back focus table for Aluma

Discussion in 'Aluma Series' started by blackdragon72, Aug 6, 2019.

  1. blackdragon72

    blackdragon72 Standard User

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    I dont see back focus for Aluma camera in back focus table. Where can I find it?
    And does FW8-aluma take the same face plate (T-thread or STL) as FW8-8300?
     
  2. Doug

    Doug Staff Member

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    A back focus table for Aluma is in the works. Each model is slightly different so we wanted to make sure we had accurate numbers for everything.

    Yes the filter wheel face plates are the same as FW8-8300 and the old FW8G-STT.
     
  3. Colin Haig

    Colin Haig Staff Member

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    One of the most common configurations is the Aluma 694 or 814 with the FW8G-Aluma self guiding filter wheel.
    This has a backfocus of 54mm, depending on the refractive index of the filters (I've assumed the filters shorten by about -0.6mm).
    The adapter ring or nose-piece will add extra distance, depending on whether the draw tube screws into the the adapter ring or stops at the front.
    Most of the adapter rings are about 3.75mm thick; if you use the 2" nosepiece into a STL thread, it usually has a base flange of about 1.65mm; for a total of
    54+3.75+1.65 = 59,4mm

    With the other sensors, the backfocus varies are few millimeters.
     
  4. blackdragon72

    blackdragon72 Standard User

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    That is very useful.

    Does 54mm number include T-thread or STL thread face plate?
     
  5. Colin Haig

    Colin Haig Staff Member

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    The 54mm is from the image sensor Silicon to the front plate of the self guiding filter wheel.
    When you attach an adapter plate to the front, it adds thickness if something that screws into it stops at the thickness of the plate.
    So for example, a draw tube without a lip or stop will go in, and not add backfocus distance.
    A nosepiece that has a lip or stop or base flange threads in, but stops at the front face of the adapter plate so adds distance of the stop and the thickness of the plate.
     

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