Resolved Backfocus

Discussion in 'STX and STXL Series Cameras' started by Jorgen Andersson, Aug 16, 2021.

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  1. Jorgen Andersson

    Jorgen Andersson Standard User

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    Sir
    A simple question for you to answer but for me difficult to understand.

    My STXL 11002 camera has 0.98 "back focus. If I have my regular focus 3" outside the tube, should I move the camera to a distance of 3.98 "from the tube or 2.02" from the tube wall?
    Best regards
    Jorgen
     
  2. Colin Haig

    Colin Haig Staff Member

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    It works like this:
    The image focal plane forms a certain distance from a reference point on the telescope. This is the "focal plane distance" or "back focus distance" or "flange focal distance" for a photographic camera lens.
    For a refractor or Cassegrain style scope, this is usually a point on the back of the telescope that is used as the reference.
    If there are accessories such as a focuser, rotator, field flatenner and focal reducer, those items consume the available distance.
    The STXL typically requires 0.948" of back focus. That is from the optical distance from the reference point (usually the face of the camera) to the sensor.

    So, if 3" is the focal plane distance, and the camera requires 0.948", then you have 3 - 0.948 = 2.052.
    I'm not sure how you get 0.98" - perhaps that includes the mounting hardware to attach to the focuser.

    To work it out:
    Available back focus = Telescope back focus distance - any distance used for the focuser and accessories - (adaptive optics + off-axis-guider + filter wheel - 1/3 of filter thickness + camera back focus).
    Ideally, you want to have enough room so that the focuser can be at the mid-point in it's range of travel, allowing for you to move the image train (camera etc) in/out to compensate for shifts in focus caused by temperature.
    If not at the mid point, at least have a few mm of room either side of focus to allow for thermal expansion/contraction.
     
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  3. Jorgen Andersson

    Jorgen Andersson Standard User

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    Thanks Collin
    You made it clear to me with your educational explanation.
    Regards Jorgen
     

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