Celestron Evo 8: Tracks, drifts, then stabilizes?

[jburley]

Until last night everything mostly worked great with my Celestron Evo 8 w/ Starsense. It would auto-align great (using hand controller only), I'd add some additional reference points, go-to my target, and it would mostly stay still (though I couldn't really do exposures for 30s+ without seeing some drift/doubling in images).

However, last night it would go-to target and have it nicely centered in frame... and then for the next 3 minutes or so it would drift very quickly (you could easily see it continuing to move in captures) until the target was all the way at the edge (or off) the frame... and then it would stabilize! In fact, it would be so stable and tracking well that I could do much longer exposures with no star drift. But... didn't matter much as my target was off-frame by the time it stabilized.

Any ideas?

Things I tried

• Added additional reference points
• Recalibrated the starsense with a known target, re-ran autoalign
• Reset several times

 

[Cosmic Geoff]

It may be nothing to do with the Starsense, and it does not seem like an electronic type fault.  It could be simple backlash in the gears, which will be particularly noticeable if you are looking at a small camera frame rather than a low power eyepiece field.

In fact my CPC800 at random did exactly the thing you describe, and I tracked this down to the mount being too stiff to rotate in azimuth, binding at certain positions. The cure was to strip down the mount and ease off a big nut that set the clearance for the azimuth turntable.  I suggest you check for backlash, or investigate the workings of the mount if the problem occurs at random rather than immediately on going to target.  If the mount had no azimuth clutch that you can slacken off to spin it, try running it continuously through 360+ deg of azimuth and see if there is any change in the motor noise.

You can also change in the setup the direction in which the mount makes the final approach to target, IIRC, which might sort your problem.

Edited May 5, 2021 by Cosmic Geoff
Test

[jburley]

Thanks for the reply! I had assumed that the backlash would show itself in the few seconds after movement, and wouldn't continue for 3-5 minutes (and then stop entirely for the remainder of the tracking), but I may completely misunderstand it. 

I'll try for the full 360 rotations as well (haven't heard any sound issues, and the whole setup is only a few months old). The other suggestions... phew, may be out of my ability, but will keep it in mind when all else fails!

[Starwatcher2001]

I agree with Geoff, it's likely backlash. At low powers it can be hardly noticable, but at high power in my SCT (say 300x) it can take the entire field of view to take up the slack. In fact with the backlash set to maximum value it's still not quite enough - which is really annoying. This is the poorest aspect of my scope, but one I've learned to live with.