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Old 18-02-2011, 10:58 AM
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pmrid (Peter)
Ageing badly.

pmrid is offline
Join Date: May 2008
Location: Bribie Is.
Posts: 3,225
Guiding at Long Focal Lengths

Celestron have yet to come out with a reducer for the HD range of scopes so imaging options are very limited - either HyperStar at F1.9 or native at F11. Guiding at F11 (which equates to a FL just a whisker under 4 metres) is a real challenge. I've tried SBS and Piggy-Back guiding but neither really gives me the stability I want.
So I am about to go back to using an OAG. Added to that, I have sourced a non-Celestron reducer (Optec "Lepus" .63) that is said to be designed for highly corrected scopes including the HD line. But I'm a bit up in the air about how to combine the reducer and an OAG.
Reducers have very specific requirements governing the distance from reducer to CCD imaging plane and the Optec Lepus is certainly no exception - so much so that you have to buy a camera-specific adapter with the reducer. Similarly, the manufacturer says, you have to position the reducer as close as possible to the back of the OTA and the corrective optics that sit inside the baffle tube of the HD scopes. That's where my dilemma comes in.
If I position the OAG in front of the reducer, the guide camera has to work at F11. I can't position it behind the reducer without screwing up the distance to CCD measurement.
Here is a rough graphic of the setup as I see it at the moment.
Can you guide at F11. I'm imagining the image scale on a QHY5 at F11 and, to say the least, it would be minute. So finding a guidestar would be awfully hit-and-miss. I'm assuming I can overcome differential focus issues by using adapters on the QHY5 and that I can get the QHY8 into focus without screwing up my guiding focus too much.
But can you guide at a FL of 4 metres? I have my doubts so I'm hoping either for reassurance or the sound of distant laughter.
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Click for full-size image (guider setup.JPG)
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