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Equipment Discussions >> Mounts

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dawziecat
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
G11 Star Elongation: ONLY NEAR THE MERIDIAN
      #5432986 - 09/21/12 03:39 PM

I get good results with exposures up to 30 minutes with my G11 as long as I don't image within 15 degrees or so of the meridian.
Then things go to Hades and I get E-W elongation.

I have done my darndest to eliminate any possibility of flex and AM USING VERY SHORT FLs (caps intended). I get this elongation even at 180mm FL. Balance does NOT seem to be the problem. I've tried both equi-balance and east-heavy to VERY east heavy with no apparent difference. Imaging load is light . . . less than 15 pounds with a 180mm camera lens.

I guide with the STi guiding kit and PHD.

I've tried all I can think of to fix this frustrating problem.

The crux of the matter is (again, caps intended), IT ONLY HAPPENS WHEN IMAGING NEAR THE MERIDIAN!

Thirty minute subs look great well off the meridian. Within 15 degrees I have to cut back to 3 minutes or so.


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Mantis707
member


Reged: 08/14/12

Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5433066 - 09/21/12 04:21 PM

Has it been like this since you got the mount or is this something that just started happening....(Changes in equipment payload, service done to the mount etc?)

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Stew57
Carpal Tunnel
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Reged: 05/03/09

Loc: Silsbee Texas
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Mantis707]
      #5433102 - 09/21/12 04:46 PM

If your balanced east heavy the amount will change depending on the angle of the ra axis.

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David Ault
sage


Reged: 09/25/10

Loc: Georgetown, TX
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Stew57]
      #5433238 - 09/21/12 06:26 PM

Terry,

Have you noticed any cable bunching, anything restricting the movement of the RA axis? What does your guiding graph look like when approaching the meridian? Is it sending really large pulses E/W to try to correct, or is it behaving normally (which indicates some form of flexure)?

What kind of power supply are you using? If your powering off a battery or 12V supply, when the load shifts and it tries to draw more current the voltage may sag causing erratic behavior. I run mine with a custom 20A 15V power supply when I've got A/C power. Otherwise, I run a car battery through a power inverter and then to the power supply to guarantee proper voltage delivery (I know this is inefficient, but I was lazy and didn't want to build my own DC/DC step up power converter for running off the battery).

In my case when running the G11 off a 12V power supply or battery I had all sorts of speed control issues and strange random slews, etc. When I moved to the 15V 20A supply all those disappeared.

Regards,
David


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JoseBorrero
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Reged: 09/04/09

Loc: Puerto Rico Island
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: David Ault]
      #5433461 - 09/21/12 09:07 PM

That problem happen to me when I tried autoguide with my AT6RC and imaging with ED80. My solution was using a side by side with Orion st 80 as a guide scope to my ED80. It seem flexure was my problem.

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dawziecat
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: JoseBorrero]
      #5433938 - 09/22/12 07:23 AM

Thanks for the replies so far but nothing falls into place.

1/ I am unsure if this was always a problem or a recent occurrence. I always had some poor subs but only recently sleuthed it out to being only when very near the meridian.

2/ Power? Not likely. Using AC.

3/ Symptoms point to flexure but I can't see how. I am away from the gear now and can't post pics. I've tried several configurations. Latest is a Casady plate bolted directly to the G11. Camera (ST8300M) is bolted directly to a Losmandy D style dovetail through the 1/4”X20 tripod mounting screw. Lens is mounted to camera and is firmly supported under its objective end. STi guide scope mounted securely “Joey” style to dovetail. Everything is TIGHT!!!! I've used other configs too that were perhaps more prone to flex. My problem led me to this which I just can't see flexing unless the whole D plate is drooping or twisting. Seems inconceivable.

Flexure only at the merdidan and with such short Fls seems highly unlikely with this configuration. Weight is absolutely trivial too considering what this mount should be able to carry with ease.

4/Guiding seems normal. Nothing I can see unusual going on in the magnitude or size of the corrections . . . again pointing to flex I suppose. But only very near to the meridian??? And at very short FLs???


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mclewis1
Thread Killer
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Reged: 02/25/06

Loc: New Brunswick, Canada
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5434121 - 09/22/12 10:02 AM

Terry,

One thing that does change as you approach and drop away from the meridian is the amount of torque required by your motors ... the amount depends on how much out of balance your setup is.

I'd vary your balance a bit and see if you can either make the situation worse of better.

Have you ever manually checked the worm/ring gear spacing or tightness around the whole circumference of the ring gear? You're looking for any tightness or binding areas as you slowly drive the worm (without any load on the ring ... and then with your normal load).


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David Ault
sage


Reged: 09/25/10

Loc: Georgetown, TX
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: mclewis1]
      #5434231 - 09/22/12 11:14 AM

Terry,

If you aren't seeing any changes in guiding, it does seem less likely to be power, gear binding or a balance shift. Power could still be an issue though if your AC/DC converter is a 12V lower amperage model. The only reason I push this point is because your issues seem eerily similar to mine when I was running with a 12V power supply. It could supply 30A, so power delivery was not an issue. This told me the G11 really needs 15V+ (and probably over 10A, at least when slewing) to run correctly.

Does the imaging lens have moving elements? I've never really though about this before, but it does seem like most camera lenses are designed to be held horizontally, not vertically, so I just wonder if something is shifting around in there.

Do you know anyone with an off-axis guider setup? It might be difficult to get the off-axis guider between your camera and lens, but you could stick their whole scope/guider/camera setup on your mount and at least rule that out.

Regards,
David


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Hilmi
Post Laureate
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Reged: 03/07/10

Loc: Muscat, Sultanate of Oman
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: David Ault]
      #5434343 - 09/22/12 12:18 PM

I can think of two possibilities.

1) Lens barrel creep. One of my lenses has this nasty habit.
2) Your mount needs to be tightened up. I had all kinds of imaging nightmares until one day I said I have had enough and I just pulled the entire mount apart and rebuilt it. After tightening everything up, my problems went away.


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dawziecat
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Hilmi]
      #5434862 - 09/22/12 06:21 PM

Again thanks for the replies.

1/ I am on the road and sort of unprepared to answer some specific questions . . . like what PS I use. It will be a week or so before I get home and have the gear available.

I have only camera lenses. I can't rule out movement of some internal elements but focus seems to hold true. Also, if this were so, a sub or two would be ruined and then the offending element should settle in and subs would be good again. Doesn't seem the case. All subs very near the meridian are poor. They become poor and they stay poor until I shift away from the meridian.

2/ Unfortunately, off axis guiding seems just not possible with EOS lenses period.

3/ I am reluctant to pull the mount entirely apart as Hilmi suggests. I have inspected the worm-gear spacing on the RA
axis and all seems well there . . . but my examination was very inexpert.

4/ I have nobody with whom I could swap gear to rule out the mount. So far I have ascertained the problem is definitely occurring with both the EF180mm f/3.5L and EF135mm f/2.0L EOS lenses. I have used other EOS lenses but have not specifically looked for this problem at the meridian yet. I will do so after the current imaging project with the EF135mm f2.0L is complete. They are of longer FL and the problem, if it occurs with them too, will be proportionately worse.

The problem I am encountering seems similar to what Steve Cannistra reported on this web page, i.e., inconsistent tracking near the meridian with his G11. His "fix" was a Temma 2.

I'm hoping for a . . . er . . . less drastic solution.


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Peter in Reno
Postmaster
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Reged: 07/15/08

Loc: Reno, NV
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5434901 - 09/22/12 06:45 PM

Every mount will always need maintenance. This including gear meshing. I don't know how difficult to adjust the mesh for G11. It seems like your mount may be most sensitive near the Meridian. The worm could be floating above the gear and if the mesh is out of adjustment or got a little loose due to wear and tear since you bought the mount, then the gear mesh may need to be tweaked.

I've owned my A-P Mach1GTO for about a year and it is just about to need gear mesh adjustment because I can feel a very slight free play in RA axis. When I first purchased it brand new, the RA axis free play was tight.

Maybe your G11 just completed a break in period and needs RA mesh adjustment. From then on, it may not need adjustment for a while.

Does it matter which part of the sky when your mount is near the Meridian?

Peter


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Peter in Reno]
      #5434943 - 09/22/12 07:05 PM

I'd rule out gear adjustment problem because it always happen near the meridian... well unless you always park your mount to CWD using HC or software in which case you are always working on the same teeth of the gear. So if you are always parking back to CWD everyday: try turning the mount on during the day for few hours just to use another section of the gear next time.
Worm adjustment rules itself out (because of the 4mn period).


I've seen balancing having some minor effect on star elongation. But if I understand correctly you can guide perfectly with 30 mn exposure until you are too close to meridian at which point 3mn are the max you can do. That is not a minor effect to me... It has to be something else. I mean, if I can 30mn perfect exposures it means that my balance really is good enough.

I always do a little bit of east heavy balance and more often that not I would image 10 degrees past the meridian without noticing any image degration in 10mn exposures. So I don't think it is a balance issue.


I am really thinking flexure also. That's pretty much the only thing left. Have you noticed if DEC was affecting the angle at which it starts messing up? I am not sure how your lenses are attached to your mount but I can imagine problems happening when the lenses are pointing straight up if they're only held by 1 screw.


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Peter in Reno
Postmaster
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Reged: 07/15/08

Loc: Reno, NV
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: zytrahus]
      #5434967 - 09/22/12 07:25 PM

Can you post a picture of your setup including the short 180mm focal length scope mounted on G11. Make sure you show everything including counterweight, guide scope, camera, etc. We will have a better understanding since a picture contains a thousand words.

Thanks,
Peter


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dawziecat
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Peter in Reno]
      #5434998 - 09/22/12 07:48 PM

I will post pics of the setup when I get back home. On the road right now. It will be a week to 10 days.

In the interim, thanks all for the replies.


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Peter in Reno
Postmaster
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Reged: 07/15/08

Loc: Reno, NV
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Peter in Reno]
      #5435000 - 09/22/12 07:49 PM

Also, can you show pictures of elongated stars as well? Maybe just a single sub is enough.

Thanks,
Peter


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Hilmi
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Reged: 03/07/10

Loc: Muscat, Sultanate of Oman
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Peter in Reno]
      #5435353 - 09/23/12 01:06 AM

Just take a set of LN keys and tighten down everything. And by the way, the G11 is so easy to strip to little bits that you would be amazed at the simplicity of it all. Only annoying bit is getting the worm block back on. I always found that part hard.

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Nebhunter
Carpal Tunnel


Reged: 10/04/03

Loc: Frostbite Falls
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5438271 - 09/24/12 03:11 PM

For what it's worth - have you drift aligned the mount and for how long?

The meridian presents a different set of variables and this became evident with my older ST-4 guider. It has a calibration routine - and at the meridian being east heavy or otherwise has very little effect due to the position of everything. The counter weights are almost hanging straight down so how do you effect east heavy?

I found that loading the Dec axis east heavy did the trick. I'm a side by side set up and hung a large C clamp on the outside edge of the Dec plate. This caused the RA to become east heavy - improved tracking, but also loaded the Dec axis to one side. Otherwise with a neutral balance you are subject to the play in the worm/pinion. The guider will command a movement but the slack in the worm prevents any movement. Next cycle the guider says - wow way off - and does another bigger move. Finally the slack catches up - and it's way overshoot. Next cycle - guider tries to bring it back, but the slack in Dec won't take up.

I hope this is not confusing. I also solved the problem by making the worm mesh in Dec very snug to where the hand controller would message Dec lag off and on. The tighter mesh would then respond quickly to guider commands. But you need to constantly adjust mesh - and release it before each slew. So the C clamp became the answer for me. It's a 4" clamp. Can't hurt by trying it. It's offsetting the weight away from the counter weight shaft.


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dawziecat
Carpal Tunnel
*****

Reged: 10/20/10

Loc: Rural Nova Scotia
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Nebhunter]
      #5475283 - 10/17/12 03:12 PM

Hi to All:

I said I would post images of the gear when I got back so here they are.

The problem persists. I have taken the G11 apart pretty much. Adjusted the RA backlash until it is so small I can no longer measure it by any deflection of the CW shaft. I have not touched dec backlash but it seems pretty minimal. Both axes rotate smoothly with no binding in evidence. Balance seems fine but I've played with it too, to no avail.

I don't see how alignment can be an issue. Twenty minute subs were fine off the meridian last night, just as they always are. As I entered within 10 degrees of the meridian, they fell off a cliff, just as they always do, and I had to cut back to 5 minutes.

Despite all my Herculean efforts to eliminate flex and my adjusting the RA backlash, quite successfully I think too, nothing has made any difference.

Guiding seems fine with nothing untoward showing up on the PHD graphs.
Len retains sharp focus which, in my mind, rules out any "barrel creep" in the lens. It is a pretty light lens in any event. It happens with other lenses too. Likely all of them but I have exhaustively determined it to be so with at least two.

I just can not image near the meridian with this gear and remain perplexed at which component is causing the problem.


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5475303 - 10/17/12 03:22 PM

There is at least one thing left... Try OAG.

Knez


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korborh
professor emeritus
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Reged: 01/29/11

Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: orion69]
      #5475439 - 10/17/12 04:45 PM

Terry, is the issue in tracking or guiding?
With only tracking, the elongation is in RA only or is it RA/DEC? Since in tracking only RA axis is moving, if elongation is in RA or other direction, then that would be good data point to investigate further.


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