AC4040 - Rectangular gradients

Discussion in 'Aluma AC Series CMOS' started by Neil M Robertson, Jan 12, 2022.

  1. Neil M Robertson

    Neil M Robertson Cyanogen Customer

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    I am getting rectangular gradients appearing in different locations depending on filter selected. s this a filter defect?
     

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  2. Doug

    Doug Staff Member

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    Please post a FITS frame. It's really hard to tell anything from a screen shot.
     
  3. Neil M Robertson

    Neil M Robertson Cyanogen Customer

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    Here is the fits file.
     

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  4. Doug

    Doug Staff Member

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    We need RAW data straight from the camera - not heavily processed like that.
     
  5. Neil M Robertson

    Neil M Robertson Cyanogen Customer

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    Doug, Here are a couple of files pre processing.
     

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  6. Doug

    Doug Staff Member

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    These sensors do tend to have some vertical/horizontal structure in calibration frames. That does not look particularly unusual.
     
  7. Neil M Robertson

    Neil M Robertson Cyanogen Customer

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    Doug,
    The vertical structure in the center is coming from the dark current. I believe the structures to the top and the sides are coming from the filters since if you rotate the filter they move by the corresponding amount.
     
  8. Doug

    Doug Staff Member

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    How are you illuminating the camera? Is that through the telescope?
     
  9. Neil M Robertson

    Neil M Robertson Cyanogen Customer

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    Yes through the telescope. I have a electroluminescent panel for my flat frames. The pictures I am taking have the same issues unless I take a picture with the filter empty. I bought all this equipment from you guys in December 2o21. Not managed to get an image yet. I have read lots of info on CMOS chips. I have the IMX571 Back-Illuminated CMOS chip on another camera which is awesome but seem to be hitting a brick wall with this new AC4040 on the calibration frames. I really need some help.Here is a dark frame and you can see that it has that vertical line up the middle which is in the frames I sent you earlier.
     

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  10. Doug

    Doug Staff Member

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    Yes that is common for 4040 dark frames.

    I strongly recommend using the Stack Pro High gain mode with this camera. You should be able to take 5-10 minute single exposures and get good results.

    As for the flats, are you saying the position of the vertical stripes depend on the filter selection? Does that show up in your final images after calibration? Try aiming for a target brightness that is in the mid-range for the sensor. In single exposure mode that is around 2048 ADU.
     
  11. Neil M Robertson

    Neil M Robertson Cyanogen Customer

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    Doug,

    Thanks. Should the target be the same regardless of binning?
     
  12. Doug

    Doug Staff Member

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    Binning basically averages four pixel values, so it should be the same.

    Obviously on longer exposures with StackPro the target would be higher.

    I don't think the rectangular shape of the pattern has anything to do with the filter wheel. It's too perfectly vertical to be optical. It's not uncommon for sensors (CMOS or CCD) to have differences in flat-field response across the spectrum - it's part of the reason we do flats for each color. Screen stretch on the flats really exaggerates the effect.
     

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