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Mosaic Artefacts
01-25-2010, 14:19
Post: #1
Mosaic Artefacts
Hello everybody !

I am trying to build a mosaic of 2x2 images observed with dithering mode. When I run Swarp with the proper images and weight maps, I get an mosaic image but with some artifacts in the joint borders of the four images ( like a cross in the center of the image).
I attach an result image where you can see that.


Do you have any idea of how could I remove or avoid those kind or artifacts ? should I have to run Swarp with some special parameter ?

Thank you for your time !!

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02-07-2010, 23:36
Post: #2
RE: Mosaic Artefacts
Hi JM,

thanks for the illustration. What you call "artifacts" look essentially like zones with a higher noise level to me. When co-adding dithered mosaic images, the gaps between detector chips end up as regions where less pixels are averaged, and therefore the amplitude of the noise is higher there. When the gamma correction is nonexistent or insufficient (which is usually the case with most astronomical display tools Sad), a higher noise level translates in an apparent increase of the average background level.
Now, it might also happen that the borders of the individual CCD images come with an overscan region which, if not trimmed or zero-weighted, may end up in the stack and/or confuse the background subtraction algorithm in SWarp and artificially raise or decrease the average sky level there.
- Emmanuel.
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