From: "Milton Aupperle" <milton@outcastsoft.com>

Date: May 28, 2008 11:52:33 AM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Field of View versus Orientation of Celestron Off Axis Guider

Hi Folks;

Any of you that have used or are using a Celestron Off Axis Guider

(OAG) may be interested in looking at this diagram I created:

http://groups.yahoo.com/group/Astro_IIDC/files/Other/OAG_Visual_Representation.png

Basically it demonstrates what a Celestron Off Axis Guider would see

as you rotate the guide camera around the main camera, from Position 1

to Position 4. The two red circles describe the "donut" Field Of View

that the OAG can see as it's rotated around the scopes axis. The inner

most Red Donut ring is with the prism orientation knob twisted  all

the way out and the outer red ring is with the prism knob all the way

in. The Moon in the center gives you an idea of scale, as it's about

31 arc minutes in size in the image.

When I used my Celestron OAG, I was always under the impression that

the prism of the OAG was "looking" at an angle of 90 degrees or less,

so that if the Guide cameras was oriented vertically above the main

camera, then that's where it looking for objects, above the guide

cameras FOV within the two red circles.

In actual use, the guide camera views what is below the main camera on

the opposite side that it is oriented on. This solves my mystery where

I always had problems finding a specific guide star that "should" be

there but I would never find it, or I would find some other star.

Hope it helps someone..

Milton Aupperle