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Equipment Discussions >> Mounts

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APshooter
member


Reged: 10/14/13

Loc: Camby, In.
G-11 tracking issues
      #6137555 - 10/14/13 05:48 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.astronomycameras.com/blog/archive/20120612/venus-black-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?


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APshooter
member


Reged: 10/14/13

Loc: Camby, In.
Re: G-11 tracking issues new [Re: APshooter]
      #6137570 - 10/14/13 05:53 PM Attachment (41 downloads)

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.

Edited by APshooter (10/14/13 05:55 PM)


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Joe Bergeron
Vendor - Space Art


Reged: 11/10/03

Loc: Upstate NY
Re: G-11 tracking issues new [Re: APshooter]
      #6137737 - 10/14/13 07:52 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.

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APshooter
member


Reged: 10/14/13

Loc: Camby, In.
Re: G-11 tracking issues new [Re: Joe Bergeron]
      #6138068 - 10/14/13 11:15 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.

Edited by APshooter (10/14/13 11:16 PM)


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blueman
Photon Catcher
*****

Reged: 07/20/07

Loc: California
Re: G-11 tracking issues new [Re: APshooter]
      #6138157 - 10/15/13 12:12 AM

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


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APshooter
member


Reged: 10/14/13

Loc: Camby, In.
Re: G-11 tracking issues new [Re: blueman]
      #6138265 - 10/15/13 02:18 AM Attachment (36 downloads)

Here are two typical shots from last night.

Edited by APshooter (10/15/13 02:20 AM)


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APshooter
member


Reged: 10/14/13

Loc: Camby, In.
Re: G-11 tracking issues new [Re: APshooter]
      #6138268 - 10/15/13 02:21 AM Attachment (24 downloads)

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.

Edited by APshooter (10/15/13 02:24 AM)


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blueman
Photon Catcher
*****

Reged: 07/20/07

Loc: California
Re: G-11 tracking issues new [Re: APshooter]
      #6138273 - 10/15/13 02:38 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


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D_talley
Carpal Tunnel
*****

Reged: 07/07/05

Loc: Richmond VA
Re: G-11 tracking issues new [Re: blueman]
      #6138464 - 10/15/13 07:51 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.

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APshooter
member


Reged: 10/14/13

Loc: Camby, In.
Re: G-11 tracking issues new [Re: D_talley]
      #6138969 - 10/15/13 12:43 PM Attachment (19 downloads)

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).

Edited by APshooter (10/15/13 12:46 PM)


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tomcody
Pooh-Bah
*****

Reged: 07/06/08

Loc: Titusville, Florida
Re: G-11 tracking issues new [Re: APshooter]
      #6139121 - 10/15/13 01:53 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

Edited by tomcody (10/15/13 01:58 PM)


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APshooter
member


Reged: 10/14/13

Loc: Camby, In.
Re: G-11 tracking issues new [Re: tomcody]
      #6140067 - 10/15/13 11:41 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.

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blueman
Photon Catcher
*****

Reged: 07/20/07

Loc: California
Re: G-11 tracking issues new [Re: APshooter]
      #6140085 - 10/15/13 11:54 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
Quote:

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.




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APshooter
member


Reged: 10/14/13

Loc: Camby, In.
Re: G-11 tracking issues new [Re: blueman]
      #6142100 - 10/17/13 12:05 AM

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.

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dragonslayer1
scholastic sledgehammer


Reged: 02/25/12

Loc: SLC, UT
Re: G-11 tracking issues new [Re: APshooter]
      #6142881 - 10/17/13 12:43 PM

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

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APshooter
member


Reged: 10/14/13

Loc: Camby, In.
Re: G-11 tracking issues new [Re: dragonslayer1]
      #6142971 - 10/17/13 01:36 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...

Edited by APshooter (10/17/13 01:54 PM)


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dragonslayer1
scholastic sledgehammer


Reged: 02/25/12

Loc: SLC, UT
Re: G-11 tracking issues new [Re: APshooter]
      #6143013 - 10/17/13 01:54 PM

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


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blueman
Photon Catcher
*****

Reged: 07/20/07

Loc: California
Re: G-11 tracking issues new [Re: dragonslayer1]
      #6143247 - 10/17/13 04:07 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


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APshooter
member


Reged: 10/14/13

Loc: Camby, In.
Re: G-11 tracking issues new [Re: blueman]
      #6144004 - 10/17/13 11:51 PM

My G-11 is from 1997, so I have no goto capability. Here's the link to the NGC max:
http://www.jmitelescopes.com/buy_ngc.htm#NGC-MAX%20Computer%20Only

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

Edited by APshooter (10/17/13 11:54 PM)


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blueman
Photon Catcher
*****

Reged: 07/20/07

Loc: California
Re: G-11 tracking issues new [Re: APshooter]
      #6145019 - 10/18/13 02:40 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


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