perennial declination stiction
Posted 11 February 2013 - 10:02 AM
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?
Posted 11 February 2013 - 10:08 AM
-- 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
Posted 11 February 2013 - 10:15 AM
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.
Posted 11 February 2013 - 10:48 AM
Putting aside the wind do you know why your mount moves randomly in DEC?
Posted 11 February 2013 - 12:44 PM
OTOH the Mach1 is also heavily damped on both axes. But it is much less sensitive to balance.
Posted 11 February 2013 - 08:17 PM
Posted 11 February 2013 - 09:03 PM
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?
Posted 12 February 2013 - 01:44 AM
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.
Posted 12 February 2013 - 04:46 PM
Posted 12 February 2013 - 05:34 PM
Posted 12 February 2013 - 06:45 PM
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.