Author Topic: Need Help with Background Checkerboard Pattern  (Read 1572 times)

Offline GordonH

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Need Help with Background Checkerboard Pattern
« on: 2017 January 10 13:39:06 »
I imaged NC925 and while processing I noticed a "checker board pattern" in the background and persists into the higher signal area of the galaxy itself. The pattern runs at an angle to the row/columns of the sensor pixels. I've attached a grossly overstretched image to illustrate the conditions.

The images were taken with a Canon 6D DSLR using an AstroTech AT10RC scope. Acquisition software is TheSkyX Camera Add-in.  I've been using this combo for about the last 9 months and have not seen the pattern previously. I did go back and check some previous images.

I initially used 38 - 8 minute subs (I image from pretty badly light polluted skies and needs lots of subs!) I dithered these subs with an off-set of 10 pixels on the guide camera (translates to about 26 pixels on the image.) After seeing the pattern I inspected the individual subs, the master bias, dark, and flat and could not discern the pattern. I then stacked groups of 10 subs from the beginning and end of the run. The pattern appeared. I initially stacked using LinearFit, but, also tried WinsorizedSigma.

My next thought was some kind of relationship to the dithering pattern (TheSkyX seems to move the same number of pixels between each sub??) I re-imaged the target about 3 weeks later (I live in Michigan - you don't get many clear nights!) this time with the dither set to 5 pixels. Same result. I then stacked all the images together, tried the different rejection techniques and then started over with all the frames and used the Bayer drizzle routine in BatchPreprocessing. That's what was used for the attached image.

I also separated and inspected  the R - G - B channels from the integrated image. The pattern appear in all three channels, but, more noticeable in the red.

If the pattern were aligned with the sensor I would blame the camera. The fact that its on an angle leaves me puzzled.

Any ideas / help will be appreciated.

Gordon

Offline oldwexi

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Re: Need Help with Background Checkerboard Pattern
« Reply #1 on: 2017 January 10 14:17:40 »
To be on the safe side -
Did you debayer before register or afterwards.
It looks like you registered the undebayered images ???

Gerald

Offline GordonH

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Re: Need Help with Background Checkerboard Pattern
« Reply #2 on: 2017 January 10 15:26:58 »
Registration was done after debater. The initial processing was done with the BatchPreprocessing. Additional integration attempts were done with the debayered-aligned images. (For clarification, the attached image was the extracted luminance channel.)

Gordon

Offline Salvatore Giambruno

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Re: Need Help with Background Checkerboard Pattern
« Reply #3 on: 2017 July 11 11:06:22 »
Gordon,

This looks like a problem I am having - the checkerboard pattern.  I am only experiencing it with Ha images - my LRGB images are fine.  Did you find a solution/answer to this problem?
--Sal

Offline pfile

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Re: Need Help with Background Checkerboard Pattern
« Reply #4 on: 2017 July 11 11:32:32 »
i have seen this problem in the past with narrowband images, but usually only from a very dark site. what happens is that there's very little background signal in the subs, just barely above the read noise floor. so when you subtract the master dark, the lights have a lot of pixels that clamped at 0. and then when you register and integrate these, you get a checkerboard pattern.

you can check your calibrated subs to see if there are a lot of 0-valued pixels. if so, usually you can get around this problem by specifying an output pedestal while calibrating the lights, say 100. then you never have any values that clamp at 0 - say for instance there are a bunch of pixels that would be -5 or -10 (meaning the dark pixel is 5 or 10 ADU above the light pixel at that location.) in the output file you'd have values of 95 or 90, but never 0. when you integrate and process the stack the pedestal is easily removed with DBE or just by moving the black point in the histogram.

rob

Offline Salvatore Giambruno

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Re: Need Help with Background Checkerboard Pattern
« Reply #5 on: 2017 July 12 10:20:23 »
Thats an Excellent answer Rob. Thank you. I will try this. I also want to add that my pattern was very strong and in hot-pixel checkerboard groups. The problem was due to using an uncalibrated frame as the Reference frame in Star Alignment. Using a calibrated frame eliminated the checkerboard blobs because the registration now worked correctly and the patterns were successfully rejected during integration.

Thanks again for posting a rock-solid answer!
--Sal