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

[K1JJ@aol.com]

In a message dated 96-08-13 21:52:48 EDT, you write:

>
>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

Hi Andy,

I can always count on you for the answer.

Well, I won't explain why now, but this F8.5 Newtonian  has the ability to
use a six inch secondary remotely at will, resulting in plenty of size to
illuminate the field. (Thats a 27% obstruction)

At what size eyepiece diameter do you feel  this six inch secondary mirror
will begin to be inadequate? 

I'm not sure I understand what you mean by the off-axis coma problem. I know
all Newtonians suffer from it to some degree, but what you're saying is that
since the field is now wider, that I'm picking up a larger outer portion of
the Airey disk...or something like that?

As ridiculous as it seems, would a coma corrector ( like TelevueParacorr) be
the solution for such a slow Dob as mine? (F8.5)....

If it did work, what would be your impression of a full 7mm+++ exit pupil
with a 3" barrell...would the field look super big? .....think its worth the
effort for the effect??

I wonder where some of these big eyepieces could be had. Also, I wonder what
easy to find lens could be put to use to make your own?


CU

Tom Cathey