Re: [APML] piggy back guide scope size

[<cschur@cybertrails.com>]

Larry, Id like to add a general formula for the guidescope is a 
magnification of 4 to 8x the taking FL in inches, and an aperture of 90mm 
as a minimum for good guide star selection.  Often a 12mm eyepiece with 2x 
barlow is about right for guiding such a setup.

Chris Schur

> --- Larry McManus <lmcmanus@clearpointadv.com> wrote:
>> Can anyone give me a minnimum size for a guide scope
>> to go on my sc. I have
>> a 6 x 6 Hasselblad connected to the visual back of
>> my 8" sc and I need to
>> come up with a setup to allow me to guide on long
>> exposures.
> 
> Hey Larry,
> 
> Kent's right. Because of the moving mirror in SCTs,
> you are best advised to use an OAG, like a GEG or
> Taurus T III. You could be guiding away perfectly
> through a guidescope and there can still be any number
> of ways that your shots will be wasted. Flexure,
> mirror rotation, focus shift, OTA contraction due to
> falling temperatures. Guidescopes on a SCT are a
> crapshoot.
> 
> Alan
> 
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