Mr. Peterson wrote:
> What are the important considerations in choosing
> a guide scope for my LX? I read some where that
> a guide scope should be of a particular focal length
> or focal ratio.

There are many who have chosen a guide scope for long exposure photography.
The pros and cons of this method have been discussed many times. The
general consensus is that guides scopes are good if you are a
novice/intermediate astrophotographer. But, they do have their limitations.
In the end, experience seems to yield to the OAG.

> I was considering the Orion short tube... but I
> also own a Meade 90mm refractor at 1000mm focal length.

The primary concern with any guide scope is flexure. It is important to
keep the auxiliary telescope secure to the primary telescope. Guide Scope
rings are the primary choice, but other options do exist.

The secondary concern is the actual scope. Since there is a budget, you are
fortunate to be exempt from some design related issues. As such, most
people have found that a 60mm to 100mm refractor works well as a guide
scope.
Important considerations for refractors:

The guide scope should have a long focal length, as you have discovered, of
at least half that of the imaging telescope. If your intention is to use an
autoguider, flexure becomes a problem again if you need a barlow to meet a
minimum focal length. An 80mm telescope with a 1000mm focal length is the
average choice. If you use the 400mm FL telescope, you will need to use a
barlow.

The guide scope should have a focus lock. This is a small locking screw
that will keep the draw tube in a set position. This is important for,
among other things, maintaining autoguider focus. Most 80mm and larger
refractors have a locking screw.

In the end, you have added a lot of weight to your system (guide scope,
autoguider, rings, and mounting plate), not to mention the camera and any
other accessories. You'll need to make sure this is all balanced. So,
you'll need to add a 2D balancing system for lateral and vertical balancing.

Yes, you should be careful about the weight. Take the precautions and do
what you think your telescope is capable of handling. The best rule to
follow: if you think it's too much weight, it probably is. Find ways to
reduce it. Your telescope can hold a lot. Just make sure you do not
introduce new flexure problems and that everything is balanced properly.

Best of luck

JDK
The AstroGarden Observatory
http://www.geocities.com/ResearchTriangle/1107/

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