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perennial declination stiction

I powered up the AP600 tonight (in spite of the clouds) and tried to characterize the guiding. I had re-lubed the DEC axis again today (for the umpteenth time) and was hoping for some improvement.

There is none. The calibration pattern in MaximDL is not an "L" shape - it is a 1 (e.g. the foot of the L is so short as to be almost invisible). GoTo pointing is not affected, so it seems that only very small DEC movements don't register.

Here's what the guiding graph looks like..

as can be seen, due to polar mis-alignment, the DEC drifts north for quite some time; the corrections aren't having any effect until the stiction is overcome; and then things are normal.

I can't count how many times I've taken this mount apart in an attempt to fix that DEC stiction; but all my efforts have obviously been for nothing.

any other ideas? incidentally i lucked out on the polar alignment and was able to turn off DEC guiding. should be good for up to 10 minutes i think. worth the effort?
3 tips or workarounds:

-- Manually Identify the direction of the drift, and just guide in that direction, this will take you 5 min or so at the begining of your imaging session
-- Maximdl guiding algorithm can identify the DEC drift automatically and prevent reversing the DEC axis when guiding.
-- Increase the guiding rate to the maximum, usually 1x sideral


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I have already done all three.

The challenge is that if there is wind or some displacement and the DEC swings to the other side, it takes forever to reverse the DEC, and then when it reverses, it has to reverse again to counteract the DEC drift.
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You cannot guide out the wind, usually if the wind hits the mount the star will move but after the wind is gone the star will go back to it's original place. If everything is properly set with 0 flexture the DEC should only drift in one direction.

Putting aside the wind do you know why your mount moves randomly in DEC?


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well there is a bit of backlash in dec.. wind or cable drag or not-quite-perfect balance all can cause random movement. it is difficult to get perfect balance because the AP600 doesn't have a bearing in DEC - it has a bronze bushing. so the DEC axis does not turn freely, it is heavily damped.

OTOH the Mach1 is also heavily damped on both axes. But it is much less sensitive to balance.
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If this is an old QMD model, AP built backlash compensation switches into the hand controller and the old manuals (still available on their web site) have info on how to adjust. Issue wasn't stiction as opposed to amount of movement needed in motor gear train to reverse direction.

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Hi Orly,

Have you tried a lighter grease? Maybe lubriplate no.105?

OT: I'm interested in the guiding graphs that I come across in these forums. Mind telling the FL of your guidescope and pixel size for the graph posted above?

hans.L My Flickr

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David, both the old QMD controller and the replacement GoTo controller I'm using have backlash compensation.

The problem is that if the compensation is too low, it does nothing (due to the stiction) and if set high enough to overcome the stiction, DEC corrections overshoot.

I have tried canola oil, Kendall 90W anti-stiction gear oil, white lithium grease, and Superlube. I could have sworn it performed best with canola oil.

Don't know where to get Lubriplate 105 here. I really ought to try that!

The focal length was 560mm and the guide camera was a Meade DSI Pro. Pixel scale is about 3" / pixel. Payload was an AT90EDT plus various dongles (probably 20lb total).

So the RA RMS guiding is about 0.9" - RA is not bad. It's DEC that is the problem. For reference, with the same payload and guide scope, I generally get about 0.1-0.2 pixel RMS with the Mach1 (so that's 0.3" - 0.6" RMS). Not wildly better, but consistent.
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Isn't this declination stickiness the same problem that everyone has made such an issue of on the CGEM?

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The declination "sticktion" of the CGEM is often cogging from the dec motor. It is there even after a "hypertune" with a very smooth dec axis.
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How do you know that you have stiction? Every part of your symptom description sounds like backlash.

You might be able to make some progress by unbalancing the mount in declination to ensure that the gears are engaged and loaded. This would give you some resilience to wind, and Jose's suggestions should prevent direction changes in the guider.


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