Light Artifact in all my subs

Discussion in 'STX and STXL Series Cameras' started by Mark McComiskey, May 13, 2021.

  1. Mark McComiskey

    Mark McComiskey Standard User

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    I was hoping the brain trust at DL could help me figure out a problem I am having with (I believe) my STX-16803. I am using an SBI FW on a Tak FSQ106EDXIV. Camera is cooled to -20C.

    On one side of my images, the first 8th of the frame (from about column ~60 to 550) has a prominent vertical light signal running from top to bottom.

    This shows up on all my subs, with all my filters, on every subject I image, and is always on the same side of the frame (i.e. columns ~60-550), independent of where in the sky an object is being imaged, pre and post meridian flips. Problem shows up in 15 minute NB subs and 3 min LRGB subs.

    I am attaching two consecutive subs (pre and post meridian flip) in case that helps in the diagnosis of the problem/solution. Happy to provide any more samples/info needed.

    Any ideas what might be leading to this? I've read and reread the manual, but can't figure out what could be causing this or how to fix it?

    Any help would be truly appreciated. At the moment, I am forced to treat the camera as if it were 25% smaller along the x axis than it is in order to avoid having any of the object fall into the problem area, which is defeating the benefit of the wonderfully huge sensor in this system.
     

    Attached Files:

  2. Doug

    Doug Staff Member

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    Could you upload a matching dark frame?
     
  3. Mark McComiskey

    Mark McComiskey Standard User

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    Attached Files:

  4. Doug

    Doug Staff Member

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    Yes, the dark subtraction removes it completely.

    It's just an artefact of the DC restoration circuit. There are compromises with the time constant of the circuit. The setting used avoids having trails behind stars, at the expense of having a small ripple at the left side of the frame - which completely subtracts out.
     
  5. Mark McComiskey

    Mark McComiskey Standard User

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    Following up on this. I have little doubt that my problems stem from user error and/or a lack of understanding of the details of my camera, but after a fair number of hours of trying, I can't seem to get rid of the artifact.

    I follow a standard calibration process for my subs - from each sub, I subtract the master bias and the master dark (which itself has been master bias subtracted). I then divide each sub by a master flat, which has itself been calibrated with bias and dark subtraction, register the files and integrate.

    When I do that, the integrated file still shows the artifact.

    Is there somethings special I need to be doing to eliminate this artifact?

    In case it helps, links to the various files being used:

    Integrated Ha: https://1drv.ms/u/s!AsHXUvMw3K77guQCkeB2-P_obEtWwg?e=Z3LCDX
    Master Bias: https://1drv.ms/u/s!AsHXUvMw3K77guQD8wx6rXREHM6yew?e=2BKyoP
    Master Dark: https://1drv.ms/u/s!AsHXUvMw3K77guQBHWO2LW5AJLNWmg?e=3cvL83
    Master Flat: https://1drv.ms/u/s!AsHXUvMw3K77guQEehIk4Jx-hYr7yA?e=C8EDxP
     
  6. Doug

    Doug Staff Member

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    These images have all been extensively processed. I can't do anything with processed frames. I need RAW data.

    I can only look at the performance of our camera. I cannot comment on your processing steps done in Pixinsight.
     
  7. Mark McComiskey

    Mark McComiskey Standard User

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    My previous response seems to have disappeared. I am not asking you to comment on processing. To clarify, all the files posted above are unprocessed. They are simply the results of integrating and calibrating.

    Please tell me what I need to provide you - I can provide several light, bias, dark and flat subs right from the camera, if that helps.

    My problem is the when the calibrated subs are integrated, the "ripple" reappears.
     
  8. Colin Haig

    Colin Haig Staff Member

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    Doug's tried to explain that he needs straight-from-the-camera bias, dark, light (and flat) frames, that have not been auto-dark subtracted or calibrated, stacked, binned, or processed in any way.
    That way we could analyze what the camera is outputing.
    Once it's had any manipulation done to it, the information is lost.
    The images should be about 32MB in size in FITS format, taken with MaxIm or CCDOps, not compressed, and 1x1 binning.
     
  9. Colin Haig

    Colin Haig Staff Member

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    One other thought:
    Can you provide a photo of your setup, showing the camera, fw, any accessories, focuser, attachment to the telescope, etc?
    A cellphone picture or two would do.
     
  10. Mark McComiskey

    Mark McComiskey Standard User

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    Thanks for following up.

    Below is a link to a folder that contains:

    10 x 600s raw Ha subs,
    10 raw flats,
    10 raw biases,
    10 raw 600s darks
    10 raw 17s darks (used on the flats in calibration)

    Binning at 1x1 on all, uncompressed, FITS header intact

    If it is of significance, I use 100 flats to make the master, 200 biases for the bias, 100 600s darks for the 600s master dark and 200 17s darks for the 17s master dark

    I can of course provide the full data set if needed.


    This is a photo of my imaging setup

    post-272943-0-99097200-1608151535.jpg
     
  11. Doug

    Doug Staff Member

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    Did you post the link to the downloads?
     
  12. Mark McComiskey

    Mark McComiskey Standard User

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  13. Colin Haig

    Colin Haig Staff Member

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    Mark - Don't beat yourself up - we've all done it - so you're in pretty good company.
    Thanks for sending, we'll check out the files.
     
  14. Mark McComiskey

    Mark McComiskey Standard User

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    Thanks much! If you need anything, let me know.
     
  15. Doug

    Doug Staff Member

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    Okay I think your problem is all in the processing. I just dropped these files into MaxIm DL via the Set Calibration and Stack commands, with standard settings, and it came out beautifully.

    As expected, the "light artifact" subtracted out 100%. It's not really light, it's a DC restoration effect in the analog chain, and proper calibration removes it completely. (The slight artifact on the right is due to image realignment during stacking.)

    Here the image is with zero processing - just contrast stretched so it'll show up in JPEG format.

    Man that image is gorgeous!!!

    Spaghetti Nebula.jpg
     
  16. Mark McComiskey

    Mark McComiskey Standard User

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    So, first of all, thank you very much for looking at this so quickly and after hours.

    Second, thanks on the image!

    If I understand you correctly, the dark area on the right of the image above, which looks like the first 250 columns or so, is darker than the rest of the frame due to the fact that the dithering I am doing is causing the frames to be sligthly misaligned? I ask, because the area that is showing up dark in the image above, is the very same area that is showing up light in my processing. Still not sure what I am doing to cause that... If so, then I need to dither a little less and crop a little more to solve this problem?
     
  17. Doug

    Doug Staff Member

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    If you dither and align, some of the pixels near the edge will see less frames than others. You can end up with lines etc. at the edge of the frame. Easiest to just crop them off.
     
  18. Mark McComiskey

    Mark McComiskey Standard User

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    Thanks very much for the attention and apologies for the delay in responding - life got in the way of astroimaging.

    After about 10 hours of trying to find anything I was doing in my processing that might explain my inability to remove the Ripple (including getting help from much better processors than me), I finally took the image you posted above and applied a stretch. The Ripple appears in the image you posted fairly clearly under a stretch, so does not appear to be removed in the processing you did. I have highlighted the edges of the Ripple (more or less) in the stretched image below. Any thoughts on how to get rid of it? SN- Stretched.jpg
     
  19. Doug

    Doug Staff Member

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    Okay, I've had a closer look.

    First off, you do have some hot columns on the chip that aren't being corrected by dark frame subtraction, and are bleeding through the stack. There are a few ways you can deal them.

    My suggested approach is to use Remove Bad Pixels - you can tell Set Calibration to run the Remove Bad Pixels command as part of the calibration process. Hit all the columns that appear in your final image, and they will vanish.

    The gradient appears to be flat-field error. Taking good flats can be tricky.

    Spaghetti Nebula.jpg
     
  20. Doug

    Doug Staff Member

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    To summarize:

    Hot columns in CCD sensors are normal, and you will get more of them appearing over time due to cosmic ray-induced radiation damage. I don't know how old your camera is - please provide the serial number.

    The overall image gradients appear to be due to imperfect flat-fields. It can be very tricky to get perfect flats, especially when you're shooting a large, faint object like this. The flat-field source has to evenly illuminate the entire front aperture of the telescope. Also you can't have any light leaks in the system, as they will cause non-imaged light spillover.
     

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