Hi,
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.
Joe
