Why are some guidescopes unsuitable for certain focal lengths?

[jacko61]

As the title says. I've been looking into adding a guidescope to my Celestron C8 as I'm going to be trying out my hand at imaging once the snow clears and I can get the roof off my observatory.  In my searching, I keep coming across statements e.g. Guidescope XXX won't work with a long focal length primary.  The question I have is 'why not'?  Surely if the guidescope is focussed on a star and whatever software is controlling the mount, the object will stay where it is in the primary's camera. 

Graeme

[Seelive]

It depends upon the ratios of the imaging scope : guide scope and the imaging camera pixel size : guide camera pixel size. At best, the guiding software will only be capable of controlling the mount to keep the guide star (NOT the imaged star) centred to within around +/- 0.2 pixel of the guide camera.  So if the focal length of your imaging scope is 2000mm and you use a guide scope of 200mm then, if the pixel size of the guide camera and the imaging camera are the same, then the imaged stars will wander about by + / -  2 pixels. If you use a guide camera with a pixel size larger than the imaging camera, it gets even worse.

[michael8554]

I believe that guide camera pixel size is only relevant in the calculation of the guide Imaging Scale, which will determine the amount of wobble that +/- 0.2 pixel will transfer to the imaging camera.

Michael

[alacant]

On 22/01/2021 at 15:50, jacko61 said:

my Celestron C8

Hi

For planets you don't really need a guide telescope. For anything else I think you're gonna need an off axis guider. We tried and failed with an old orange tube c8. The mirrors just don't stay put. But at the time, we had little experience and of course YMMV.

I think the rule of thumb is to use a guide telescope of focal length not less than 1/4 the fl of the main telescope and -to guide a c8- a guide camera with small pixels. 

HTH

 

Edited January 23, 2021 by alacant