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It was the OTA - not the mount!
Well I have been pondering the tracking accuracy of my Vixen Atlux mount for quite some time now, so I finally tried something different, polar aligning using a Canon 400D attached to a 127mm (1500mm focal length) MAK rather than the C9.25 SCT.
PEMPro said I was West of the SCP and I eventually got it to less than 0.3 arc minutes of error. Then I tracked Jupiter for 16 minutes before clouds rolled in. I put the Canon's central focusing dot dead centre on Jupiter - it was about 1/4 Jupiters diameter. After 15 minutes the dot wandered South in DEC maybe 1/5 of Jupiters diameter but RA was dead centre.
PEMPro doing 10 Worm cycles said PEC was less than +/- 2 arc seconds - so that is pretty fine (if I didn't screw up a reading somewhere) and RA and DEC drift per worm was like 0.00014 something?
So I tried imaging Eta Carina, unguided for a bit over 16 minutes (969 seconds) before the clouds came in - attached is what I got (Photoshop CS2 levels and curves - no sharpening or deconvolution of stars). I also include a 512 second shot of Eta Carina.
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So this indicates I have have mirror flop on my SCT if I keep seeing star trails (as a 2350mm focal length SCT thru a Lumincon OAG gives about 1570 mm focal length - almost the same as the MAK). Strange mirror flop as it has always appeared as a constant star trail about 1-2 arc seconds per minute...
Question good folk - how to best eliminate this? I seem to remember someone racked their mirror all the way back then added a 10:1 SCT focuser at the rear of the OTA to achieve focus. Racking the mirror all the way back I supposed stiffened it considerably to eliminate nearly all flop, then you just have to play with focus tube extenders at the rear of the focuser to achieve focus I guess.
Alternatively you could remove the mirror, fit a spring around the baffle tube and replace the mirror - so that the mirror cell is always under expansion tension to minimise movement. Anyone else got clever ideas? I have a feather touch 10:1 fine focuser, so I can't add a Hutech mirror lock. What might folk here suggest?
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