[Numpy-discussion] Finding Star Images on a Photo (Video chip) Plate?

Sat May 29 00:16:18 EDT 2010

On Fri, May 28, 2010 at 8:31 PM, Anne Archibald
<aarchiba at physics.mcgill.ca>wrote:

> On 28 May 2010 23:59, Wayne Watson <sierra_mtnview at sbcglobal.net> wrote:
> > That opened a few avenues. After reading this, I went on a merry search
> with
> > Google. I hit upon one interesting book, Handbook of CCD astronomy (Steve
> B.
> > Howell), that discusses PSFs. A Amazon Look Inside suggests this is
> mostly
> > about h/w. I tried to figure out how to reach the scipy mail list, but,
> as
> > once a year ago, couldn't figure out the newsgroup GMANE connection. This
> > search recalled to mind my Handbook of Astro Image  Processing by Berry
> and
> > Burnell. It has a few pages on the PSF. In the ref section for that
> > material(PSFs) there's another ref to Steve Howell that may be of use:
> Astro
> > CCD Observing and Reduction Techniques, ASP, Pacific Conf. Series, vol.
> 23,
> > 1992. There are further Berry and Burnell refs that may be applicable.
>
> Ah, sorry, I've been at an astro conference all week, I should have
> expanded that acronym. PSF is short for "Point Spread Function"; the
> idea is that with an optically good telescope, a point source anywhere
> in the field of view produces a blob of characteristic shape (often
> roughly a two-dimensional Gaussian) in your detector. The shape and
> size of this blob is set by your optics (including diffraction) and
> the atmospheric seeing. A star, being intrinsically a point source,
> produces a brighter or less bright version of this blob centered on
> the star's true position. To accurately measure the star's position
> (and often brightness) one usually fits a model blob to the noisy blob
> coming from the star of interest.
>
> I should note that this requires you to have more pixels than you
> "need", so that even a point source is spread over many pixels;
> without this it's impossible to get subpixel positioning (among other
> things). Older consumer digital cameras often lacked this, since it
> was difficult to put enough pixels on a CCD, but fortunately megapixel
> mania has helpfully ensured that no matter how sharp the focus, every
> feature in your image is smeared over many pixels.
>
> > I probed IRAF, SciPy, and Python, but it looks like a steep learning
> curve.
> > The SciPy tutorial page looks like overkill. They have what looks like
> very
> > large tutorials. Perhaps daunting. I did a quick shot at pyraf, a
> tutorial
> > page, but note it has a prereq of IRAF. Another daunting path.
>
> Wait, you think SciPy has too many tutorials? Or that they're too
> detailed? Just pick a short, easy, or sketchy one then. Here's one
> that's all three:
>
> >>> import scipy.stats
> >>> scipy.stats.norm.cdf(3)
> 0.9986501019683699
>
> That's the value of the CDF of a standard normal at three sigma, i.e.,
> one minus the false positive probability for a one-sided three sigma
> detection.
>
> > Well, maybe a DIY approach will do the trick for me.
>
> I haven't used IRAF yet (though I have data sets waiting), and I do
> understand the urge to write your own code rather than understanding
> someone else's, but let me point out that reliably extracting source
> parameters from astronomical images is *hard* and requires cleverness,
> attention to countless special cases, troubleshooting, and experience.
> But it's an old problem, and astronomers have taken all of the needed
> things listed above and built them into IRAF. Do consider using it.
>
> Anne
>

Plus, if you're in the field of astronomy, knowing py/IRAF will be a *big*
gold star on your resume. :-)

DG

> On 5/28/2010 5:41 PM, Anne Archibald wrote:
> >
> > On 28 May 2010 21:09, Charles R Harris <charlesr.harris at gmail.com>
> wrote:
> >
> >
> > On Fri, May 28, 2010 at 5:45 PM, Wayne Watson <
> sierra_mtnview at sbcglobal.net>
> > wrote:
> >
> >
> > Suppose I have a 640x480 pixel video chip and would like to find star
> > images on it, possible planets and the moon. A possibility of noise
> > exits, or bright pixels. Is there a known method for finding the
> > centroids of these astro objects?
> >
> >
> >
> > You can threshold the image and then cluster the pixels in objects. I've
> > done this on occasion using my own software, but I think there might be
> > something in scipy/ndimage that does the same. Someone here will know.
> >
> >
> > There are sort of two passes here - the first is to find all the
> > stars, and the second is to fine down their positions, ideally to less
> > than a pixel. For the former, thresholding and clumping is probably
> > the way to go.
> >
> > For the latter I think a standard approach is PSF fitting - that is,
> > you fit (say) a two-dimensional Gaussian to the pixels near your star.
> > You'll fit for at least central (subpixel) position, probably radius,
> > and maybe eccentricity and orientation. You might even fit for a more
> > sophisticated PSF (doughnuts are natural for Schmidt-Cassegrain
> > telescopes, or the diffraction pattern of your spider). Any spot whose
> > best-fit PSF is just one pixel wide is noise or a cosmic ray hit or a
> > hotpixel; any spot whose best-fit PSF is huge is a detector splodge or
> > a planet or galaxy.
> >
> > All this assumes that your CCD has more resolution than your optics;
> > if this is not the case you're more or less stuck, since a star is
> > then just a bright pixel. In this case your problem is one of
> > combining multiple offset images, dark skies, and dome flats to try to
> > distinguish detector crud and cosmic ray hits from actual stars. It
> > can be done, but it will be a colossal pain if your pointing accuracy
> > is not subpixel (which it probably won't be).
> >
> > In any case, my optical friends tell me that the Right Way to do all
> > this is to use all the code built into IRAF (or its python wrapper,
> > pyraf) that does all this difficult work for you.
> >
> > Anne
> > P.S. if your images have been fed through JPEG or some other lossy
> > compression the process will become an utter nightmare. -A
> >
> >
> >
> > Chuck
> >
> >
> > _______________________________________________
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> >
> >
> >
> >
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> >
> >
> > --
> >            Wayne Watson (Watson Adventures, Prop., Nevada City, CA)
> >
> >              (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
> >               Obz Site:  39° 15' 7" N, 121° 2' 32" W, 2700 feet
> >
> >                There are no statues or memorials dedicated to
> >                Thomas Paine for his substantial part in the
> >                American Revolution.
> >
> >                 -- An observation in The Science of Liberty
> >                       by Timothy Ferris
> >
> >
> >                     Web Page: <www.speckledwithstars.net/>
> >
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