Re: ATM 3" 75mm or 4" 100mm eyepieces

[epabcc@epa.ericsson.se (Bratislav Curcic)]

> cheap 3" Meade refractor and use it? How do you get the required 75mm
> or 100 mm out of say an F10  3" or 4" refractor lens system?

You can't. The focal length of the "normal" objective is far too long
to be used in an eyepiece. I suggest check Edmund or Melles Griot catalogs
and make a Kellner (it should work quite well at f/8.5). The exact optical 
formula can be found in many optical books, but the design is quite 
forgiving and will tollerate a lot. Basically you need a plano convex
lens (or even biconvex, with one side weak) and a (smaller) achromat.
Symmetrical eyepiece is also very simple. You need two achromats ~80mm
dia, at around 150 to 200mm f.l. Place them with crown elements inside,
at about 10-20 mm spacing. The eye relief will be enourmous, so you may
need to make a rig to keep the eye at exit pupil similar to what Clave
does with their long f.l. eyepieces.
Give us a buzz if you need more help.

Bratislav