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

[K1JJ@aol.com]

In a message dated 96-08-19 21:18:47 EDT, you write:

>
>> 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

Hi Bratislav,

I will definately see if Edmund has those available and do some
experimenting. 

My GOAL here is a wider field of view at reduced power for comets, enjoyment,
etc. If I were to use a 3" diameter 75mm  with the 22" F8.5 this yields 61X
with an exit pupil of 8.8.

What I'm not sure of is: What will the view be like. I realize I will
vignette anything over 7mm at best with my eye, but.... will I see a BIGGER,
 wider field of view than a 55mm 2" eyepiece, (6.5mm exit) or is there
something here that outweighs the improvement and makes the whole project a
waste? (like exit pupil).....(My diagnol/focuser system here can handle up to
about a 4" diameter w/o vignetting.

I don't care if things do not get brighter, I just want wider, and
bigger.....

I am confused as to how far you can take the diameter of an eyepiece. Assume
no vignetting in your optical system.... As an extreme, ridiculous example:
 Say you had a 10" diameter 245mm eyepiece giving a mag of 18X and exit pupil
of 30mm!  What would the view look like here with a Kelner? Again, I know the
exit pupil is wasted, but how about contrast, field of view, can your eye
handle this big eyepiece diameter, and eye relief,  etc??

I'm real curious.

Thanks in advance.

Tom Cathey