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Guide scopes and optical quality


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I'm considering having a go at guiding my rig in the near future, and if possible i would like to do it relatively inexpensively.

I plan to do the LX mod on my spc880 and use that as the cam, but i'm unsure of what is suitable as a guidescope.

I gather that refractors are preferable due to lack of mirror flexure, but can i just buy a bargain basement 70 - 80mm?

Also, are there any rules of thumb regarding focal lengths? I think i read somewhere once that you need to be using at least half the focal length of your scope to maintain accuracy, am i on the right lines?

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The significance of the ratio of focal lengths between imaging and guiding scopes is now obsolete. The autoguiders are so accurate that the ratio of FLs can effectively be ignored. You wouldn't guide the Keck with a finder-guider but in amateur terms you can forget about it.

What now matters is the f ratio of your guidescope, especially if using a not very sensitive webcam. Faster is better. The ST80 is cheap and effective and with the supplied Barlow you throw away the lens and use the rest as an extension tube. You also want a FOV wide enough to pick up stars so a moderate FL (the ST80 is 400mm) is to be preferred. Since guide rings are so expensive it might be better to dispense with them and pay more for a sensitive guider which did not need them.

Olly

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To sort of backup what Olly's saying.. I get away with using a Slow 500mm f8 telephoto lens because I use a reasonably sensitive guidecam a Meade DSI IIc and have never had to move the guide scope or alter framing to find a guidestar with exposure of 2s max...

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To sort of backup what Olly's saying.. I get away with using a Slow 500mm f8 telephoto lens because I use a reasonably sensitive guidecam a Meade DSI IIc and have never had to move the guide scope or alter framing to find a guidestar with exposure of 2s max...

Ditto...Ive just sold my guide rings, using the ST80/QHY5 combo I found I didn't need them.

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Gah... on the subject of guidecams, as much as i love my mac, i really feel like i shot myself in the foot when it comes to astro imaging. it doesn't look like i have very many options when it comes to compatible guidecams.

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