Re: [APML]: Guiding Guidance

[Chuck Vaughn <aa6g@aa6g.org>]

Hi Robin,
>  
> 1. What are the pros and cons of using an off axis guider vs. using a
> guide scope--say an ETX with a barlow.

The last time I stuck my neck out on this subject I just about got it
cut off so I'll be very careful what I say this time. :-)

In most cases it's much easier to find a suitable guide star when
using a separate guidescope. The problem is flexure between the
guidescope and the photographic instrument is a common problem
especially with exposures over an hour. It's not necessarily
impossible to solve but it can be difficult and may not be worth
the frustration in my opinion. 

An off-axis guider (OAG) will usually solve a flexure problem. The
drawback of using an OAG is that the choice of stars is limited
because of a smaller field and many instruments deliver poor star
images in the OAG's field of view. The ST-4 is not able to "see"
a comatic or otherwise distorted star image as easily as a (nearly)
perfect on axis image. In other words, you'll get a much higher
brightness
number with a pinpoint as opposed to a blob. 

A partial solution is to choose a design for your photographic telescope
that has good off axis star images. A refractor and to a lesser extent,
the Ritchey-Chretien, are examples of designs that have off axis star
images with which the ST-4 works well. Admittedly, the refractor is
likely to be the telescope most likely to work with a separate
guidescope.

Chuck Vaughn
<aa6g@aa6g.org>