Flats - True or False

Discussion in 'STX and STXL Series Cameras' started by Alan Karty, Feb 12, 2019.

  1. Alan Karty

    Alan Karty Cyanogen Customer

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    1. For a refractor, dust on the exterior surface of the outer lens does not produce donuts in flats, whereas dust on the filters in the filter wheel do. True or false?

    2. If I don't disassemble my image train, then the dust pattern remains the same and I don't need to recreate flats for every imaging session. True or false?

    3. If the pattern of the flat is symmetric around the axis of rotation, then I don't need to take a separate set of flats after rotating after meridian flip. True or false?

    Thank you.

    Alan
     
  2. Colin Haig

    Colin Haig Staff Member

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    1. Mostly true. Depends on the length of the refractor whether the donuts are visible - they may be present but faint. Closer to sensor = smaller dust circle. So, tiniest ones are on top of the sensor, then top of chamber window, then on filters. If you have a field flatener, then it will also create donuts (usually a set of 2-4 lenses on back of scope).
    2. True. However, if you move the focus or things aren't mechanically rigid, some of the effects like dust circles and vignetting may change.
    3. Meridian flips are tricky. Often there is detectable sag in the imaging train, and it becomes more obvious after the weight shifts around to the extremes of a meridian flip. Also, if you have a rotator, you will need to do flats at different positions of the rotator.
     
  3. Alan Karty

    Alan Karty Cyanogen Customer

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    Thank you Colin.
     
  4. Colin Haig

    Colin Haig Staff Member

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    You're welcome. Thank you for being a customer!
    Also, as I looked at your images in the other thread, I noticed you haven't set up the optical parameters of your scope / guide scope arrangement. It's under File... Settings... Site and Optics.
     

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