Quote:
Originally Posted by Retrograde
Hi Rob,
As no-one else more knowledgeable has responded yet I thought I'd jump in and ask the obvious question - have you considered going for an off-axis guider?
Imaging through an RC at a focal length of 2000mm might not be well suited to guiding through a guidescope given the risk of differential flexure (no matter what guidescope you choose). Some people make a guidescope work when imaging at long focal lengths but most with setups similar to yours would go the OAG option I believe
It might difficult to know whether the guidescope option will work or not till you try it. Perhaps a small ED/Apo refractor that could also be put to use as a wide-field imaging scope &/or grab & go visual scope might be an option in case the guidescope path doesn't give satisfactory results?
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Quote:
Originally Posted by kencas
Take this with a grain of salt as I'm an absolute newbie, but I was under the impression that a guide scope of nominally 30/120 is only effective up to approximately 1000mm focal length, and up to 50/150mm or 60/240 and longer up to approximately 1500mm focal length. But these would also be dependent upon the pixel pitch of the guide camera, and also at their "best" performance on a completely rigid mount.
For longer focal lengths Off Axis Guiders (OAG) are sourcing their targets through the same image chain as the primary optical image pathway, This means that any movement within the scope (such as SCT mirror slip) can be compensated for, whereas an external rigid guide scope can't "see" any changes to the primary optical path and can't compensate.
But also, an OAG is also using the focal length (give or take a bit) of the primary scope itself, and so a 1500mm SCT would act somewhat like a 1500mm guide scope by diverting a "very small" part of the primary optical path to the guide camera, whereas a guidescope is only around the 150-700mm focal length.
But again, I am only too happy to be corrected by those who know a hell of a lot more about it than I do!!!
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Thanks for the replies.
Not sure about using a OAG. My thinking is, and I am happy to be corrected. I would have thought using a guide scope of the focal length and aperture I am looking at would give a wider field of stars to lock onto with the guiding software (e.g. PHD), whereas the OAG would only give you the view the OTA is seeing or part there of. Would that be wide enough?
In regard to flexure, my OAT has a Losmandy "D" style (3inch) dovetail plate on the bottom and the top, thus the guide scope would be secured on top of the OTA, would this be rigid enough to reduce flexure
Cheers and thanks