Resolved Bahtinov Mask

Discussion in 'Image Capture' started by Kenn Hopkins, Dec 19, 2018.

  1. Kenn Hopkins

    Kenn Hopkins Standard User

    Joined:
    Feb 21, 2015
    Messages:
    10
    Sir,

    Please find attached All-Filters.jpg file showing four images through my Celestron C-8 HD, SBIG STF8300m with filter wheel using AstroDon Series II filters and a Starlight Instruments FeatherTouch focuser.

    The images show the Red and Green filters with three straight fingers while the Luminance and Blue filters show a ‘broken finger’ on the right finger.

    I sent this image to Celestron to explain why the Luminance and Blue show a ‘broken finger, their response was: “I personally think you should try getting another filter and mask, to ensure that is not the issue because the issue only arises with those specific units and nothing else. I recommend getting those items replaced from the manufacture and testing them out before bringing in your current scope, to ensure those are not the items causing the issue.”

    When I brought up the scope to Celestron, the person I spoke with suggested the problem was either the Feather Touch or even the SBIG Product. I then told him when I used my 1975 Orange Tube C-8 all filters showed straight fingers.

    Have you ever experienced your product showing this type of ‘broken finger’ when using a Bahtinov mask. I still think it is something to do with the design of the C-8 HD scope with a corrector lens in the baffle tube, but I’m just trying to touch all bases.

    Kenn
     

    Attached Files:

  2. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    6,057
    It's definitely not the feather touch; this is an optical issue, not a mechanical one. Also definitely not caused by the camera.

    My suspicion is that you are seeing spherochromatism. There is some residual color in a single-corrector-plate Schmidt system (this is why the Palomar Schmidt has a two-element corrector).

    The Celestron telescopes are pretty good in this respect because their master plate manufacturing technique allows them to optimize the corrector shape; however, there will be some color difference across the spectrum. If your focus test is sensitive enough then it will be visible.

    The giveaway here is that the effect is worse in the Blue, and even worse in the Luminance. That is indicating a focus shift at the blue end of the spectrum. The reason you're seeing lines instead of dots in the blue frame is that the defocus is worse at the bluer end of the spectrum.

    Whether this matters in real-world imaging is something you should evaluate. Compare the FWHM of stars under good seeing in a Luminance frame versus a red frame.
     
  3. Kenn Hopkins

    Kenn Hopkins Standard User

    Joined:
    Feb 21, 2015
    Messages:
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    Thank you for the excellent and understandable response.
    Kenn
     

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