Re: ATM Eyepieces

["Albert Highe" <ahighe@ix.netcom.com>]

JW,

I'd stay away from the longer focal length eyepieces. There are a few
factors that you haven't mentioned, like the size of your diagonal, the size
and height of your focuser, the diameter of your tube, and the amount of
back focus. These all determine the size of your illuminated field. Take a
look at the recent Sky and Telescope issue.

I would forget ANY eyepiece with a focal length longer than 25-26mm. The
field of view will usually be quite narrow. You most likely will also have a
very dim field at the edge. I also wouldn't waste money on a wide angle in
this focal length. Begin considering wide angle eyepieces at about 19mm.
So, my recommendation is to have a 20 - 26mm Plossl for your lowest power.
Your highest useful magnification will be about 240 X (40X per inch). Even
then, seeing will usually limit going this high. A 4-5mm eyepiece will give
you this magnification. However, most eyepiece designs will have too little
eye relief (your eye will be pressed right up to the objective). I find the
eye relief of Plossls are too little for anything below 13mm. Here is where
a good 2 to 2.5X Barlow works well. Alternatively, consider the Lanthanum
eyepieces such as offered by Vixen and others. They have very long eye
relief at very short focal lengths. Depending on what you like to observe,
you'll find you have a favorite magnification. For many objects, I like
observing at around 100X. You might consider a more expensive eyepiece in
the range you use the most. I like the TeleVue Radians. I would give the
10mm or 12mm a high recommendation. Even though I think the 9mm Nagler is a
great eyepiece, it and other Naglers will probably be too heavy for your
scope, unless you have a convenient way to shift balance quickly between
eyepieces.

So, my recommendations are:

a 20 to 25mm Plossl to give you roughly a 1 degree fov for scanning and
large open clusters
a 15mm Plossl
a 10mm Radian
4-5mm Lanthanum or a 2X Barlow.

The 45 degree image erector is for refractors. It will be useless with a
Newtonian.

Albert

----- Original Message -----
From: jw <ntent@win.bright.net>
To: ATM <ATM@shore.net>
Sent: Friday, August 11, 2000 12:25 PM
Subject: ATM Eyepieces


>
> Hello, All:  Just wondering what the list would recommend for a set of
> eyepieces for a 6" f7.5 dob.  I'm thinking of 4 or 5   1 1\4"eyepieces,
and
> a 2x Barlow and.... what else?  Is it a good idea to use a 45 degree
"image
> erector" for this 'scope  (I don't think so.. but I thought I'd throw that
> up to you all for comment   I'm pretty sure those are best used on
> refracting scopes.)   Thanks in advance for the input.     JW
>
>
>