[APML] Fun with JMI NGF-SE and focal reducers.

[Duncan Munro <dmunro@sfu.ca>]

Playing around with my NGF-SE last night, I discovered that if you 
remove the rubber grip ring around the Celestron F6.3 reducer that it 
(the reducer) can be used as a replacement for the SCT small format 
adapter ring.   This means that you can insert the reducer into the rear 
of the NGF and lock it in place using the 3 set screws.  The reducer/NGF 
is then threaded onto the rear cell of the SCT


I calculated that the reducer will be a minimum of 12mm closer to the 
focal plane so this should help with scopes with limited back focus (IE 
a C-8). The reducer can be inserted even further into the JMI focus tube 
but then you begin to loose focus travel.  At full insertion the 3 set 
screws will grip the reducer on its rear rear raised rim which is a nice 
snug fit. With full insertion focus travel  is limited to about  4mm but 
you can get the reducer about 22mm closer to the focal plane. This 
should also allow the reducer to be placed closer to it's optimal 
position for field correction and flatness.


Procedure: Place the NGF face down on a flat surface with the barrel 
fully retracted. remove the SCT adapter. Remove the rubber grip ring 
from the f6.3 reducer (it's  basically a thick rubber band) and place 
the male end into the NGF.  The reducer will fall into the focus tube 
about 12mm but will be perfectly flat since its resting on the end of 
the focus tube. Tighten the 3 set screws evenly and thread the whole 
assembly onto your scope.

I'm going to see about having the reducer machined down so it can fit 
entirely into the focus tube so that it can be positioned exactly at the 
correct distance from the focal plane while I use the large format 
adapter to fit the NGF-SE onto my 10" SCT.

Duncan Munro


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