#21  
Old 07-04-2011, 11:52 AM
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Exfso (Peter)
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I am using the Tak Clamshell, this is built like a tank. The guidescope is an ED 80 and it has fixed rings bolted to a Losmandy dovetail plate, which is in turn fixed to a Losmandy side by side saddle. (also built like a tank)
I was using the Tak guiding mount , which is not up for sale, for the ED80, but it is built to withstand a Hiroshima type attack I reckon and was just too bulky for my system. Eliminating this has not eliminated the problem. Obviously something in the imaging train is moving slightly, and it is just a process of elimination to find out what it is.
Definitely a frustrating process. What I have found out is that the stars are elongated in and E-W direction, so that may give me something to work on.
Thanks for your ideas, I will keep plodding along with this.
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  #22  
Old 07-04-2011, 11:53 AM
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Exfso (Peter)
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Robin, the F/L of the Tak is F7.7 and the ED80 is about F7.5 from memory
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  #23  
Old 13-04-2011, 04:53 PM
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A23649 (Nathan)
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is the focuser all tight and good? ie no slop?
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  #24  
Old 13-04-2011, 09:10 PM
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Exfso (Peter)
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Nathan, no hassles with the focuser, it is a Takahashi, they are built like tanks.
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  #25  
Old 13-04-2011, 10:40 PM
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Tandum (Robin)
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Have you tried swapping the cameras between scopes or can you borrow an oag?
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  #26  
Old 13-04-2011, 11:43 PM
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Eggy stars - oh yes the bane of the astrophotographer.

Firstly, 15 minutes is probably too long for a guidescope setup like that. try 10 minutes and you'll see an improvement. 15 minutes is big ask for a guidescope setup with no self guiding or no offaxis guide scope. Flexure shows more in the longer exposures.

Secondly, depending on the centre of gravity of your setup sometimes you may be nicely in balance with the scope horizontal but nastily out of balance when at say 40 degrees. This is caused by a top heavy centre of gravity that is too far away from the axis of the mount. The heavier the gear for the mount the more precise your balance needs to be.

Check for that.

Cable drag. I have had subs ruined by cables dragging on one side of the meridian and not the other. Or a cable suddenly lets go and drops a bit and pulls on the imager.

Flexure. Tighten everything. The guide camera must be firmly in the guide scope. The guide scope's focuser needs to be rigid. The main scope must be firm. Make sure your guide scope and main scope are parallel as best you can get. Try to get them both seeing the same object in an image. You don't want your guide scope not square with your imaging scope.
Use shorter subexposure times.

Polar alignment. In your case if you are getting good round stars sometimes then it suggests polar alignment is good. But 90% of guiding woes are not good enough polar alignment and poor balance.

Periodic Error. I had it once where a bit of rubbish got in the gear of my Tak NJP which for some reason has an opening where you can see one of the gears. It made the autoguider hop when it hit that spot. I simply cleaned it off.

If you have grease on your clutch could it be your clutch is slipping when under load in some angles?
Make sure clutches are tight.

Pier/tripod levelling and stability. Are you imaging with a permanent pier or a tripod? If a tripod, then how stable is the ground? Could one leg be slowly sinking under the weight? Your setup should be level.

Squareness of the system is something to strive for.

Autoguider settings are not ideal. With CCDsoft when you cross the meridian you need to check the "reverse X" box as corrections now need to be reversed. If you don't your X errors will just grow and grow.

Callibrating the autoguider. It sometimes pays to callibrate the autoguider on the other side of the meridian or if the autoguider is moved in any way.

Setting max/min move on the autoguider settings.

I set the max move so that errors larger than usual are ignored so the mount doesn't correct for seeing or a bit PE or a bit of wind. I also set min so if its too low its best to not do a correction at all.
Backlash settings. Maxim DL manual has a good writeup of how to set the backlash settings.

Aggressiveness. This can vary but I usually use 10 or if the setup is giving really low errors and the seeing is good then 5.

Autoguiding exposure times.

This is trial and error for your mount. My NJP invariably gives the best tracking with 1 second or less exposures. If the seeing is bad 2 seconds.
Anything longer and the errors are worse. My Paramount though is best at 3 seconds or 4 second exposure times.

Guide star selection:

You want a bright enough guide star so if a thin cloud comes over the guider still sees through it. But you don't want an overly bright bloated star and you don't want a double star. Nor do you want a guide star which has another star close by that is about the same brightness. This will confuse the software about which one is the guide star.

The first thing I do if I see larger than normal errors in guiding is I try another guide star. Its amazing how much the errors change by selecting a tighter, rounder star. The errors can be 1/3rd with the right star.

For guiding shorter focal lengths like your TOA130, a short focal length guide scope will work fine. Too long a focal length and you'll have trouble with finding guide stars. Are you using a finder scope for guiding?
Finder scopes are usually low quality and the eyepiece holder plastic or something weak. How strong is it?

Your problem is almost certainly flexure so look for where that is coming from. Your guide scope focuser and guide camera tightness in the guidescope would be the first check. Then the mounting of the guide scope. If you had any other of the above wrong then you wouldn't get good results sometimes and bad others. It would always be bad.

If you are using a mirrored scope for the guide scope then it could be mirror flop.

If you can't find it then you may need to use an off axis guider which is far superior to a guide scope anyway so its worthwhile. Unless your clutches are slipping.

Greg.
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  #27  
Old 14-04-2011, 02:50 AM
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Exfso (Peter)
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Thanks Greg, that list is rather comprehensive, should keep me busy. Only thing I really have to double check is that all the fixings are as tight as they should be. As I said before, I re positioned all the cables so no cable drag, cleaned the clutch pads, always have clutch tight enough so that there is not slippage. Checked guidescope focuser is tight too. As far as long subs go, when the stars go egg shaped can happen on shorter exposures, less than 5 mins as well as longer.
I am on a permanent pier setup. My finderscope is an ED80 and is clamped into position via fixed rings. I was using a Tak Guiding mount for adjustment, but this weighs around 6lbs on its own, and I thought this may have been partly to blame, but with it removed the problem still exists.
Seeing the moon is approaching full and the weather is crud, I may pull all the scopes off and check all fixings are tight etc. I doubt there would be any flexure in the Losmandy SBS saddle plate. I have always used this and it is only recently the problem has arisen.
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  #28  
Old 14-04-2011, 04:07 AM
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Tandum (Robin)
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And double check polar alignment. I pulled the sbs bar off mine tonight and went back to stacked scopes and thereafter all subs where eggy. I'd buggered up the alignment in the process.
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  #29  
Old 14-04-2011, 01:21 PM
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Exfso (Peter)
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Robin, my polar alignment is very good, I set it up with startarg, a great program.
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