Re: Specifying the Optical Reference Point on any Focal Reducer ?

Joe Zeglinski

Hi again,
    Please ignore  my initially posted thread. Upon re-reading, I found my answer  at the AP webpage, in AP’s very well written and detailed document on using their “Telecompressors” (focal reducers).

    “Astro-Physics Telecompressors - 27TVPH & CCDT67 Compression and Coverage”

    Some of my confusion came from reading tech specs on a competitor’s website – like Celestron’s  reducers for the HD-1100 and HD-1400, where they provide back spacing but NOT the FR Optic’s focal length. There was also further confusion in initial info whether that was measured from the base of the output threads (i.e. right from the final lens in the group, inside the threaded flange), or from the rear “flange” (which is the correct reference point). The HD Reducer’s body length is unpublished as well.
   Even though the required SPACER Length is indeed provided, without knowing the back focus taken up by their HD accessory, one can’t determine its impact on an OTA’s known available back focus distance.HD FR is assumed to be directly attached to the back of their HD OTA.
    So, with two unknown factors,  “optic focal length and accessory overall length”,  there would insufficient detail to use Rolando’s “formula”.

   “FR COMPRESSION = ( Optic Focal Length – Distance from FR Optic Center to “Primary” Focal Plane ) / Optic Focal Length”

All this is fine if the FR is used only with their HD scope, but not enough detail if this optic is purchased for use on other brand scopes.
    Doesn’t seem to be an industry standard for stating all the detailed specs, perhaps an oversight at Celestron, and others.
Thanks for your patience.

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