Hot Spots turning Black

Discussion in 'STF Series CCD Cameras' started by Kenn Hopkins, Apr 18, 2019.

  1. Kenn Hopkins

    Kenn Hopkins Standard User

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    Feb 21, 2015
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    Using STF8300m with its 5 filter wheel, CCDSoft v5.00.217 to acquire images and CCDStack v2.90.5589.16047 to process the images.

    Lately I’ve been seeing black spots where ’hot spots’ occur in my images, they appear with all the filters except HA. Purchased the SFT8300m a few years ago and I always set the temperature to the ‘recommended’ -5.

    I recently took a set of 10 – 8 minute darks and a new set of bias using CCDSoft . In CCDStack I checked the images, 10 8 minute M66, and zeroed in on one of the ‘hot spots’. I then ran the ‘Calibrate’ tool using the new darks and bias along with my flats. The ‘hot spot’ turned black. I then cleared out the stack of M66 and renew the stack then ran the calibrate tool with just the flats—no black spots and the ‘hot spot’ was still there. I then ran the ‘data register’ tool in CCDStack and selected the ‘remove hot spots’ it located the ‘hot spot’ and then removed it.

    Of course the black spots are OK in the background but when they are in the nebula it causes more work.

    Is there something wrong with my STF8300m or would it be a software problem? Interested in finding out why my ‘hot spot’ is turning black when I use my darks and bias.

    Kenn
     
  2. Doug

    Doug Staff Member

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    Hard to say without data. Please post a set of raw FITS frames so we can have a look.

    Also you should be able to run colder than -5. Just don't get too close to 100% power.
     
  3. Kenn Hopkins

    Kenn Hopkins Standard User

    Joined:
    Feb 21, 2015
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    Please find attached the 8 minute Dark, Bias Master, Flat, the 8 minute of the Leo Triplet. Of interest is M-65 in the Upper right part of the image. It has a few 'hot spots' that turn black when I calibrate the image using the Dark/Bias/Flat on the main image. M-66 has a few 'hot spots' also but they seem to blend in better but still noticeable.

    Any and all comments are welcomed.
    Kenn
     

    Attached Files:

    Last edited: Apr 21, 2019
  4. Doug

    Doug Staff Member

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    I see hot pixels, which disappear when the calibration is applied.

    There are a small number of hot pixels that are fully saturated at 65,535 at this exposure time and temperature. So when they are subtracted they go to the background level. That is what is producing your "dark spots".

    Some recommendations:
    1. Increase the cooling as much as possible, subject to keeping the cooler power < 95% at all times. This will reduce the severity of hot pixels.
    2. Dither - this means moving the telescope very slightly between exposures. Then you use a sigma clip or SD Mask process to combine the frames. The hot pixels will be completely suppressed.
    I'm not sure if CCDSoft has dither and sigma clip capability. Other packages such as MaxIm DL can do it.
     

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