NB Flats

Discussion in 'Filter Wheels' started by Alan Karty, Sep 12, 2019.

  1. Alan Karty

    Alan Karty Cyanogen Customer

    Joined:
    Aug 29, 2016
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    I am uncertain of the quality of my Ha and Sii flats. The OIII seems acceptable. All three are Astrodon 50 mm round unmounted 3nm. All Astrodon LRGB flats are OK.
    System specs: FSQ-106, Gemini Rotator/Focuser, SBIG FW8G-STXL Filter Wheel, SBIG STXL 11002 camera, flat illumination provided by Alnitak Flip-Flat.
    An image of each NB flat is attached. The Ha and Sii don't have the classic smoothness and boundary. I received an exchange on the Ha filter but it didn't fix matters. I changed slots on the Ha filter and it helped a little bit - moved Ha to slot 6 and OIII to slot 5. Previously all I had for Ha was a bright arc. OIII was unaffected. What can you advise? Perhaps nothing is wrong and the flat is accurately portraying my optics. But why only Ha?
    Thank you for your assistance.
    Alan
    P.S. I also tested Ha in slot 1 - no improvement.
     

    Attached Files:

  2. Colin Haig

    Colin Haig Staff Member

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    Oct 27, 2014
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    Am sure @Doug will chime in.
    Changing filter slots shouldnt make much difference. Tilt of the flip flat or of the filter could have an impact.
    Perhaps try a sky-flat (twilight/dawn) or T-shirt flat (white t-shirt) to compare, just to see if its flip-flat related...
    Optec may have a suggestion as well.
    Are these "new" Astrodon filters or "old" Don Goldman Astrodon filters?
     
  3. Doug

    Doug Staff Member

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    Perhaps you have a scattered light problem? Or a light leak? Narrowband filters are very dense, so if there are any sneak paths for light to get around the filter that will be a LOT more obvious than for an LRGB filter.

    If you are doing the flats in the daytime then there's a lot more light in the observatory. Make sure the observatory is as dark as possible, given that you have a flat light source in there.

    Try covering the camera / filter / focuser optical train in a dark cloth - except make sure air can circulate through the camera's fan / heat sink.

    Try adding some baffling in front of the filter wheel. If you could temporarily mount a piece of black construction paper with a suitable hole in front of the wheel, for example, that would make an effective baffle. If that is effective then you could make something more permanent.

    Can you post an image of a "good" LRGB flat?
     
  4. Alan Karty

    Alan Karty Cyanogen Customer

    Joined:
    Aug 29, 2016
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    Thank you for your replies.

    These are the "new" Astrodon filters.

    I will try covering the camera/filter/focuser optical train as you suggest.

    I will contact Optec as well.

    Attached is an image of a L flat.
     

    Attached Files:

  5. Doug

    Doug Staff Member

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    Sep 25, 2014
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    Okay, so comparing the two it really looks to me like you're getting scattered light that's getting around the filters. Covering it and/or adding some baffling should help.
     

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