Resolved Back focus and vignetting with STF 8300

Discussion in 'STF Series CCD Cameras' started by Peter Bresler, Feb 12, 2020.

  1. Peter Bresler

    Peter Bresler Standard User

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    I just used the STF 8300 with the filter wheel for the first time last night. I used a 2 inch adapter to fit the filter wheel into my Optec focuser on my Celestron Edge HD 11 inch with the focal reducer. I was able to get focus, but the mirror was close to the far limit. I do not think there was sufficient back focus to use the OAG. I also noted quite a bit of vignetting, which I was not expecting. Is this to be expected?

    Peter
     

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  2. Colin Haig

    Colin Haig Staff Member

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    As far as the vignetting goes, what focal reducer are you using, and how big is the fully illuminated image circle from it? What is the focal plane distance from the back of the focal reducer?
     
  3. Doug

    Doug Staff Member

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  4. Colin Haig

    Colin Haig Staff Member

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    I just spent a couple minutes with your image, and am not seeing much vignetting - more signal is needed to really tell what is going on.
    You'll need to do a flat frame FITS and post here.
    eg image the twilight sky / a white T-shirt, or an evenly illuminated panel. You want the ADU count up around 35,000 to 45,000 or so.

    The sample M51 you provide - the stars could be sharper. Part is focus, part may be drift - the stars are a bit elliptical.
    I could also use a field with a bunch more stars to analyze the image.
    The histogram is quite low in your image - a longer exposure (5-10minutes) would help, but of course guiding will probably be needed.

    A photo of your setup would help.
    As I envision what you're saying, your setup is something like this:
    C11 Edge HD -> Possibly the Celestron 1100 0.7x Focal Reducer -> Some Optec focuser -> An adapter from optec to 2" -> Filter Wheel -> Camera
    Perhaps you could fill in the blanks with specifics and dimensions.
    What you want is:
    C11 Edge HD -> Possibly the Celestron 1100 0.7x Focal Reducer -> Optec focuser -> adapter -> Off-Axis Guider -> Filter Wheel -> Camera

    Let us know.
     
  5. Peter Bresler

    Peter Bresler Standard User

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    I have the Celestron 0.7 focal reducer designed for the scope. The camera filter wheel is flush against the Optec focuser...I do not think it could be closer, only farther with an adapter. That shot may not have been guided, but subsequent ones are. Here is another sub. I was having some collimation issues, maybe not fully resolved. It was nearly full moon.
     

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  6. Peter Bresler

    Peter Bresler Standard User

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    The required back focus for the scope is 146 mm. What is the distance from the front of the filter to wheel to the camera sensor?
     
  7. Doug

    Doug Staff Member

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  8. Peter Bresler

    Peter Bresler Standard User

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    The filter wheel, Optec focuser I am using now and camera just meet the required backfocus for the scope at 146mm. That means that I cannot add an OAG to the train. The filter wheel appears to have a port for an OAG. What would I need to get?
     
  9. Doug

    Doug Staff Member

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    Sorry, the FW8-8300 does not have a built-in OAG port.
     
  10. Peter Bresler

    Peter Bresler Standard User

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    Does that basically mean that I cannot use an OAG with the camera and filter wheel?
     

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  11. Colin Haig

    Colin Haig Staff Member

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    Our SBIG StarChaser SC-2 off-axis guiding camera is about 0.8" / 20.3mm thick. (Not sure if you have ordered one).
    If you found some way to reduce the thickness of that stack of stuff (adapters, draw tube, Optec), you might be able to fit it in.
    Basically, something's gotta give. You could ditch the reducer, or replace the old Optec TCF-S with something slimmer like their LEO.
     

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