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unpublished as well.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.
Joe
