Advice for Taking Flat Frames

Discussion in 'My Astrophotos' started by Mike Hambrick, Nov 19, 2022.

  1. Mike Hambrick

    Mike Hambrick Cyanogen Customer

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    I have read a lot of instructions about how to take flat frames, but they all seem to deal with how to get proper illumination and skip over some of the more basic details. Can someone help to answer the following questions ?
    1. Does the flat frame have to be precisely focused at the exact same focus point as the imaging train ?
    2. Do I need to take a different flat frame for each filter that I will use ?
    3. What is a typical exposure time for a flat frame image ? Please don't answer that it should be whatever it takes to reach a certain saturation level. I am looking for a starting point. Is a flat frame image typically in fractions of a second, tens of seconds, or hundreds of seconds.
    Mike
     
  2. Doug

    Doug Staff Member

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    1. Yes, focus needs to be as close as practically possible to the in-focus position. If the scope is a tiny bit out-of-focus due to normal thermal expansion/contraction, that's probably okay... but check the results of your flat-fielding to be sure - especially around dust donuts.
    2. Yes.
    3. Unfortunately that really depends on how bright your light source is, and how dense your filters are. For narrowband filters you will likely need a longer exposure and/or more light. I'd start with the Luminance/Clear filter. Adjust your exposure to get 1/3 to 2/3 of saturation (in my experience it's better to be at the higher end of that on CCDs, and the lower end of that for CMOS sensors). Then check your RGB and narrowband filters and see if they need an exposure adjustment.
     
  3. Mike Hambrick

    Mike Hambrick Cyanogen Customer

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    Thanks for the quick response, Doug.
    So, for a STXL16200 camera with Astrodon filters in a FW8G filter wheel can you suggest a starting point for an exposure time ? I am looking for an order of magnitude; 0.5 second, 0 seconds, or >60 seconds.

    Mike
     
  4. Doug

    Doug Staff Member

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    No idea. It also depends on the telescope and how bright your flat-field illuminator is.
     
  5. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    I use this plugin to somewhat automate the process and to calculate my exposure times.

    http://winfij.com/maximdl/skyflats.html

    Between different binning and different filters my flat exposures range from less than a second to over 60 seconds, there are many variables and there is no one answer as Doug stated.

    You really need to learn how to calculate the proper saturation of your CCD with your particular setup and for each filter. That is a really nice camera you have, learn to get the most out of it.
     

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