trouble with flat frames

Discussion in 'STT Series' started by Mitch Green, Mar 20, 2016.

  1. Mitch Green

    Mitch Green Cyanogen Customer

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    Hi,

    I'm having troubles with flat frames. I cleaned the luminance filter with a lenspen, not sure if that's appropriate.

    I have an SBIG STT-8300M and the flat doesn't correct my photos. I included a photo after applying flats as well.
    [​IMG]
    [​IMG][/URL][/IMG]
    before cleaning
    [​IMG]
    after cleaning
    [​IMG]
    luminance filter with flat applied.

    Another thing is the blue filter seems to be flawless, so that would rule out the sensor being dirty.

    All in all, i can't even use the data i have because of background is too messed up. Any help would be appreciated.

    Thanks,

    Mitch
     
  2. Doug

    Doug Staff Member

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    Are you taking flat-darks? In other words, dark calibrating the flats themselves? If you don't do that then the flat will not work correctly. At the very minimum you should subtract a bias frame.
     
  3. Mitch Green

    Mitch Green Cyanogen Customer

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    I'm taking bias frames but i'm not taking dark flats. Would taking flat darks solve this? I'm not sure how to take flat darks. also i'm taking darks frames but not flat darks.
     
  4. Doug

    Doug Staff Member

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    A flat-dark would be of the same duration as the flat.

    Bias should be sufficient for short exposures (1 sec or less). For anything longer you should use a flat/dark.

    What software are you using?
     
  5. Mitch Green

    Mitch Green Cyanogen Customer

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    Im using pixinsight.
     
  6. Doug

    Doug Staff Member

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    Okay I'm not familiar with it, but I'm sure it can handle flat-darks.

    The reason you need to calibrate the flat-fields is that any overall offset will affect the cancellation of the dust donuts etc. It comes down to simple math; you can't swap the order of multiplication and division. (a + b) / c is not the same as a + b/c.
     
  7. Mitch Green

    Mitch Green Cyanogen Customer

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    I'm not sure how to apply flat-darks either. But if you can, could you (or others) try and see if you get the same result i'm getting. I uploaded my files here

    lights: https://www.dropbox.com/s/rdkjbsv6t5q0aqv/light luminance frames.zip?dl=0
    flats: https://www.dropbox.com/s/i7g03arvlyqqci6/luminance flats 2x2bin-037lflatatpoint.zip?dl=0
    Darks: https://www.dropbox.com/s/u01runxuagybood/darks 420 2x2bin-002Dark.zip?dl=0
    Bias master: https://www.dropbox.com/s/2jl9xxk3mbfpbnt/Bias Master.fit?dl=0
    Thanks
     
    Last edited: Mar 21, 2016
  8. Doug

    Doug Staff Member

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    The bias master isn't usable outside of Pixinsight. Please send raws.
     
  9. Mitch Green

    Mitch Green Cyanogen Customer

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  10. Doug

    Doug Staff Member

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    Comparing a single flat frame to a single light frame reveals that they are nothing alike. There's no way it could work.

    Creating good flat frames can be challenging. It appears you're not illuminating the telescope properly for your flat-field frames. What are you using for your light source?
     
  11. Mitch Green

    Mitch Green Cyanogen Customer

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    Thanks for looking.

    I'm using Alnitak Flat-Man XL Flat Fielder. http://www.optecinc.com/astronomy/catalog/alnitak/flatmanxl.htm

    Its suppose to be even illuminated. I put it on my Ritchey Chretien. I wonder if i could move it out further, would it be any better. Or maybe take sky flats, not sure what to do now.
     
  12. Doug

    Doug Staff Member

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    There are quite a few challenges with flat-fielding. The entire aperture of the telescope must be evenly illuminated. If the flat-man is slightly smaller than your entrance aperture then it won't work properly. (For the record it has to be bigger than you primary mirror! It has to be at least as big as the entrance aperture.)

    Some RC systems are open truss. That can be a problem for in-dome illuminators in the dome because scattered light can get into the system bypassing the optics. That will definitely mess up your flats. In fact any sort of light leak can give you problems.
     
  13. Mitch Green

    Mitch Green Cyanogen Customer

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    The flat man is as big as the entrance aperture. The Ritchey Chretien is 14 inches and the flat man is 18 inches x 18 inches. I put a shroud on the truss system and tried flats and got the same results as before. only thing i can do is cropped my photos so you can't see the shadows, which is not a real solution but all i have.
     
  14. Doug

    Doug Staff Member

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    You must have a light leak past the shroud. It takes surprisingly little to mess it up.

    If you have a dome then sky flats or twilight flats might work better.
     

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