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Dark Flats vs. Bias

Michael Clark
in General
It is mentioned that Bias effectively stays consistent through to 30's exposures, I think this was covered in the WBPP videos. Before seeing this information, I heard that Flat Darks are essentially Bias unless they are longer than 1 second. Based off that information I have always just tried to get my flats to shoot between 2-9 seconds and maybe thats not enough? Should I be shooting Flats over 30s(!) to get a true Flat Dark that isn't just a Bias frame? 

What's your target exposure times for Flats?

Comments

  • BlockHeadBlockHead
    Hi Michael,

    Hmm...not quite? (It isn't what I tried to communicate in my videos.)
    Dark frames for flats are necessary for some cameras where there is an extra electronic signature or behavior that is in addition to a linear response (like dark current). Typically this is the amp glow. 
    If there isn't any extra thing- then there isn't a need to match the dark frame for flats exposure time with the flats themselves. 

    As far as the exposure is concerned- a bias captures the bias level and no measurable dark current. So how long can a flat be in order to be calibrated by a bias frame? How long does it take the sensor to accumulate significant/measurable dark current... maybe many seconds for most sensors today. 
    So a bias frame is just a short exposure. A dark frame can be a short exposure. There is no difference... what matters is the number of electrons you gather in addition to the bias level. If there is some... you have a dark frame. You can rename a dark frame to a "dark current measuring frame." 

    You do not need to get longer exposure flats. If a short exposure dark/bias calibrates the flat - you are done. There are reasons, not dealing with dark/bias frames to take longer than at least 2 seconds for flat exposures. Anything above this as long as you get a good amount of illumination is fine. 

    Please see my video on this again for a more detailed explanation.

    Thanks,
    -the Blockhead 
  • Michael Clark
    edited December 2022
    Thanks Adam for the clarification. Not sure why that element of your video stuck out to me as I was doing my Flats yesterday, but it did. I figured I was missing something small details as to why we do Flat Darks over just using a Bias.

    What got me to thinking of calibration is that I seem to still have glow on my edges, after DBE, that I am assuming is amp glow not being calibrated out. It's nothing major that a small crop can't take care of, but for mosaics, or frames that I don't want to crop, this is starting to question my process.  Was thinking that maybe I am not getting the amp glow to translate in my flat darks and I am needing longer exposures for my flats....which made me consider the bias discussion like 30s, which seems like a tough exposure time to get a good flat. 

    Is amp glow typically handled with darks, or flats and flat darks?
     
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