Re: [APML]: Off axis Guiders

[Chuck Vaughn <aa6g@aa6g.org>]

Rich,

> What should I reasonably expect regarding guide star image quality using
> an off axis guider with a SCT (f/10 or reduced to f/6.3). My first
> impressions have been that the potential guide stars, are like looking at
> dim fuzzy stars through a poorly collimated scope.

What Peter told you is exactly what I used to see in my C14 when using a
Giant Easy Guider - terrible comatic stars, seagulls is a good
description.
Since that time I've had a Giant Easy Guider on a 7"-f/9 StarFire. The
off-
axis stars were wonderful...plenty to choose from. Since then I had an 
off-axis guider specially made for my Ritchey-Chretien which uses the
2.7"
AstroPhysics focuser. The center of the prism is located about 30mm from
the
center of the field and does not block the corner of a 35mm frame. The
images
I see off-axis in the R.C. are far superior to what I saw in the C14 but
not
as good as the refractor. The guide star selection can sometimes be
marginal.
My criteria for marginal is that I have to set the ST-4 for an
integration
time of more than 2 seconds and/or a boost of 3. I try to stay with 1 or
2
seconds and a boost of 1 or 2.

> Is this inherent in the
> design of a OAG and a result of capturing only a small part of the light
> cone or is it a bad OAG. Help!
> 
The prism is not necessarily capturing only a small portion of the light
cone. It could be capturing the whole thing for a particular star. It 
depends on how far the focal point is from the pick off prism, where the 
star is in the field, how large the prism is and the f/ ratio. Try this
little test: Put a decently bright star in the off-axis eyepiece and 
defocus it until you see the bright disk. Is it a disk or is chopped off
on one side or both? If you can see the whole disk then you're using the
whole aperture. If not, you're guide star brightness is suffering.

Chuck Vaughn  <aa6g@aa6g.org>