Why is the bias frame higher ADU than the dark frame?

Discussion in 'Image Capture' started by Elizabeth, Sep 24, 2015.

  1. Elizabeth

    Elizabeth Standard User

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    I expected that the bias frame would return the lowest possible values from the sensor, yet it is 1030 ADU of a 100 pixel diameter roi while the dark is 956 ADU.

    My model is STF8300M.

    Elizabeth
     
  2. Tim

    Tim Staff Member

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    Were the bias and dark frames taken under exactly the same conditions?

    Can you send FITS images of each type for us to examine?
     
  3. Elizabeth

    Elizabeth Standard User

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  4. Tim

    Tim Staff Member

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    What software are you using to capture these frames?
     
  5. Elizabeth

    Elizabeth Standard User

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    We have our software using the SBIG universal driver. Specifically this is the call I am using with option 2 for format
    SBIG_FILE_ERROR CSBIGImg::SaveImage(constchar *pFullPath, SBIG_IMAGE_FORMATfmt/* = SBIF_DEFAULT */ )
     
  6. Tim

    Tim Staff Member

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    ..but what software exactly? Is any post processing done on the images?

    Does the same issue occur if you take images using CCDOps?
     
  7. Elizabeth

    Elizabeth Standard User

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  8. Tim

    Tim Staff Member

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    Can you send the serial number written on the back of the camera?
     
  9. Elizabeth

    Elizabeth Standard User

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    83H30093.
    But that is contained in the FITS header I believe.
     
  10. Francis Wilkin

    Francis Wilkin Standard User

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    I have the same problem with my STXL11002 camera: the biases have larger means than the darks, and it is perplexing me and the 12 students of my observational astronomy course. We're taking the images with MaximDL 6 and saving them in fits format, with no post processing.
     
  11. Tim

    Tim Staff Member

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    There seems to be something in CCDOps causing this. When I take an image in CCDOps, I can see the issue using my STF-8300M. If I try to reproduce the issue in MaxIm DL using the same parameters, i.e. a bias frame and a 6s dark frame both at -5C, then they both appear to be the same ADU count. Would either of you be able to confirm this using the MaxIm DL demo perhaps:

    http://www.cyanogen.com/maximdl_dl.php
     
  12. Elizabeth

    Elizabeth Standard User

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    Tim,

    I tried your experiment with Maxim DL and the bias still higher by about 35 ADU than the 6s Dark.

    Elizabeth
     
  13. Doug

    Doug Staff Member

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    That doesn't make much sense. A bias frame is just a short-possible dark frame.

    In MaxIm DL, try taking the following dark frames:

    0.01 second
    0.1 second
    1 second
    10 second

    ...and see what happens with the offset level.
     
  14. Elizabeth

    Elizabeth Standard User

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  15. Doug

    Doug Staff Member

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    Odd. The short exposures are all pretty much the same, but the 10 second exposure is lower.

    What do you get if you do a 100 second exposure at the same temperature?
     
  16. Elizabeth

    Elizabeth Standard User

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    The 100s frame is even lower. This goes against my understanding of how CCD ought to behave. Which should be an integrator of photons.

    http://1drv.ms/1kzDzQN
     
  17. Tim

    Tim Staff Member

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    Do you have "Auto Bias Level Correction" turned "off" in the "SBIGDriverChecker64" -> "Config Drv" window?
     
  18. Elizabeth

    Elizabeth Standard User

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    Auto Bias Level Correction is checked, that is it's on.
     
  19. Tim

    Tim Staff Member

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    What happens if you turn it off?
     
  20. Elizabeth

    Elizabeth Standard User

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