Skymaker oversampling
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08-01-2007, 12:33
Post: #1
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Skymaker oversampling
Hi Emmanuel,
I have two questions about skymaker. 1) playing with the skymaker input params I found that if you increase the PSF_OVERSAMP value you get a FWHM smaller then the input SEEING_FWHM. That sounds strange to me. Any hint about a reasonable value for that parameter? 2) setting SEED_MOTION and SEED_STARPOS to non zero values I found that the images produced without changing any params are not exactly the same (few stars in the difference image). Thanks in advance for the answer Lino |
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08-01-2007, 15:54
Post: #2
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RE: Skymaker oversampling
Hello Lino,
thanks for posting on the forum. Quote:1) playing with the skymaker input params I found that if you increase the PSF_OVERSAMP value you get a FWHM smaller then the input SEEING_FWHM. That sounds strange to me. One side effect of PSF_OVERSAMP is that it decreases the extent of the PSF model on the simulated image (if PSF_MAPSIZE is kept constant). So if the PSF FWHM is measured with SExtractor's FLUX_RADIUS, with a PSF_MAPSIZE too small for such a high sampling, there is definitely the possibility to have an impact on the half-light radius (mostly because of missing wings). To simulate ground-based images, for a given PSF_MAPSIZE (which basically sets the speed at which each object is computed), it is sufficient to adjust PSF_OVERSAMP to an odd value, in such a way that the upper boundary of the pupil range is higher than Telescope_diameter + 2 times "r0" (Fried radius). In other words, increase PSF_OVERSAMP until the WARNING message disappear! Quote:2) setting SEED_MOTION and SEED_STARPOS to non zero values I found that the images produced without changing any params are not exactly the same (few stars in the difference image). This should not happen. You simulate only stars? Cheers - Emmanuel. |
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08-01-2007, 16:04
(This post was last modified: 08-01-2007 17:10 by Emmanuel Bertin.)
Post: #3
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RE: Skymaker oversampling
Hi Emmanuel,
Emmanuel Bertin Wrote:Hello Lino, Yes, only stars. ciao Lino |
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08-01-2007, 17:11
Post: #4
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RE: Skymaker oversampling
Gloops. I will investigate and report back to you!
Cheers - Emmanuel. |
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08-03-2010, 02:40
(This post was last modified: 08-04-2010 02:50 by natsushi.)
Post: #5
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RE: Skymaker oversampling
Hi Emmanuel and Lino,
(08-01-2007 17:11)Emmanuel Bertin Wrote: Gloops. I will investigate and report back to you! I also have same problem on random seed of stars; with exactly same random seed, the star positions differ run by run. This happens if one compiles with multi-threads mode(maybe default?). The quick solution is explicitly disabling the multi-threads option, i.e. ./configure --disable-threads. Also be sure to have single thread mode FFTW library. -Atsushi, |
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08-13-2010, 10:18
Post: #6
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RE: Skymaker oversampling
Hi Atsushi,
what version of SkyMaker are you referring to? This should have been fixed since V3.2.1. - Emmanuel. |
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07-20-2013, 09:06
Post: #7
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RE: Skymaker oversampling
(08-01-2007 15:54)Emmanuel Bertin Wrote: One side effect of PSF_OVERSAMP is that it decreases the extent of the PSF model on the simulated image (if PSF_MAPSIZE is kept constant). So if the PSF FWHM is measured with SExtractor's FLUX_RADIUS, with a PSF_MAPSIZE too small for such a high sampling, there is definitely the possibility to have an impact on the half-light radius (mostly because of missing wings). Very important note! Thank you. Probably should introduce a new WARNING message. That oversampling is too high. Too high oversampling leads to a shift in position: |
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07-20-2013, 15:06
Post: #8
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RE: Skymaker oversampling
There are some problems with PSF_OVERSAMP <5 and large PSF_MAPSIZE.
Quote:sky -M1_DIAMETER 0.508 -M2_DIAMETER 0.1778 -ARM_COUNT 0 -MAG_ZEROPOINT 20.8208 -EXPOSURE_TIME 300 -PIXEL_SIZE 0.1 -SEEING_FWHM 0.5 -IMAGE_SIZE 4096 -STARCOUNT_ZP 5e3 -BACK_MAG 21 -WAVELENGTH 0.5 -PSF_MAPSIZE 8192 -MAG_LIMITS 12.0,20.0 -PSF_OVERSAMP 5 Quote:sky -M1_DIAMETER 0.508 -M2_DIAMETER 0.1778 -ARM_COUNT 0 -MAG_ZEROPOINT 20.8208 -EXPOSURE_TIME 300 -PIXEL_SIZE 0.1 -SEEING_FWHM 0.5 -IMAGE_SIZE 4096 -STARCOUNT_ZP 5e3 -BACK_MAG 21 -WAVELENGTH 0.5 -PSF_MAPSIZE 8192 -MAG_LIMITS 12.0,20.0 -PSF_OVERSAMP 3 |
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08-27-2013, 22:04
Post: #9
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RE: Skymaker oversampling
Hi bmv,
thanks for the bug report. It is now fixed in the latest development version (v3.10.5). - Emmanuel. |
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08-27-2013, 22:21
Post: #10
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RE: Skymaker oversampling | |||
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