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G-11 tracking issues

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#1 APshooter

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Posted 14 October 2013 - 04:48 PM

Hello all. I'm having a tracking problem with my Losmandy G-11 mount and I need your help. I bought the mount in 1997 and shot fairly successfully with film for several years. In 2000 I bought a Starlight express HX516 and managed to get one fairly decent shot with it. Lately I've tried some long exposure experiments with my Imaging Source DMK 41AU02.AS. Here's some shots I did with it:
http://www.astronomy...ck-drop-effect/

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?

#2 APshooter

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Posted 14 October 2013 - 04:53 PM

Here's a shot of M31 I made with the G-11 mount using film back around 2001. Three exposures, Hypered Pro 400 in a Pentax 6x7 camera.

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#3 Joe Bergeron

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Posted 14 October 2013 - 06:52 PM

I'm having similar problems with my 20-year-old G11. I frequently get double stars and elongated stars, even now that I'm finally autoguiding the thing. I haven't yet gone to the expense of an Ovision worm, and I'm disappointed that it hasn't fixed your problem. Makes me think I might be wasting my money to try that or a Losmandy one-piece worm block. I have tried some of the other things you've tried. At this point I'm at a loss. The PHD graphs I see, with an RMS value commonly around 0.50, do not inspire confidence.

#4 APshooter

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Posted 14 October 2013 - 10:15 PM

When I tested the Ovision mod the first night, it was with a canon EOS attached to the saddle with a ball mount. The stars looked very round, even after five minutes of unguided exposure. Now that there is a real load onboard, I'm still at a loss to explain the double stars. Forums talk suggests the worm is sticking to the drive gear, causing jumps that cause the errors. One thing I have not done is swap the drive gear from the Dec to the RA, so see if it makes any difference...
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.

#5 blueman

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Posted 14 October 2013 - 11:12 PM

Are the double stars oriented East-West or North-South?
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.
Blueman

#6 APshooter

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Posted 15 October 2013 - 01:18 AM

Here are two typical shots from last night.

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#7 APshooter

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Posted 15 October 2013 - 01:21 AM

And the second shot: It's hard to tell from these images, but when looking closely at the computer monitor, every star appear as double, with a fainter companion next to each bright star.

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#8 blueman

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Posted 15 October 2013 - 01:38 AM

I will guess from the photo that the elongation is East/West, based on the M-27 position.
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.
Blueman

#9 D_talley

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Posted 15 October 2013 - 06:51 AM

The problem I started having with my older G11 was the oldham coupler was loose, causing double stars. The set screws had backed out.

#10 APshooter

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Posted 15 October 2013 - 11:43 AM

Great tips guys, I will look into both of these possibilities! As for what I'm shooting with:
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).

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#11 tomcody

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Posted 15 October 2013 - 12:53 PM

Try switching the two stepper motors out Dec and Ra, the new RA motor may have a problem with one of its windings that is causing uneven stepping.
Or
You could have a bad driver chip on the digital Drive board causing the same effect.
Why did you change out the RA motor?
Rex

#12 APshooter

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Posted 15 October 2013 - 10:41 PM

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.

#13 blueman

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Posted 15 October 2013 - 10:54 PM

A friend was just having this problem and he changed out the hand controller cable and it stopped.

I am talking about the reverse direction when pressing a button. It happened mostly if he did a quick movement.
Blueman

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.



#14 APshooter

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Posted 16 October 2013 - 11:05 PM

The inadvertent direction flipping only occurred on the RA drive. The Dec always responded correctly. I'm currently on my 3rd hand controller :). Losmandy rebuilt the drive after I sent it in...seems to work fine. I will try all the checks and tips tonight to see what flexure I can eliminate.

#15 dragonslayer1

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Posted 17 October 2013 - 11:43 AM

can I just ask a question sense seem to have Losmandy brain trust here; If you cant access polaris does the Losmandy offer a similar option like all Star Polar Align? Am looking for a larger mount and weighing options, thank you, Kasey

#16 APshooter

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Posted 17 October 2013 - 12:36 PM

Not that I'm aware of. I use a JMI NGC Max to locate objects. It has a polar align feature that will help dial in Polaris accurately, although I have never used that function. I have found that the polar scope works quite well once it's set up. I recently put in the new reticle, good til 2030.

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...

#17 dragonslayer1

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Posted 17 October 2013 - 12:54 PM

Thank you AP. It looks like tha NGC is an Gemini/ or Gemini 2 GOTO for earlier models? thank you,
Kasey

#18 blueman

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Posted 17 October 2013 - 03:07 PM

IF you can get fairly aligned, then you can use one of two things in Gemini.
Polar Align Assist
Polar Axis Correction
Check the manual for how to do it.
Blueman

#19 APshooter

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Posted 17 October 2013 - 10:51 PM

My G-11 is from 1997, so I have no goto capability. Here's the link to the NGC max:
http://www.jmitelesc...X Computer Only

It is only a pointing aid and not associated with Losmandy.

#20 blueman

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Posted 18 October 2013 - 01:40 PM

IF you have a Gemini, then you should have Polar Axis Correction and Polar Alignment Assist, but only with Gemini.
If you do not have Gemini, then you will pretty much have to drift align.
Blueman

#21 dragonslayer1

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Posted 18 October 2013 - 03:37 PM

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

#22 tomcody

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Posted 18 October 2013 - 04:19 PM

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 servo motors are only for Gemini users, the digital drive uses stepper motors.
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.
Rex

#23 dragonslayer1

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Posted 18 October 2013 - 04:29 PM

Thank you Rex,, the Losmandy seems to be in a class all by itself. Is it just the older ones that require tinkering? I also copied a statement about the motor difference, from what I got its saying the servo is an improvement?
""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,
Kasey

#24 Stew57

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Posted 18 October 2013 - 05:00 PM

If you are pulse guiding or driving the servo via pulse ie. CGEM/DX than the statement of the difference between servo and stepper does not apply.

#25 tomcody

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Posted 18 October 2013 - 09:23 PM

Real quick:
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.
Rex






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