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?
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.
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.
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,
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.
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.
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...
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.
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).
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.