G-11 tracking issues
Posted 14 October 2013 - 04:48 PM
The issue I'm having is the stars on long exposure ccd images the stars are in groups of two (doubled). Without going into too much history I'll outline the fixes I've tried so far:
1. Had the RA drive stepper replaced about a year ago.
2. Bought the Ovision worm upgrade for the RA.
3. Got a new polar reticule for the polar scope, cleaned and reinstalled.
4. New cables for the Celestron drive RA and Dec
5. Cleaned out the mount bearings, relubed both axis.
6. Installed new clutch pads both axis.
Last night I spent several hours trying different balance points. First I tried fully balanced, then East heavy, then West heavy. I gave the Ovision worm a little more play, thinking it might be binding. (I had set it up according to the instructions for about 1mm play over a 30" swing). I'm still getting oblong stars at every turn, not sure what to try next. I know some folks have beaten the curse of the oblong stars, hoping for an inexpensive fix. Anyone have any ideas to try?
Posted 14 October 2013 - 04:53 PM
Posted 14 October 2013 - 06:52 PM
Posted 14 October 2013 - 10:15 PM
I have tried various guiding times with my ST-4, from 1 sec to 10 sec, to see if guiding was causing it. Doesn't seem to be.
Posted 14 October 2013 - 11:12 PM
If you have too much DEC play in the worm, it can rock back and forth if balance is very close. The DEC reverses direction where the RA does not, so it is prone to this problem.
The other possibility would be binding or grit in the grease of the worm.
If you were guiding then even a double star as a guide star can cause this.
So you see it is not that easy to give you an answer, especially without an image to see the problem.
Posted 15 October 2013 - 01:18 AM
Posted 15 October 2013 - 01:21 AM
Posted 15 October 2013 - 01:38 AM
So, this is the direction of the elongation and that would look just like flexure to me.
You have not stated what your imaging scope is, if it is an SCT or reflector, then it could be mirror shift.
If you are using a refractor, then differential flexure would be my best guess. This could be either the guide scope, guide scope mounting or the main scope focuser sagging.
Of course this is mostly a guess, but having dealt with flexure myself over the years, it sure looks like it.
Posted 15 October 2013 - 06:51 AM
Posted 15 October 2013 - 11:43 AM
Astrophysics 130mm F6 Starfire refractor in a double set of rings. Televue 90mm guidescope. St-4 autoguilder. G-11 mount. Imaging source camera (temp camera for now).
Posted 15 October 2013 - 12:53 PM
You could have a bad driver chip on the digital Drive board causing the same effect.
Why did you change out the RA motor?
Posted 15 October 2013 - 10:41 PM
Posted 15 October 2013 - 10:54 PM
I am talking about the reverse direction when pressing a button. It happened mostly if he did a quick movement.
The RA motor wasn't performing correctly...it was stalling and reversing directions when using the hand controller. So, hit left, and the motor would go right instead, or sometimes left...totally at random. I can swap out the motors easily enough. I'll also try and see what flexure issues I can eliminate.
Posted 16 October 2013 - 11:05 PM
Posted 17 October 2013 - 11:43 AM
Posted 17 October 2013 - 12:36 PM
On a side note, I have corrected and tweaked everything on the mount and am ready for a good night under the stars to test the tracking. Awaiting better weather...
Posted 17 October 2013 - 12:54 PM
Posted 17 October 2013 - 03:07 PM
Polar Align Assist
Polar Axis Correction
Check the manual for how to do it.
Posted 18 October 2013 - 01:40 PM
If you do not have Gemini, then you will pretty much have to drift align.
Posted 18 October 2013 - 03:37 PM
Posted 18 October 2013 - 04:19 PM
The servo motors are only for Gemini users, the digital drive uses stepper motors.
It looks like the G-2 is the latest and greatest.. Do even the owners w/o Gemini upgrade to the servo motors anyway? I also saw where it is suggested to get the worm block upgrade? Kasey
The one piece block is usually only needed for imagers (who need as low a PE [periodic error] as they can get) and only to make the worm adjustments easier to do and maintain.
Posted 18 October 2013 - 04:29 PM
""Servo motors run with a continuous smooth rotation, rather than discreet steps as with stepper motors.
They also have very high torque, even at high slewing speeds""
The one piece worm block would be for someone wanting what,, mabey 2-3 hour tracking? It seems that the mount can do 1-2 minute unguided pretty well? thank you,
Posted 18 October 2013 - 05:00 PM
Posted 18 October 2013 - 09:23 PM
The older G11's did not have as accurate worm gears as the newer ones and combine that with two piece worm blocks ( which have six axis of possible movement compared with two possible axis on the one piece blocks ) and you have more tinkering.
Stepper motors have a single torque out put and turn in discrete moves or steps, compared to servos which have a continous drive current and a variable current with relationship with position. This makes servos smoother for long fl imaging and able to handle positioning changes like a wind load on the ota ( when the wind moves the ota, the servo senses the position change and puts out more torque ( depending on the servo drive design) to move back to position.