STT and STXL backfocus - manuals versus backfocus table

Discussion in 'STT Series' started by EricC, Dec 20, 2016.

  1. EricC

    EricC Cyanogen Customer

    Joined:
    Jan 31, 2015
    Messages:
    167
    The STT manual at http://diffractionlimited.com/wp-content/uploads/2016/04/stt_manual_080112b.pdf on page 48 says the mechanical backfocus from the front of the camera to the CCD is 16.77 mm but the backfocus table at http://diffractionlimited.com/wp-content/uploads/2016/11/backfocus.993x0.gif lists it as 16.2 mm, a difference of 0.55 mm.
    On page 50 of the manual the mechanical backfocus with the self-guiding filter wheel is 54.06mm but the backfocus table lists 53.5, a difference of 0.56 mm.

    For the STXL, the manual (last updated this month) has 25.31mm mechanical backfocus from CCD to mounting surface of camera but the backfocus table has 24.1 mm, a difference of 1.2 mm. And the manual has mechanical backfocus to FW mounting surface of 61.84 but table has 60.6, a difference of 1.24 mm.

    I'm about to order several adapters from Precise Parts and need to be sure I order the correct size.
    Are the manuals correct or the backfocus table? If the later, when will the manuals be updated with the correct numbers?

    Thanks - Eric
     
  2. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    8,372
    I didn't write up either document, but my suspicion concerns the term "mechanical backfocus". That's the physical distance to the sensor. However there is a window in there, and the glass reduces theeffective distance a little due to the effects of refraction.

    http://dougkerr.net/Pumpkin/articles/Glass_Plate.pdf

    Assuming a typical index of refraction of 1.5, the distance is reduced by 1/3 the thickness of the glass.
     
  3. Tim Puckett

    Tim Puckett Guest

    Eric
    Go by the chart as the manuals are being updated and corrected.



    backfocus_993x0.gif
     

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