Finding Star Images on a Photo (Video chip) Plate?
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? -- 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 Timoth Ferris Web Page:<www.speckledwithstars.net/>
On Fri, May 28, 2010 at 5:45 PM, Wayne Watson <sierra_mtnview@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. Chuck
On 28 May 2010 21:09, Charles R Harris <charlesr.harris@gmail.com> wrote:
On Fri, May 28, 2010 at 5:45 PM, Wayne Watson <sierra_mtnview@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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On 28 May 2010 23:59, Wayne Watson <sierra_mtnview@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
On 5/28/2010 5:41 PM, Anne Archibald wrote:
On 28 May 2010 21:09, Charles R Harris <charlesr.harris@gmail.com> wrote:
On Fri, May 28, 2010 at 5:45 PM, Wayne Watson <sierra_mtnview@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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-- 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
On Fri, May 28, 2010 at 8:31 PM, Anne Archibald <aarchiba@physics.mcgill.ca>wrote:
On 28 May 2010 23:59, Wayne Watson <sierra_mtnview@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@gmail.com>
wrote:
On Fri, May 28, 2010 at 5:45 PM, Wayne Watson <
sierra_mtnview@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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-- 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
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-- 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)
I managed to find some FORTRAN code that is relevant. Algorithms for CCD Stellar Photometry,K. Mighell, Kitt Peak, <http://www.adass.org/adass/proceedings/adass98/mighellkj/>. He discusses PSF and provides a few "starter" programs, centroid and peak determination. He discusses sky background (, correction, cleanup?) and points to his photometry package). My chip is not big at all, about 1/2". No large chips for what I'm doing. I don't mind starting with simple methods to see how far they get me. That's a learning experience that often provides a better understanding of the whole process. At this point, this is about exploration to see how setting up an image to analyze can be accomplished. Once done, then well known methods are available for my purpose. Where this is going at the moment is to do source extraction. Towards that end I found an interesting source that does a good job of summarizing how one does it. Here's an excerpt. ===================================== Star-Extraction Algorithms The conventional star-extraction algorithms[4,5] widely used for ground-based data reductions are comprised of two basic steps. The first step is to fit the sky background in order to detect the star pixels from the background. The second is to search for pixels that belong to the same star, and group them together to compute the center coordinates of the star. A typical implementation of such an algorithm is described as below. 1) Background Estimation First, the sky background in each pixel is estimated in order to seperate the object signal from the background signal[4]. The image is divided into many grid cells, each including a number of pixels. In each grid cell, an estimate of the background level is made and is assigned to the center pixel as its nominal value. A relatively "coarse" mesh of the sky background distribution is thus constructed. The value at any other pixel can be obtained from 2-dimensional spline-fitting interpolation of the mesh. 2) Searching for Star Pixels After the interpolated sky background is subtracted from the image, the sky contribution to any star pixel becomes negligible. Only small residues remain in the sky pixels and the mean residue value can be taken as the standard deviation of the background. After convolution with a Gaussian filter, the image is traversed to search for pixels whose values are N times higher than the background standard deviation. Each group of connected high-value pixels are joined together to form an extracted star. The conventional algorithms described above are time-consuming, although they have the advantage of detecting sources with very low signal-to-noise ratios (SNRs). They are not suitable for satellite onboard data reductions since the onboard computers are usually several hundred times slower than even a normal personal computer available on the ground. SciPy. Hmm, this looks reasonable. <http://scipy.org/Getting_Started>. Somehow I missed it and instead landed on <http://www.scipy.org/wikis/topical_software/Tutorial>, which looks fairly imposing. Those are tar and gzip files. In any case, the whole page looks quite imposing. Where did you find the simple one below? =============================== On 5/28/2010 8:31 PM, Anne Archibald wrote:
On 28 May 2010 23:59, Wayne Watson<sierra_mtnview@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
On 5/28/2010 5:41 PM, Anne Archibald wrote:
On 28 May 2010 21:09, Charles R Harris<charlesr.harris@gmail.com> wrote:
On Fri, May 28, 2010 at 5:45 PM, Wayne Watson<sierra_mtnview@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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-- 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
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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/>
Sat, 29 May 2010 11:29:56 -0700, Wayne Watson wrote: [clip]
SciPy. Hmm, this looks reasonable. <http://scipy.org/Getting_Started>. Somehow I missed it and instead landed on <http://www.scipy.org/wikis/topical_software/Tutorial>, which looks fairly imposing. Those are tar and gzip files.
There's also the PDF file that contains the main text.
In any case, the whole page looks quite imposing. Where did you find the simple one below?
The scipy.org wiki is organically grown and confusing at times (I moved the page you landed on to a more clear place). The correct places where this stuff should be is http://docs.scipy.org/ http://scipy.org/Additional_Documentation -- Pauli Virtanen
Good. I'll latch onto them (bookmark) them soon. I like the organic description. On 5/29/2010 12:09 PM, Pauli Virtanen wrote:
Sat, 29 May 2010 11:29:56 -0700, Wayne Watson wrote: [clip]
SciPy. Hmm, this looks reasonable.<http://scipy.org/Getting_Started>. Somehow I missed it and instead landed on <http://www.scipy.org/wikis/topical_software/Tutorial>, which looks fairly imposing. Those are tar and gzip files.
There's also the PDF file that contains the main text.
In any case, the whole page looks quite imposing. Where did you find the simple one below?
The scipy.org wiki is organically grown and confusing at times (I moved the page you landed on to a more clear place).
The correct places where this stuff should be is
-- 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 "Science and democracy are based on the rejection "of dogma." -- Dick Taverne, The March of Unreason Web Page:<www.speckledwithstars.net/>
One can also try to use photometry software like Daophot, it uses MIDAS by ESO http://www.eso.org/sci/data-processing/software/esomidas// , which everyone can download. It seems that Daophot http://www.star.bris.ac.uk/~mbt/daophot/ isn't for free :-(, I never cared, since it's installed on our institute's computers .... Here is first a *very* introductory (german-language) page I used that time: http://members.galev.org/rkotulla/mrmiller-stepbystep-daophot.php If you want to know every gory detail about Daophot, http://www.astro.wisc.edu/sirtf/daophot2.pdf is a good point to start. It's the reference docu by the author if I'm not mistaken. I can give some little advice with using Daophot and MIDAS if you want to. My knowledge is that gained by a conscientous project in studies during several monthes, where I analysed an open star cluster. (With cosmic rejection etc. pp. blah blah) I would not recommend using a DIY approach, since the thing is in my opinion quite cumbersome. An approach used by Daophot to reject artifacts is to calculate sharpness and roundedness parameters, see the Daophot docu above for reference how it works. Friedrich
Thanks for the info. By, DIY I mean borrowing only what seems necessary to get started. For example the FORTRAN code that I mentioned, I suppose the opposite of this is trying to understand what's going on by trying to learn some library of functions that go far beyond my needs. Sort of just getting down to a basic approach rather than trying to encompass large amounts of info on a subject. It's sort of like learning to ride a tricycle before learning to ride a bicycle. On 6/1/2010 11:28 AM, Friedrich Romstedt wrote:
One can also try to use photometry software like Daophot, it uses MIDAS by ESO http://www.eso.org/sci/data-processing/software/esomidas// , which everyone can download.
It seems that Daophot http://www.star.bris.ac.uk/~mbt/daophot/ isn't for free :-(, I never cared, since it's installed on our institute's computers ....
Here is first a *very* introductory (german-language) page I used that time: http://members.galev.org/rkotulla/mrmiller-stepbystep-daophot.php
If you want to know every gory detail about Daophot, http://www.astro.wisc.edu/sirtf/daophot2.pdf is a good point to start. It's the reference docu by the author if I'm not mistaken.
I can give some little advice with using Daophot and MIDAS if you want to. My knowledge is that gained by a conscientous project in studies during several monthes, where I analysed an open star cluster. (With cosmic rejection etc. pp. blah blah)
I would not recommend using a DIY approach, since the thing is in my opinion quite cumbersome.
An approach used by Daophot to reject artifacts is to calculate sharpness and roundedness parameters, see the Daophot docu above for reference how it works.
Friedrich _______________________________________________ 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 "Science and democracy are based on the rejection "of dogma." -- Dick Taverne, The March of Unreason Web Page:<www.speckledwithstars.net/>
I have used sextractor in the past : http://terapix.iap.fr/rubrique.php?id_rubrique=91/ Johann On 05/29/2010 08:29 PM, Wayne Watson wrote:
I managed to find some FORTRAN code that is relevant. Algorithms for CCD Stellar Photometry,K. Mighell, Kitt Peak, <http://www.adass.org/adass/proceedings/adass98/mighellkj/>. He discusses PSF and provides a few "starter" programs, centroid and peak determination. He discusses sky background (, correction, cleanup?) and points to his photometry package).
My chip is not big at all, about 1/2". No large chips for what I'm doing. I don't mind starting with simple methods to see how far they get me. That's a learning experience that often provides a better understanding of the whole process. At this point, this is about exploration to see how setting up an image to analyze can be accomplished. Once done, then well known methods are available for my purpose.
Where this is going at the moment is to do source extraction. Towards that end I found an interesting source that does a good job of summarizing how one does it. Here's an excerpt. ===================================== Star-Extraction Algorithms The conventional star-extraction algorithms[4,5] widely used for ground-based data reductions are comprised of two basic steps. The first step is to fit the sky background in order to detect the star pixels from the background. The second is to search for pixels that belong to the same star, and group them together to compute the center coordinates of the star. A typical implementation of such an algorithm is described as below. 1) Background Estimation First, the sky background in each pixel is estimated in order to seperate the object signal from the background signal[4]. The image is divided into many grid cells, each including a number of pixels. In each grid cell, an estimate of the background level is made and is assigned to the center pixel as its nominal value. A relatively "coarse" mesh of the sky background distribution is thus constructed. The value at any other pixel can be obtained from 2-dimensional spline-fitting interpolation of the mesh. 2) Searching for Star Pixels After the interpolated sky background is subtracted from the image, the sky contribution to any star pixel becomes negligible. Only small residues remain in the sky pixels and the mean residue value can be taken as the standard deviation of the background. After convolution with a Gaussian filter, the image is traversed to search for pixels whose values are N times higher than the background standard deviation. Each group of connected high-value pixels are joined together to form an extracted star. The conventional algorithms described above are time-consuming, although they have the advantage of detecting sources with very low signal-to-noise ratios (SNRs). They are not suitable for satellite onboard data reductions since the onboard computers are usually several hundred times slower than even a normal personal computer available on the ground.
SciPy. Hmm, this looks reasonable.<http://scipy.org/Getting_Started>. Somehow I missed it and instead landed on <http://www.scipy.org/wikis/topical_software/Tutorial>, which looks fairly imposing. Those are tar and gzip files. In any case, the whole page looks quite imposing. Where did you find the simple one below? ===============================
On 5/28/2010 8:31 PM, Anne Archibald wrote:
On 28 May 2010 23:59, Wayne Watson<sierra_mtnview@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
On 5/28/2010 5:41 PM, Anne Archibald wrote:
On 28 May 2010 21:09, Charles R Harris<charlesr.harris@gmail.com> wrote:
On Fri, May 28, 2010 at 5:45 PM, Wayne Watson<sierra_mtnview@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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-- 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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participants (7)
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Anne Archibald -
Charles R Harris -
David Goldsmith -
Friedrich Romstedt -
Johann Cohen-Tanugi -
Pauli Virtanen -
Wayne Watson