Autoguiding aggrivation

[robulaic]

Hi everyone

I'm hoping smarter minds than my own will provide a solution I'm missing

I'm having issues guiding in dec on my CGEM DX. I've noticed this on my c9.25 - the camera is a QHY5-LII and the guide scope is a Stellarvue 60mm finderscope.

You will see on the attached graphs that it guides in dec, then moves a huge amount then suddenly pulls back.

My polar alignment is pretty close, I've done the polar alignment routine, made sure the mount is level (it's on a pier), facing true south, and set the angle to match my lattitude. So although my polar alignment probably isn't absolutely perfect it should be fairly close.

The things I've tried with this scope - moving the scope north before PHD calibration, changed PHD settings, kept the scope unbalanced north or south all to no effect.

The thing that has frustrated me most, is I've used a WO Megrez 72 on this mount and I know it was unbalanced in RA and Dec but my guide lines were very good comparatively.

I've also noticed no change between the ASCOM driver and native PHD support


So if anyone could make suggestions I would be quite happy to take them on board

[Rod771]

Hi Robert

Some CGEM's suffer from drive cogging in the servo motor. It may be what's causing your spikes. However if the mount is ok with the other scope then your probably ok in that respect.

This link show's a great way to check if your dec motor is affected

http://www.youtube.com/watch?v=bVqW8ZGi7_4

The issue has also been discussed heavily in this thread.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5537628/page/3/view/collapsed/sb/5/o/all/fpart/1

Celestron are close the releasing a new MC firmware version which should fix the problem.

You could also try higher anti backlash settings in dec.

I'm very interested that your polar alignment procedure worked. Do you have the Nexstar + Hand control? The polar align feature hasn't worked for me from day one. Derik form celestron has recently confirmed that there is a bug in the Nexstar + HC when performing the ASPA in the southern hemisphere.

It hopefully be fixed when the dec issue is completed.

Hope this helps as I wouldn't consider myself a smarter mind in any case :P

[toc]

Its probably worth your while using the Beta MC. See the message from
svtdoug in this thread on cloudynights:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5537628/page/3/view/collapsed/sb/5/o/all/fpart/2/vc/1

Im using it on my CGEM, and havent noticed anything bad so far :) (although I havent had much of a chance to actually test if it AG any better)

[toc]

ITs always worked a treat for me. What was happening when you tried it?

[multiweb]

There are two issues there. The first being your polar alignment. Try to refine it to avoid DEC drifting. The other is that you have PHD set to auto and probably some backlash. That pattern is typical of a guiding direction reversal in DEC. Going down are pulses being sent (up) to the mount while still in the backlash, then the spike up when it's caught up, then reverse direction at the top and the same looping sequence in the backlash again. So guide only again the drift in DEC and you'll be alright or slightly unbalance your scope in DEC as well. Another thing is your maximum duration in RA and DEC. I'd get them down to 500ms and ramp up the agressiveness to 80. If you spend 1s sending pulse in RA the DEC won't guide for that same period and vice versa.

[g__day]

I think Marc hit the nail on the head for you, be interested in knowing from that graph what is your imaging scale (how many arc seconds does each pixel / vertical bar represent), what did you set as minimum movement before a guide pulse is issued (pixels or arc seconds) and what you set your hysteresis value as (I generally go above 30% to try and cancel out seeing fluctuations).

Your RA looks reasonably good - I'd guess that is indicative of smooth gears and reasonable seeing, so yes DEC looks like the interplay of polar mis-alignment and backlash with the lag in PHD and your control system trying to overcome the drift.

It would be interesting if you switched the DEC routine from auto to North or South only - in one direction the mount should just drift off, in the other I would expect to see a similar but improved version of what you have posted above!

Matt

[Rod771]

When syncing on a star (east) to adjust ALT and AZ, the mount should slew a little away in order for me to adjust the star manually back to center. Instead of doing this the mount slews/flips all the way around to the west.

Detailed here http://www.iceinspace.com.au/forum/showthread.php?t=101528

Derik has advised for some reason the bug only effects the Nexstar + HC .
The original Nexstar HC is ok.