> From: Eric Jensen <ejensen1 at swarthmore.edu>
> To: Astronomical Python mailing list <astropy at python.org>
> Subject: Re: [AstroPy] Rebin FITS images and preserve or recalculate
>         WCS?
> Message-ID: <4A90B8D1-DCF3-4BCB-8D57-90E20781CC86 at swarthmore.edu>
> Content-Type: text/plain; charset="utf-8"
>
> Thanks, Gary and Petr!  I appreciate the answers so far.  A couple of
> additional notes:
>
> I should have been clear that I realize that the basics of the binning
> part are hard with numpy operations, though I appreciate the clear code
> from Gary.   There are possibly edge-of-image issues, but those are
> probably best handled by just restricting the binning factor to numbers
> that divide evenly into the array dimensions.
>
> However, there are potential bit-depth issues - if I add together several
> pixels with 50,000 counts (previously stored as 16-bit unsigned integers)
> I?ll need to change the bit depth.  Maybe astropy?s FITS-handling routines
> make that automatic when writing an array to a new FITS file.
>
> That said, it?s mostly the subtleties of the WCS part that I?d like to
> handle with vetted tools if possible.
>
> For example, if I look at a FITS header from an image solved by
> astrometry.net, it has not just CRVAL1,2 + CRPIX1,2 + a CD matrix, but it
> also has SIP distortion coefficients.  It?s possible that those are
> negligible for our images (I haven?t yet checked them to work out the
> details of the standard to figure out how much difference they make) and
> they could just be ignored.  (And our images straight off the telescope,
> using MaximDL, have some kind of proprietary/undocumented distortion
> correction scheme that I?ll just need to ignore anyway, I think, so maybe
> the distortion issue isn?t important.  I?ll have to test that.)
>
> But it highlights the general issue that WCS has lots of permutations, and
> it seems better to handle it (if possible) with tested tools such as
> already exist in astropy.  The question is whether there?s a useful way to
> deploy the astropy (or other library) WCS functionality here, or whether it
> is indeed best just to do it manually with the linear coefficients
> following the solution outlined by Gary and Petr already.
>
> One specific question:
>
> > On Aug 4, 2020, at 6:33 AM, Petr Kub?nek <petr at kubanek.net> wrote:
> >
> > And if you happen to have WCS in CD_ matrix, divide it by 2 (scalar, eg.
> all members).
>
> These would need to be *multiplied* by the binning factor, yes?
>
> If there is no general solution already implemented and tested, I
> certainly can dig into it more myself - I think the basic outline (minus
> distortion) is already given, modulo subtleties I haven?t though of.  But
> in general I?ve encouraged my students to use tested libraries rather than
> re-inventing the wheel when possible, so I?m trying to take my own advice!
> :-)
>
> Thanks for any additional thoughts,
>
> Eric
>
>
> > On Aug 4, 2020, at 6:33 AM, Petr Kub?nek <petr at kubanek.net> wrote:
> >
> > And if you happen to have WCS in CD_ matrix, divide it by 2 (scalar, eg.
> all members).
> >
> > Petr
> >
> >> 4. 8. 2020 v 11:44 dop., Bernstein, Gary M <garyb at PHYSICS.UPENN.EDU>:
> >>
> >> Hi Eric -
> >> I don?t know whether this exists but in this case it might be faster to
> write than to do the Google search.  The block summing can be done in numpy
> by reshaping the array.  In the case where the original image shape is
> divisible by the shrinkage factor s, you?d do
> >>
> >> m,n = img.shape
> >> img = np.sum( img.reshape(m//s,s,n), axis=1)  # Contract along y
> >> Img = np.sum( img.reshape(m//s,n//s,s),axis=2). # Contract along x
> >>
> >> And to fix the WCS, you need to divide CRPIXn by s, and multiply CDELTn
> by s.  There is some slight complication to the first operation because
> FITS assumes 1-indexed pixels, and the integer marks the center of a pixel,
> so its really CRPIXn -> (CRPIXn?0.5)/s + 0.5
> >>
> >> I think this should work universally because the linear transform is
> always the first step of a WCS mapping.
> >>
> >> Cheers,
> >> Gary
> >>
> >>> On Aug 3, 2020, at 11:01 PM, Eric Jensen <ejensen1 at swarthmore.edu>
> wrote:
> >>>
> >>> Hi all,
> >>>
> >>> We are looking at purchasing a new CMOS camera that has 4-micron
> pixels, which would significantly undersample our PSF. (The camera is
> otherwise excellent for our needs, e.g. excellent QE.)  So I?m looking at
> whether there would be a straightforward way in scripted post-processing to
> reduce the image file sizes while not losing information.  The existing
> camera driver doesn?t support more than 2x2 binning at this time.
> >>>
> >>> Is there available Python code that can take a FITS image with a valid
> WCS in the header and rebin it, and output a FITS image that still has a
> valid WCS for the rebinned image?  The rebinning should also preserve flux,
> though I think that?s easier than the WCS part.  Nothing obvious turns up
> in a little searching, but I could easily have missed it.
> >>>
> >>> It seems that Montage might be able to do this (mShrink) but if
> possible I?d prefer a pure-Python solution, as it would be easier to
> implement under Windows on our observatory control computer.  (A fallback
> would be to install a Linux distro on top of the Windows Subsystem for
> Linux, but if it?s possible to do it purely in Python that would be a lot
> simpler.)
> >>>
> >>> Thanks for any thoughts,
> >>>
> >>> Eric
> >>>
> >>> Eric Jensen
> >>> Professor of Astronomy
> >>> Dept. of Physics and Astronomy
> >>> Swarthmore College
> >>> _______________________________________________
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> >>> AstroPy at python.org
> >>> https://mail.python.org/mailman/listinfo/astropy
> >>
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> ------------------------------
>
> Message: 2
> Date: Tue, 4 Aug 2020 09:18:25 -0400
> From: Eric Jensen <ejensen1 at swarthmore.edu>
> To: Astronomical Python mailing list <astropy at python.org>
> Subject: Re: [AstroPy] Rebin FITS images and preserve or recalculate
>         WCS?
> Message-ID: <0799D0A8-35B3-4BF3-9F28-61CF2DC10E76 at swarthmore.edu>
> Content-Type: text/plain; charset="utf-8"
>
>
> > On Aug 4, 2020, at 9:16 AM, Eric Jensen <ejensen1 at swarthmore.edu> wrote:
> >
> >  I realize that the basics of the binning part are hard with numpy
> operations,
>
> Sorry, this should have said ?are *not* hard??
>
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-- 
Jonathan D. Slavin
Astrophysicist - High Energy Astrophysics Division
Center for Astrophysics | Harvard & Smithsonian
Office: (617) 496-7981 | Cell: (781) 363-0035
60 Garden Street | MS 83 | Cambridge, MA 02138
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