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Old 07-10-2014, 07:59 PM
Tony_ (Tony)
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Double stars during autoguiding

I've had a few problems lately with guiding, this is the current one that I want to resolve.
CGEM mount, SCT9.25", ST80 guidescope + celestron nexguider.

I keep getting regular movement creating double stars. The movement is is the declination direction (elongation in the RA direction is my next issue). The movement is fairly cyclical - average every 9th image (30 second exposures). Also the 2 images are quite discrete - so the movement from one point to the other is quite fast. Maybe some sort of backlash issue - but that doesn't explain the cyclical nature? Overall movement in RA is cyclical but the dec should be more or less random?

Note: I think polar alignment was very good - there was virtually 0 drift over 4 hours (GOTOs still centered). Also I didn't seem to have any flexure issues.

I am thinking the solution may be to not have the balance too good in the declination? I usually have it balanced as well as possible - but now I think maybe I should have some excess weight at one end - what do you think? I've read that it's best not to have the balance too good in RA either - how much out of balance should it be?

Any other ideas?

Regards,
Tony.
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  #2  
Old 07-10-2014, 09:57 PM
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Mirror flop?

If you're PA is good the mount shouldn't be moving in Dec.

Cheers
Stuart
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Old 08-10-2014, 05:48 AM
Tony_ (Tony)
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Probably not mirror flop - that would usually only happen once, particularly when crossing the meridian. This is happening every 9 or so subs. I have only ever seen real mirror flop once and the movement was a lot more than this - it seems to be controlled by using Celeston's suggestion of focusing upwards.
Maybe I need to re-grease my worm drives? I was surprised when I found that the movement was in the dec direction - particularly so regularly. If it is cyclical that seems to indicate an issue with the RA worm drive.
Next time I will watch the autoguider to see how much adjustment it is making in both axes. I've had the mount for 2 1/2 years - this problem only started a few weeks ago.
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Old 08-10-2014, 06:33 AM
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Merlin66 (Ken)
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Hmmm
Try a guide session with no Dec corrections.
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Old 08-10-2014, 05:04 PM
Tony_ (Tony)
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Thanks Ken,

That sounds like a great idea - at least I can confirm (or eliminate) the DEC axis. As long as I have good PA the guiding in the RA should compensate for PE. Now - just to wait (and wait) for a clear night.

Regards,
Tony.
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Old 09-10-2014, 08:11 PM
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How long is the worm cycle? It may be 5 or so minutes showing up on the 9th image. It may be a spec of dirt on your worm or a rough spot on it.

Greg.
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Old 09-10-2014, 11:12 PM
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Merlin66 (Ken)
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Greg,
The OP says the jump is in Dec....that's what threw me.
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Old 10-10-2014, 04:50 PM
Tony_ (Tony)
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Thanks for your input Greg and Ken,

The RA worm cycle is about 8 minutes. My exposures were 30 secs with a 4 sec delay. The jump occurred about every 9 images = 5.1 minutes so it doesn't quite relate to a factor of the worm cycle.
It might be clear tonight or tomorrow night and the moon is still bright so it's a good time to experiment with different settings to try to solve this issue and maybe try different RA settings to improve my guiding.

If I could get 2-3 minute subs with round stars I'd be happy - lately I can barely get 30 sec subs with round stars even with guiding.

Tony.
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Old 10-10-2014, 04:56 PM
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Tony,
I use a C9.25 and a C11 on the NEQ6pro mount.
Sure, a LITTLE out of balance to the east on RA helps steady things, but I haven't seen any "bounce" in Dec.
Edit: You don't say which guider program you use? I have success with AstroArtV5 and PHD2.
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Old 10-10-2014, 05:15 PM
Tony_ (Tony)
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The nexguider is stand alone - built in software. Also it can't be used with any other software. It isn't as accurate (eg. at a sub pixel level) as other guiders. It is capable of acceptable guiding but I have to solve these issues.

Tony.
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Old 11-10-2014, 10:39 PM
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Possible that your guide star has another star very very close to it that you may not be able to see with your eyes.
I had this exact issue when I was using Alnitak to guide with when imaging the Horsehead Nebula. There is another very small star right next to it. Chose a different guide star and problem was gone...
However that does not explain your cyclical issue... perhaps atmospheric disturbance . Don't know...
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Old 12-10-2014, 10:27 AM
Tony_ (Tony)
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Thanks Scott,

I thought of that as a possibility too.
I took 2 images last night with 45 sec exposures. I haven't processed them yet - but they look okay. I increased the noise level on my autoguider to reduce the possibility of fainter stars being picked up - it also made the guide star smaller.
For about 30 minutes I had the dec guiding off - no double stars and almost 0 drift. I then turned it on - I got one image with double stars - probably the first one when the dec started to correct.
The next images (subs) I had the dec guiding on - there were no double stars. I will keep monitoring it.
One good thing - apart from the guider not fully compensating for PE, I had virtually 0 drift on both images. I usually have issues with flexure - I tighten everything a bit more now.

Tony.
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Old 16-10-2014, 09:31 PM
Tony_ (Tony)
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My next set of images Tuesday night also had no double images in Dec - so maybe the problem is solved. I guess I'll know after a few more nights.
The image had a little elongation in the RA direction and there was about 3' drift over 1.5 hours - probably from flexure. The image is noisy and I am trying to get out of the habit of darkening the sky to much and clipping the black.
This is about 70 x 60 sec subs, c9.25" F/10 centralds canon 60d @ -16C iso1600.

Things I have done to try to improve the problem:
- increase noise on autoguider to reduce the possibility of a fainter star being picked up.
- offset the RA balance slightly to the East.
- offset the Dec balance to the mirror end.
I need reduce the RA elongation - I suspect it is caused by the guider not properly compensation for PE? I can either increase/decrease aggression or guiding speed - any suggestions (apart from trial and error).

Tony.
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Old 17-10-2014, 11:18 AM
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Hey Tony, can you post a picture of your complete imaging rig, especially the guiding setup. I would like to see how your guide scope is aligned and attached with the main imaging scope.
I have some thoughts about what is going on, but want to see what your rig actually looks like.

Cheers, DJ Scotty
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