[APML] Guide scope advice

[Emmanuele Sordini <vega@bloomingstars.com>]

Hi folks,
I am to choose a new guidescope for my film deep-sky work. Presently, I am 
using an MTO telephoto lens (1000 mm, f/10 Maksutov), which I want to dump 
for the following reasons:

1) Hooks to the guiding platform only with a single photo tripod 3/8" 
screw. As tight as the screw can be, this can lead to flexure (rotation) 
problems especially when the platform is high respect to the ground.
2) Being a telephoto lens, focusing is performed by rotating the front 
wheel (which holds the whole corrector plate): this is a pain in the butt 
especially when the lens is tightly hooked to the platform. Moreover, 
misalignment can easily result.

The "light bucket" is a 500mm f/4.5 Pentax telephoto lens, the mount is a 
GM-8 and guiding is done both with an illuminated reticle eyepiece and with 
a SBIG ST-4. I wanted my new guidescope to be in the 500-1000 mm range of 
focal length, which I think to be adequate for a 500 mm lens. Given the 
above picture, I thought about the following options:

1) My old 60/700 refractor. Pros: focal length is OK, cheapest alternative 
(I already have it!). Cons: small aperture and narrow focuser is (.965" 
instead of 1.25", though I have an adapter).

2) A Chinese (Skywatcher) 90 mm Maksutov-Cassegrain reflector. Pros: fair 
price, good aperture, already suited for astronomy, focal length OK. Cons: 
focusing by primary mirror movement, which can cause guiding errors. A 
bettefr solution would be a Maksutov telescope of the same size, but with a 
rack-and-pinion focuser.

3) An 80mm guidescope, such as a short tube. Pros: fair price, fair 
aperture. Cons: focal length too short (maybe). Longer FL tubes in the same 
diameter league are available on the market, even if they aren't easy to 
find and are often quite long, which could mean higher torque on the 
declination axis and higher sensitivity to vibrations and wind.

What do you AP gurus out there think of this reckoning? Any 
insight/suggestions/ideas will be highly appreciated.

Thanks in advance and clear skies,
Emmanuele Sordini

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