Specifying the Optical Reference Point on any Focal Reducer ?

Joe Zeglinski

A question on Optics ...
    Does anyone know, for certain,  if there is any “optics industry standard” which defines the exact point on a focal reducer lens barrel,  to measure the length to the required spacer , in order to get the published “reduction factor”?
     I have assumed that as with all lenses, the distances are always measure from the vertical axis of the CENTER of the LENS, rather than from the back end flange of the lens cell itself. Since Barlows and focal reducers are often a compound lens, it makes it somewhat difficult to be sure that the  ...  “Unmarked” optical reference point ...  might be at the “mid-point” of say the 27TVPH lens barrel, rather than the “Scope side’s, flange edge”. In fact, a compound lens may not necessarily be positioned exactly at mid-barrel, so the “effective” optical center for the reducer spacing may be imprecise.
    It might be that it would take a bit more effort to “stamp” the optical centre point, so FR makers simplify by quoting the shortened, offset,  (slightly reduced length) of required spacer size, by referring  to the END to the lens barrel. As these are seldom offered together as a set, the precise measurement is lost in actual use.
    Perhaps, one such example, excerpted below, of the confusing results for lens measurements,  was in Russel Croman’s widely published and much read paper, where he seemed to have some difficulty in ascertaining the “correct starting position” to measure the required spacer to get an EXACT  0.75x focal reduction – using the AP’ 27TVPH lens in his test.
Proper Positioning of a Focal Reducer
on a Ritchey-Chrétien Cassegrain Telescope  ... (Copyright 2002) by Russell Croman

   “ Note: the AP web page for the 0.75x reducer states that the distance from the edge of the 2” adapter should be about 65mm for 0.75x reduction. However, measurement of the reducer and 2” adapter combination yields a distance of about 60mm from the back of the optics to the back edge of the 2” adapter. Thus, according to the above computation, an additional 115mm is needed to achieve 0.75x. Using a 65mm spacing from the adapter to the focal plane will yield approximately 0.82x reduction.”


    So, my question to experts in the group ... At which reference position on an FR  lens cell barrel does one start to measure the required “Spacer separation”?  

And ... is this reference position – (front, back, or center) -  common to  all brands of Focal Reducers & Barlows?

Thanks for any advice, as I am in the same quandary in my present project – and guessing it might be at the geometric center of the lens barrel.


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