On Fri, May 28, 2010 at 8:31 PM, Anne Archibald <aarch...@physics.mcgill.ca>wrote:
> On 28 May 2010 23:59, Wayne Watson <sierra_mtnv...@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.har...@gmail.com> > wrote: > > > > > > On Fri, May 28, 2010 at 5:45 PM, Wayne Watson < > sierra_mtnv...@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 > > > > > > _______________________________________________ > > NumPy-Discussion mailing list > > NumPy-Discussion@scipy.org > > http://mail.scipy.org/mailman/listinfo/numpy-discussion > > > > > > > > > > _______________________________________________ > > NumPy-Discussion mailing list > > NumPy-Discussion@scipy.org > > http://mail.scipy.org/mailman/listinfo/numpy-discussion > > > > > > > > -- > > 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/> > > > > _______________________________________________ > > NumPy-Discussion mailing list > > NumPy-Discussion@scipy.org > > http://mail.scipy.org/mailman/listinfo/numpy-discussion > > > > > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > -- Mathematician: noun, someone who disavows certainty when their uncertainty set is non-empty, even if that set has measure zero. Hope: noun, that delusive spirit which escaped Pandora's jar and, with her lies, prevents mankind from committing a general suicide. (As interpreted by Robert Graves)
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