Resolved Gain and Offset

Discussion in 'STF Series' started by Kenn Hopkins, Jan 31, 2020.

  1. Kenn Hopkins

    Kenn Hopkins Standard User

    Joined:
    Feb 21, 2015
    Messages:
    22
    I saw in CCDOps the Gain of my STF8300m camera is 0.37 and the Offset was 135. Would it be possible to obtain an explanation of what the Gain is and what the Offset is?

    I think Gain has something to do with how the chip processes the light eg: Gain 0.20 would produce a darker image than a Gain of 0.37 and a Gain of 0.50 would produce a 'brighter' image.

    I have no idea what relationship the Offset is to the Gain.

    I have no way of setting this as they are 'grayed' out, I'm just curious what they are in relation to the image.

    Curious
    Kenn
     
  2. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    6,480
    Those are settings for the A/D converter subsystem.

    The Gain number controls the analog gain of the converter.

    The Offset number controls the zero point.

    During final factory testing, we put the camera in a test fixture, which provides even illumination across the sensor. Then we run special software to optimize the two numbers. It is an iterative process.

    You want the gain adjusted so when the CCD sensor hits saturation, the A/D converter is outputting 65,535.

    You also want the offset adjusted so when there is no light (bias frame) the average pixel value is around 1,000. This ensures that no pixels ever go negative due to read noise, which would cause clipping and make dark frame subtraction not work properly.

    The trick is that adjusting the offset causes the maximum A/D output to change also. So you then have to tweak the gain again, and then adjust the offset again, until it converges on a final answer. Once the final numbers are determined, the values are written into the camera's EEPROM (permanent memory), and then the EGAIN (photoelectrons per ADU) is measured and also stored. The EGAIN is reported in the FITS header for each image.

    The values are greyed out in CCDOPS because we don't allow customers to change them them, because it is difficult to set them up properly. We do have a special tool that we can use remotely to reset those values if for any reason they get corrupted.
     
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  3. Kenn Hopkins

    Kenn Hopkins Standard User

    Joined:
    Feb 21, 2015
    Messages:
    22
    Thank you for that explanation. I know a little knowledge is dangerous and I have no desire to 'mess' with the settings I was just curious and wanted to understand what the values meant.

    Thanks again
    Kenn
     

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