Joe Huber wrote:
>
> How do I use this information to select which eyepieces to use?
You multiply the f/ratio of the scope by the eye's pupil size. This
gives the longest eyepiece for which you will not lose light.
For instance, with my 4.25 inch f/4 scope, and my (say) 6mm eyeball, I
should avoid eyepieces longer than 24mm. In the 8 inch f/6, I could go
to a 36mm eyepiece.
Another way to say it is that the exit pupil of the scope will not
exceed the entrance pupil of your eye. This points out another issue
with too-big exit pupil: the shadow of the secondary becomes
bothersome; you'll have a vague shadow moving around interfering with
part of the field of view. In some cases you can find a position for
which the field of view is unobstructed.
If I'm using the RFT during the day with a 2mm eye pupil, an eyepiece
longer than 8mm will have the shadow effect.
In some cases you may choose to use a longer eyepiece, for instance to
get a wider true field. But you'll know that you are trading off
light.
At night, the shadow effect is not especially noticeable, but it could
be hiding your intended object.
Regarding the shadow business, some eyepiece aberrations can cause a
similar effect, making eye placement very critical, both in lateral
position and in distance from the eyepiece. The original 13mm Nagler
was guilty of this.
Clear Views!
-- Jim Van Nuland, San Jose (California) Astronomical Association <a href="http://www.svpal.org/~jvn/ ">JVN's home page</a>Received on Sat Jul 31 11:54:24 2004