Re: RA rate drift (revisited)
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Hi Keith, Very nice summary of the system...? My guesses: 1. Not clutch slippage, not anything wrong with the drive system. ? 2. My guesses are: ? ? a. slight error in polar alignment. ?(most likely cause).? ? ? ? ? (Did you do a long drift test for your polar alignment... as long as your ?sub exposure time? ? I use the PHD2 Drift Alignment tool and this helps very much. ) Reason: ?I am not sure that your polar alignment which you wrote as " < 1 ' " is good enough, for the 8 inch SCT (f/10?)?has a high angular mag, and the 80mm guide scope (f/6?) is probably wide view with low angular mag.? So a single pixel shift on the guide scope, to cause a PHD error correction, already would result in many pixels shift on the main scope image. ? ?Some folks put a Barlow on the guide scope to improve its error detection. ? Other possible effects:? ? ? b. you may have a difference between the center star in your guide scope and the center star of your main telescope image frame. ? This will cause the autoguider to perfectly track the guide star, but the main image will seem to rotate. ? and/or ? ? c. you have some differential flexure between your Guide Scope to SCT coupling.? These are awfully hard to eliminate due to changes in the gravity pull on the guide scope as the mount tracks. ? and/or ? ? [d. you may have some mirror flop in your SCT. ?(SCTs are prone to this due to the way the primary mirror is "floating" by a gap around its moveable shaft, even if the shaft is "locked" in place.) ?A flop would give a quantum jump in image though...not a gradual one. ] My experience was this: I never was able to get perfect tracking/autoguiding with an external piggyback 80mm guide scope on my 10 inch f/10 SCT, even when assuring they were pointing to the exact same center image.? The reason (for me at the time) was that my polar alignment was imperfect, and the guide scope is guiding the mount to rotate about the guide scope guide star.? Any slight polar alignment error will then make the Main scope image look like it is drifting. Recommendation: I gave up on piggyback guiding, and went to an OAG.? For me, the large prism and solid construction of the Lumicon giant easy guider was superior to other ones that have many adjustment screws and a small prism.? The OAG method this eliminated the problems of autoguiding... at least for me. ? But the polar alignment drift is a necessary step... when I try to skip that step I may the price... it is easy to knock the mount slightly, and mess the alignment up. ? ? Hope these comments help you get to the finish line! Michael On Mon, Aug 14, 2017 at 9:43 AM, keithnakonechny@... [Losmandy_users] <Losmandy_users@...> wrote:
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Michael Herman