Chip "Voids" - Black spots...

Discussion in 'STX and STXL Series Cameras' started by sixburg, Jan 3, 2017.

  1. sixburg

    sixburg Cyanogen Customer

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    Please have a look at this. I'm curious as to the range of possibilities. I can't catch a break with these cameras.

    The cover glass is clean. This void appears across all filters and is rotation independent.

    The image train = 16803 >> FW7 >> MMOAG >> AO-X >> RCOS

    upload_2017-1-3_14-31-1.png
     

    Attached Files:

  2. Doug

    Doug Staff Member

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    Could be dust, or maybe frost. The easy way to tell with frost is to raise the temperature a bit - if they go away they are frost.

    Can you post a FITS frame?
     
  3. sixburg

    sixburg Cyanogen Customer

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  4. sixburg

    sixburg Cyanogen Customer

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    More information:

    1. The artifacts persist with the sensor at room temperature which is about -0.7C today so it could be something frozen. I doubt it.
    2. They appear in flats
    3. They do not appear in darks
    4. They do not appear in bias
     
  5. Doug

    Doug Staff Member

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    Yes they appear to be dust motes. Do they flat out?
     
  6. sixburg

    sixburg Cyanogen Customer

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    Agreed on the dust motes; however, they are typically are donut shaped. The size of the donut is easily calculable and is a function of the distance between the dust and the sensor. Going with that, it says the dust is right on the sensor which is not really possible--it has to be on the cover glass. Perhaps it's just very small. Hell if I know. I've cleaned it as per Bill's direction several times.

    So far, no joy on flatting out. Perhaps a different rejection algorithm / settings would remove the black spots, but so far I have not found a combination that works.

    Flats have been particularly problematic with this system. Some images response properly and others do not--it's a processing issue and not a camera issue, but the inconsistency is troubling. I've figured the flat ADU for this sensor to be around 49,000. Does this sound about right (I realize they're all a bit different)?

    Thanks,
    -Lloyd
     
  7. Doug

    Doug Staff Member

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    I'm guessing the dust spots are on the CCD cover glass. They can be invisible to the human eye; in fact, we use a fancy microscope and lighting setup to inspect windows, and even then they can be hard to see.

    Personally I'd aim for 40,000 just to make sure I'm not anywhere near saturation (49,000 is probably okay though). Bias/dark has to be accurate or it won't subtract properly. Make sure you have the latest camera firmware, which includes improved offset compensation. (If you do update firmware, redo your calibration frames.)

    Flat-fields have to be done carefully. You can't have any light leaks in the system. The front of the telescope has to be illuminated exactly as for regular imaging - if it's not fully and evenly illuminating the entire aperture then it will not work properly.
     
  8. sixburg

    sixburg Cyanogen Customer

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    Thanks, Doug.

    I might need to send it in for proper cleaning. I think these spots are indeed very, very small.

    Attached a master flat (PA 226E) just for kicks.
    https://www.dropbox.com/s/d8d8lztzh5grdh1/Flat 49K LE_PA226.fit?dl=0

    It ain't pretty but effective for the 226PA lights (except for the voids).

    I can back off to 40K ADU...the 49K comes from a process I adapted from P. Prendergast and others.

    I'm unclear where in the heck there could be a light leak....it's pretty much straight through. It's an RC so the light goes down the baffle...that's it. The image train is all threaded and dovetailed (16803 > FW7 > MMOAG > AOX > RCOS PIR > Baffle)...no place for light to get in other than the baffle. Plus we normally do flats in low ambient light conditions.

    The problem I'm seeing is some dust motes flat out and other don't (regular, big motes on the filters most likely). I'll get this figured out. I've 6 other systems of various apertures, flat panels, sky flats, camera brands, etc. and flats are the least of my concerns on those. This one is a problem though.
     
  9. Doug

    Doug Staff Member

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    It's certainly on the flat. I can't experiment on it without raw flat and appropriate darks (and/or bias as appropriate).
     
  10. sixburg

    sixburg Cyanogen Customer

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    Hi Doug. I can give you direct access to all the calibration and light frames. I just need a personal email address. I can be reached at deepskywest@deepskywest.com.
     

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