ST-402ME, nonlinearity, flatfields

Discussion in 'Image Capture' started by Maneesh Yadav, Aug 19, 2016.

  1. Maneesh Yadav

    Maneesh Yadav Cyanogen Customer

    Joined:
    Jul 12, 2016
    Messages:
    68
    Hello friends,
    Some silly beginner questions on obtaining flats. Just to get used to things I set the camera up in room with all the lights off in front of of whitebox, I wanted to measure the CCD response to resolve the conflicting advice on target ADU for flats (some sources suggest 30% of 64k = ~20k ADU, other calculators suggest ~7.5k ADU):

    http://www.hiddenloft.com/notes/CCDAP02.pdf
    http://www.telescopesupportsystems.com/thrushobservatory.org/Tips/Digital Imaging/flatfieldcalc.htm


    I thought I would be able to clearly see the non-linearity in the response by graphing exposure time vs. sampled ADU (I just waved the mouse around the image in MaximDL to guesstimate a mean value).

    The data are surprisingly linear, I don't see an obvious point where things start to bend (there is noise from my ad hoc sampling method). I can see the image start to degrade on the long exposures. Is the non-linearity that subtle that I shouldn't see it on the graph?


    Table pasted below:
    Exposure (s)Intensity (ADU)
    0.21800
    0.42700
    0.63400
    0.83800
    14300
    1.25000
    1.46200
    1.66700
    1.88100
    28700
    2.29600
    2.410000
    2.610100
    2.812000
    312100
    3.212800
    3.413900
    3.614500
    3.814600
    416000
    4.216500
    4.417500
    4.617700
    4.818700
    520000
    5.221000
    5.420600
    5.622000
    5.822200
    623000
    6.224500
    6.424600
    6.625700
    6.826000
    726700
    830000
    934000
    1037400
    1141200
    1245200
    1556600
    1865500
     

    Attached Files:

    Last edited: Aug 19, 2016
  2. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    6,862
    Yes, the linearity should be excellent, until you are very close to saturation.
     
  3. Maneesh Yadav

    Maneesh Yadav Cyanogen Customer

    Joined:
    Jul 12, 2016
    Messages:
    68
    Thanks Doug,
    In that case, the target ADU for the flat doesn't really matter as long as I'm not ridiculously close to saturation?
     
  4. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    6,862
    As long as you are in the linear region, the flat should work fine. The usual suggestion of 50% is conservative.
     

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