Re: [APML] Guide scope advice

["Stuart Heggie" <stuart.j.heggie@sympatico.ca>]

Emmanuele, I don't qualify as a guru but here is the advice I've received over the years:

1. Never use a guidescope with a movable primary so the Mak-Cass route should be abandoned.
2. Guiding should be done near or above the focal length of the imaging scope. However, people are
all over the map on this and you will find some people successfully using 200mm camera lenses to
guide much longer focal length rigs. Anyway, you could go with the short-tube refractor and put a
barlow in it and you'd be fine I think. Many fine imagers use 60mm guidescopes - try that before
buying anything.

Beauty in things exists in the mind that contemplates them.

Stuart
http://www3.sympatico.ca/stuart.j.heggie/Stuart.J.Heggie/
Flesherton, Ontario, Canada

----- Original Message -----
From: "Emmanuele Sordini" <vega@bloomingstars.com>
To: <astro-photo@seds.org>
Sent: Sunday, August 29, 2004 7:03 AM
Subject: [APML] Guide scope advice


> 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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