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

[andydtg@alpha1.phoenix.net]

Re:

>I am looking for a commercial, military or even home made 75mm 3" eyepiece
>and
>a 100mm 4" unit. The diameters  of  3" and 4" respectively must be the
>minimum, though
>the 75mm and 100mm specs can vary somewhat.
>
>Otherwise...Does anyone know of a source of these or way to build them? 
>
>I am working with a 22" F8.5  Newtonian  with a universal size focuser holder
>clear aperature to four inches, and want a couple of lower power views for
>enjoyment....A 55mm yields 83X,  which is a 6.47 exit pupil....The lowest
>power I presently have now.
>
>A  75mm eyepiece  would yield  61X, with exit pupil of  8.83mm
>
>A  100mm eyepiece would yield 46X, with exit pupil of 11.7mm 
>
>I realize that any exit pupil over about 7mm is useless for increased
>brightness,  (thats about my pupil size) but does anyone have an idea what
>the view looks like thru a 3" or 4" wide eyepiece? Does your eye have to move
>around to see the complete image? Lets assume the eyepiece is a common Erfle
>type, not a wide field Nagler Type II style.

The Celestron C-16 we used at the NASA-JSC observatory came with a 75mm Erfle
with a 3" dia field stop, gave 60x at f10 or so. It was obviusly a military
surplus item. Unfortunately these things were originally intended to be used to 
view radarscope screens & as such were not really infinity corrected eyepieces,
the off-axis aberrations were terrible & the eyepiece is still gathering dust
on the shelf, as far as I know.

Besides, the off axis coma even at f8.f would be bad & your secondary size to 
illuminate the field of such an eyepiece would be rather large also.

Andy Saulietis / DTG Alt-Az-Fp Drive Systems
12617 Harriet Ln
Santa Fe, TX 77510 USA
409-925-8854 Voice/Fax