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  1. #1's Avatar Guest

    Default sensor/system calibration: dark = bias + thermal

    I read(*) that a standard practice is to collect a set of "bias"
    frames and average them together. You then collect another set of
    "dark" frames (at some fixed exposure), remove the aforecollected
    bias, averge them up and you obtain a "thermal" frame which can be
    scaled by exposure time.

    My question: is there a need to collect the bias frames at all? If
    instead you collected a set of "dark" frames at a number of exposure
    times (say 1s, 2s, 4s, 8s, ..., 32s, 64s, etc), could you not extract
    the "bias" information from them, as well as obtaining the thermal
    scaling? If so, then you not only save the time needed to collect the
    bias frames, store and process them, but you can also measure a
    goodness-of-fit to the assumed linear model that "dark = bias +

    Or is there some Mystery to this process that makes the extra time and
    space for separate bias frames desirable?

    (*) various from, as well as Berry/Burnell "Handbook of
    Astronomical Image Processing", 2000.

  2. #2
    Mark Ritchie's Avatar
    Mark Ritchie Guest

    Default sensor/system calibration: dark = bias + thermal


    My take on this is that the bias frame is used to get the noise that is
    constant (noise caused by the actual readout of the CCD data), and therefore
    does not change with exposure setting or temperature. I think you should be
    able to create one master bias frame (a zero second exposure) and use it
    with all of your exposures.

    Now, the thermal frame gets the noise that varies with time and temperature,
    and I guess is assumed to be linear (otherwise you couldn't scale it). I
    think you should take a set of dark frames for each imaging session so that
    you get the same temperature as the real exposure, but if you end up using
    the bias frame, then you don't have to have a dark frame of exactly the same
    exposure as your real frames. You would subtract the bias frame from the
    dark frame, and then scale the resulting thermal frame. You then subtract
    both the bias frame and the scaled thermal frame.

    I just take a set of dark frames of the same exposure time as my real
    images, and then just subtract them without any scaling, but then again, I
    am a rank beginner using a SAC8 (modified security cam).

    Mark Ritchie

    <> wrote in message om...

  3. #3
    Richard Crisp's Avatar
    Richard Crisp Guest

    Default sensor/system calibration: dark = bias + thermal

    "Mark Ritchie" <> wrote in message

    Actually it is the bias that varies with time. In fact bias can vary by a
    few ADU counts over a few hours.

    See "The Handbook of Astronomical Image Processing" by Berry and Burnell
    chapter 4 for more details


    What you really want to do is to take a set of master darks and master dark
    biases one day. You median or otherwise combine the biases to create a
    single master. Ditto for the Dark. Then subtract the Master Bias from the
    Master Dark and keep the bias subracted Dark as your library dark.

    When you take images on another day, you simply shoot maybe 10 biases which
    represent that bias on that particualar day. You combine them together and
    add that to your Library Dark. Then you use that bias-added Library Dark for
    your Calibration Dark.

    Think about it: Taking 10 darks of , say, 30 minutes takes a lot of time. On
    the other hand 10 biases are much faster: just download time.

    There's little point in keeping library biases and taking darks each time.
    The idea behind keeping a library of darks is to save time.

    I find that so long as I am operating at the same temp, I can use
    bias-subtracted library darks for months. I just shoot a set of biases each
    imaging session.

  4. #4
    Mark Ritchie's Avatar
    Mark Ritchie Guest

    Default sensor/system calibration: dark = bias + thermal

    I just got "The Handbook of Astronomical Image Processing" book and
    software for Christmas, and I did read Chapter 4, but obviously I didn't
    understand it ! I re-read the chapter, and I now see that I had it pretty
    much backwards. I guess I was reading it with a 'biased' opinion of how the
    bias frames work.

    I am taking such short exposures (30 seconds max), that I have never done
    anything more complicated than just taking darks that match my real images,
    so I have never tried to actually use a bias frame, which I guess should
    have made me think twice about responding to the original post in the first

    Anyway, thanks for correcting me on this, I certainly try not to spread bad

    Mark Ritchie

    "Richard Crisp" <> wrote in message
    news:MaqLb.8732$ m...

  5. #5
    Richard Crisp's Avatar
    Richard Crisp Guest

    Default sensor/system calibration: dark = bias + thermal

    I am happy I could help Mark.

    Tell me have you teamed up with any one named Chretien yet?

    (yes I know that it needs to be spelled Ritchey, but I couldn't resist).

    "Mark Ritchie" <> wrote in message



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