Re: Tracking issues with AP900

Christopher Erickson

If I were having this problem, here is what I would probably do, in rough order...

1. Replace the ancient CP1 with a CP4, if possible. Get atmospheric refraction correction and a bunch of other new and cool functionalities.
2. Check RA axis backlash and axis balance. If your backlash has loosened up over time and the mount is very closely balanced and the worm lube is very dry, the scope could be "bouncing along in the backlash." Make the mount more imbalanced to "wrap up the backlash." This is usually more noticeable near the West and East horizons. Not so much close to the North or South horizons. Still worth checking.
3. Triple-check all of the optical train bits for something loose and flexing/sagging/shifting.  Especially focusers and all set screws and lock screws.
4. Triple-check the polar alignment using the drift method in SharpCap.
5. Carefully re-lube both axis worm drive assemblies and carefully tune the worm mesh/backlash afterwards.
6. Always use on-axis or off-axis guiding, never piggyback guiding. Piggyback guiding only leads to tears. Especially with compound scopes with floating optical elements.
7. Check for loose or broken bolts and knobs on the mount and dovetail bars/brackets/saddles.
8. If this scope is accessible by multiple users, take NOTHING for granted. Check EVERYTHING. Especially the DEC axis disassembly/assembly dovetail lock knobs.
9. When in the Northern hemisphere and observing things near the Southern horizon, the OTA is tipped a considerable amount on its side from the DEC axis. A slightly-loose dovetail lock knob could wreak havoc. Not to mention putting the OTA and cameras in great jeopardy.

I hope this helps.

On Fri, Aug 7, 2020 at 10:08 AM Chanan Greenberg <chanan2009@...> wrote:
Hi Rolando,

Really appreciate the effort to help. Just as a backdrop, I have been doing astrophotography and drift alignment for 12+ years. 
When I did my drift alignment with a high power illuminated eyepiece in the imaging scope I had zero drift in both axis for 7 minutes. This is why I do not believe this has anything to do with polar alignment. Even with poor polar alignment an AP900 should be able to track stars well without auto-guiding for several minutes. 
I am able to get that performance in any direction I point with the exception of pointing south. There it seems to be unable to track for even 30 seconds without drifting in both RA and DEC. 

Which is why i suspect - balance, flexure, mirror flop or something wrong with the mount itself.

Hope this is more complete info.


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