freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Good Guiding - What got you over the hump?
[Re: Tim C]
#2993220 - 03/19/09 07:36 PM
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Hi Tim-
The first thing I did was take all autoguiding advice with a grain of salt. It tends to be very anecdotal, non-scientific, and non-quantitative. I recommend listening to methods people use, but focus on results relevant to your equipment, and that show a clear benefit of good guiding - not just a pretty picture.
Many approaches in autoguding make perfect sense from a theoretical perspective - but the realities of making a crude mechanism track with the needed level of accuracy reveal surprising subtleties. So, be prepared to change your mind, as I have on things such as polar alignment.
Here are things that helped me:
1) Focus on fwhm in arc-seconds - in your results and others. Not just "small round stars" but - how small?
2) Advice that works for a high-end mount, such as "minimize the number of corrections," may not be best for a cheaper mount.
3) Many people, including me, abhor guiding an SCT or other mirror scope with a guidescope. It's not just guidescope flexure, but internal flexure in the sct. Switching to off-axis guiding greatly improved my results and their consistency with my C11.
4) I thought that polar alignment wasn't important for a mid-range mount (e.g. my cge) but I found that once my RA guiding was tuned (to the 2" fwhm level or so) I was getting oblong stars in dec. I tried several things in software and mechanically - but I found that my dec. guiding greatly benefited from good polar alignment.
5) PE is highly overrated since, for a mid-range mount, there are other noise sources that have to be chased at the 1-second level or so. I use video guiding and one second guide intervals with negligible latency. Nonetheless, I do use PEC to remove the fundamental and harmonics - because it does seem to reduce the work of the autoguider.
I first guided with guidedog and it was kinda fun, but I was curious just how well I could guide my cge with my own software. I was interested in Airy pattern imaging for collimation, and doing that made me realize that centroiding is full of subtleties that aren't appreciated. This led me to write MetaGuide and apply the same techniques to autoguiding.
So - here are my results, and some of my methods. Much of what I do is quite contrarian and goes against the herd - but I'm happy to show my results at the 0.5" pixel scale.
MetaGuide images with CGE, some showing raw fwhm's in the sub 2" from NE U.S.
Frank
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David Pavlich
Transmographied
Reged: 05/18/05
Loc: Mandeville, LA USA
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Re: Good Guiding - What got you over the hump?
[Re: freestar8n]
#2993283 - 03/19/09 08:15 PM
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I used the iterative method as Craig had mentioned. It works quite well and takes less time than a drift. I did 5 or 6 iterations before I got both stars centered, but it's a worthwhile endeavor.
David
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Tim C
scholastic sledgehammer
Reged: 11/11/07
Loc: Marietta, GA
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Re: Good Guiding - What got you over the hump?
[Re: freestar8n]
#2993295 - 03/19/09 08:20 PM
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thank all - lots of good advice in this thread and I really appreciate it. Frank - I like your comment about listening to different approaches and focusing on what works for you. That's exactly what I was trying to do here. As soon as I can get some hardware for my SV105 I'll give that a try and see if my results improve (they should, no mirror shift and shorter focal length). Thanks again and keep the advice coming if others want to chime in.
Tim
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JerryWise
Postmaster
Reged: 12/26/03
Loc: Lexington, SC
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Re: Good Guiding - What got you over the hump?
[Re: Craig]
#2993316 - 03/19/09 08:31 PM
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............
1) Software that didn't make me think to hard. That's why I wrote PHD in the first place.
............
Thanks Dude. I thought nothing would make me chunk my STV but the Orion package with your software is embarrassingly good. I think your work and Pavel's mask deserve the CN Nobel prize. 
I don't seem to have a problem using a little FS-60c. But the mount is pretty stable and may have something to do with it.
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Jared
Post Laureate
Reged: 10/11/05
Loc: Piedmont, California, U.S.
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Re: Good Guiding - What got you over the hump?
[Re: Tim C]
#2993368 - 03/19/09 09:03 PM
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What got me over the hump of consistently good guiding? I'll give you two answers--one long and one short.
First the short answer: Money. I bought a better mount and virtually all of my autoguiding problems disappeared.
Now the long answer: I struggled with a mount that was at the limits of its capacity for a little over a year. I had no problems with differential flexure or mirror flop since my camera is internally guided. Ultimately, what it came down to for me was that my mount, when loaded with 22 pounds of imaging gear, simply didn't track that well. There were shifts of 1-2 arc seconds in RA that occurred over time frames that were too fast for an autoguider to keep up with. I was able to make incremental improvements through adjusting the worm/wheel meshing, adusting the couplings between the drive and the worm, making sure the mount was well balanced (meaning a few ounces East heavy), etc., but I never got the results I thought my scope was capable of. FWHM numbers simply weren't where I wanted them to be. Then I received my ridiculously expensive, very high quality mount and all my guiding problems went away instantly. When using my old mount, I typically got RMS errors between guide exposures of 0.4 pixels to 0.6 pixels with my refractor (1.2 to 1.8 arc seconds). I put that same scope on the new mount, and the first night out--a night of particularly good seeing--I got RMS errors between guide exposures of 0.02 to 0.1 arc seconds. The high end mount took away the struggle.
Can you successfully autoguide with a mid priced mount? Sure. Can you autoguide well with a mid priced mount? I never did, but I have seen plenty of evidence that others do. How can you get truly good autoguiding results with a minimum of effort? Buy a better mount and don't overload it.
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Tim C
scholastic sledgehammer
Reged: 11/11/07
Loc: Marietta, GA
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Re: Good Guiding - What got you over the hump?
[Re: Jared]
#2993463 - 03/19/09 10:12 PM
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What got me over the hump of consistently good guiding? I'll give you two answers--one long and one short.
First the short answer: Money. I bought a better mount and virtually all of my autoguiding problems disappeared.
Now the long answer: I struggled with a mount that was at the limits of its capacity for a little over a year. I had no problems with differential flexure or mirror flop since my camera is internally guided. Ultimately, what it came down to for me was that my mount, when loaded with 22 pounds of imaging gear, simply didn't track that well. There were shifts of 1-2 arc seconds in RA that occurred over time frames that were too fast for an autoguider to keep up with. I was able to make incremental improvements through adjusting the worm/wheel meshing, adusting the couplings between the drive and the worm, making sure the mount was well balanced (meaning a few ounces East heavy), etc., but I never got the results I thought my scope was capable of. FWHM numbers simply weren't where I wanted them to be. Then I received my ridiculously expensive, very high quality mount and all my guiding problems went away instantly. When using my old mount, I typically got RMS errors between guide exposures of 0.4 pixels to 0.6 pixels with my refractor (1.2 to 1.8 arc seconds). I put that same scope on the new mount, and the first night out--a night of particularly good seeing--I got RMS errors between guide exposures of 0.02 to 0.1 arc seconds. The high end mount took away the struggle.
Can you successfully autoguide with a mid priced mount? Sure. Can you autoguide well with a mid priced mount? I never did, but I have seen plenty of evidence that others do. How can you get truly good autoguiding results with a minimum of effort? Buy a better mount and don't overload it.
Yep, I figured that a better mount is the ultimate way to get over the hump - I may do that eventually. The CGEM was supposed to be my better mount but after I bought it I realized that it's tracking accuracy is really no better than the LXD75 I sold although I had to tinker with the LXD75 for a while to get it decent results. It is a much better mount in just about every other respect though (very solid, good go to's, nice software). I learned after the fact that I shouldn't expect low PE out of the box with a mount in this price range. I confess I was disappointed to learn that but I see plenty of folks getting good results out of this mount class and hopefully I'll get there (with a few frustrations thrown in to keep me humble I'm sure).
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Good Guiding - What got you over the hump?
[Re: Tim C]
#2994182 - 03/20/09 11:16 AM
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I learned after the fact that I shouldn't expect low PE out of the box with a mount in this price range. I confess I was disappointed to learn that but I see plenty of folks getting good results out of this mount class and hopefully I'll get there (with a few frustrations thrown in to keep me humble I'm sure).
I wouldn't worry too much about PE, but I would aim to train the PEC using the PECTool and 8-10 turns of the worm. You should be able to do well with this mount, and things will be much more forgiving with a shortish refractor.
Keep an eye on the shape and size (arc-seconds) of the stars you get in raw, 5-15 minute exposures. If stars are oblong, determine if it is flexure or a guiding problem. Make sure your focus is tight.
A mount like this can be guided well, but it does involve "chasing the mount" - which is a good thing to do. This means low-latency and short guide exposures, with prompt, well-tuned corrections. Keeping the weight down may help, and a lightweight OAG with a lightweight SX camera has an advantage over guidescopes and much heavier cameras. I don't think you need OAG to guide a refractor, though.
Frank
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Alph
Pooh-Bah
Reged: 11/23/06
Loc: Melmac
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Re: Good Guiding - What got you over the hump?
[Re: freestar8n]
#2994317 - 03/20/09 12:23 PM
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I thought that polar alignment wasn't important for a mid-range mount
Could you please quantify how the good polar alignment should be. For a good autoguider, the 10' or 15' polar misalignment error is not a problem.
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PE is highly overrated since, for a mid-range mount, there are other noise sources that have to be chased
Not really. A mount with a low PE will most likely have 'other noises' low too and the opposite is true as well.
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Good Guiding - What got you over the hump?
[Re: Alph]
#2994559 - 03/20/09 03:00 PM
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Could you please quantify how the good polar alignment should be. For a good autoguider, the 10' or 15' polar misalignment error is not a problem.
It's hard for me to put a number on the misalignment. What I am concerned about is the actual dec. drift rate - which depends on the object placement in the sky and the polar misalignment. A guess would be around 5' misalignment and perhaps 0.5"/minute dec. drift. The issue isn't field rotation, but tight guiding in dec. at the 1.5-2" fwhm level. It is very forgiving at 3-4" fwhm, but for best results I find runs with less dec. drift to have tighter stars in dec.
I certainly don't aim to have a clear drift one way or the other, which some people recommend and has never worked for me.
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A mount with a low PE will most likely have 'other noises' low too and the opposite is true as well.
Well - PE comes from the worm and the wheel, and the other noise comes from the gearbox and connecting gears. So they are independent entities. A high end mount uses expensive components for both - but a mid-range mount may have a mixture.
The PE itself should be readily removed by PEC, leaving the other noise to be chased - so it's the other terms that better characterize one mount over another in terms of how well it can be autoguided.
Frank
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HunterofPhotons
scholastic sledgehammer
Reged: 04/26/08
Loc: Rhode Island, USA
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Re: Good Guiding - What got you over the hump?
[Re: Alph]
#2994719 - 03/20/09 04:30 PM
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....Could you please quantify how the good polar alignment should be. For a good autoguider, the 10' or 15' polar misalignment error is not a problem.....
Here's an applet that can help you answer that question.
dan
photonhunter.com
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Alph
Pooh-Bah
Reged: 11/23/06
Loc: Melmac
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Re: Good Guiding - What got you over the hump?
[Re: HunterofPhotons]
#2994900 - 03/20/09 06:18 PM
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Here's an applet that can help you answer that question.
Thanks. I know the applet and I quite often refer people to it when they get paranoidal about polar alignment. However what I wanted to know is the maximum polar misalignment error that can be guided out by a good autoguider. Clearly, there must be a practical limit to it besides field rotation.
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Tim C
scholastic sledgehammer
Reged: 11/11/07
Loc: Marietta, GA
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Re: Good Guiding - What got you over the hump?
[Re: freestar8n]
#2994942 - 03/20/09 06:44 PM
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I learned after the fact that I shouldn't expect low PE out of the box with a mount in this price range. I confess I was disappointed to learn that but I see plenty of folks getting good results out of this mount class and hopefully I'll get there (with a few frustrations thrown in to keep me humble I'm sure).
I wouldn't worry too much about PE, but I would aim to train the PEC using the PECTool and 8-10 turns of the worm. You should be able to do well with this mount, and things will be much more forgiving with a shortish refractor.
Keep an eye on the shape and size (arc-seconds) of the stars you get in raw, 5-15 minute exposures. If stars are oblong, determine if it is flexure or a guiding problem. Make sure your focus is tight.
A mount like this can be guided well, but it does involve "chasing the mount" - which is a good thing to do. This means low-latency and short guide exposures, with prompt, well-tuned corrections. Keeping the weight down may help, and a lightweight OAG with a lightweight SX camera has an advantage over guidescopes and much heavier cameras. I don't think you need OAG to guide a refractor, though.
Frank
How does PECTool work? I tried Pempro early on using about 8 worm cycles and it didn't seem to improve the PE much (30 to 26 peak to peak). I only tried that one time and probably did something wrong. Haven't had the heart to try it again yet.
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Good Guiding - What got you over the hump?
[Re: Tim C]
#2995006 - 03/20/09 07:26 PM
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How does PECTool work?
Hi-
You can get it (free) from the celestron site. It should work for your cgem. You just hook it up and tell it the number of worm periods you want to average. Then you start autoguiding and it logs each period, then averages them and loads them into the mount. It's very simple, but effective for my cge. I do 8-10 worm periods on a night with decent seeing. I find the resulting PEC log lasts a long time.
You do want to have your guiding tuned pretty well when you do it, though.
Frank
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Tim C
scholastic sledgehammer
Reged: 11/11/07
Loc: Marietta, GA
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Re: Good Guiding - What got you over the hump?
[Re: freestar8n]
#2998791 - 03/22/09 07:37 PM
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How does PECTool work?
Hi-
You can get it (free) from the celestron site. It should work for your cgem. You just hook it up and tell it the number of worm periods you want to average. Then you start autoguiding and it logs each period, then averages them and loads them into the mount. It's very simple, but effective for my cge. I do 8-10 worm periods on a night with decent seeing. I find the resulting PEC log lasts a long time.
You do want to have your guiding tuned pretty well when you do it, though.
Frank
Thanks very much. Almost sounds like you have to be getting pretty good guiding results already before you can get the most out of this tool then? I may give it a try. Think I'm going to start working with shorter focal lengths and see if my results improve. Here is my work from Friday night by the way. These are just stacks of two minute subs of the Cigar Galaxy and the M65/66 galaxy pair with the 8"SCT. Stars aren't perfect but good enough for my "average" astrophoto gallery:
Cigar:
http://tcardin.zenfolio.com/img/v5/p218953302-5.jpg
M65/66:
http://tcardin.zenfolio.com/img/v6/p2803857-5.jpg
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waassaabee
Post Laureate
Reged: 11/26/07
Loc: Central California Coast
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Re: Good Guiding - What got you over the hump?
[Re: Tim C]
#2998899 - 03/22/09 08:26 PM
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I'm still waiting to get TO the hump... much less over it.
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DeanS
Carpal Tunnel
Reged: 07/12/05
Loc: Central Kentucky
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Re: Good Guiding - What got you over the hump?
[Re: waassaabee]
#2998956 - 03/22/09 09:02 PM
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Tim, I struggled even after I got my AP1200GTO, well not nearly as much but I still had issues that I knew had to be operator error cause it wasn't the mount.
So besides the new mounts, the next best thing I have done to improve my guiding is doing away with a guide scope.
Yes this is another against the grain opinion. But I am doing a much better job of guiding using an eFinder.
This helped, then my next improvement was to spend a little more time on drift alignment as others have said. I do use PemPro and it makes it quicker.
The next thing was to "under guide." I had been trying to guide too much all the time. I set my min move limit up where as before I was trying to guide on the seeing.
And I try to guide with a 4 second exposure to help average out the seeing. May not work on all mounts but does fine with mine.
That is how I got over the hump and I find that now I seldom have to discard an exposure unless I snag a cable, or bump the mount. Guiding is such a non-issue that I can concentrate on other things.
Dean
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Tim C
scholastic sledgehammer
Reged: 11/11/07
Loc: Marietta, GA
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Re: Good Guiding - What got you over the hump?
[Re: DeanS]
#2999606 - 03/23/09 08:42 AM
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I wonder if under guiding will work with a mount like mine that has 25-30 PE peak to peak. I tried more aggressive guiding Friday night starting with 2 sec exposure times and 50 percent guide rates and going down to 1 sec exposures and up to 80 % guide rates. Didn't seem to make any difference. Can't hurt to experiment with the under guiding next time out so will try that. Btw- what is an efinder?
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Good Guiding - What got you over the hump?
[Re: Tim C]
#2999623 - 03/23/09 09:00 AM
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I tried more aggressive guiding Friday night starting with 2 sec exposure times and 50 percent guide rates and going down to 1 sec exposures and up to 80 % guide rates. Didn't seem to make any difference.
Hi Tim-
Definitely try experimenting with different options - but make sure your focus is tight and judge guiding quality by the size of the stars in the image - not what the autoguider error says. The actual errors on the image plane can be very different from the errors measured based on the guide star.
I can believe that a high end mount like AP1200 can and should be guided with much less frequent corrections - but a mount with lower grade components can't be 'trusted' to coast at all. You need an accurate centroid and low latency to chase it aggressively.
What were your results like on Friday?
Frank
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wsuriano
super member
Reged: 11/13/07
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Re: Good Guiding - What got you over the hump?
[Re: freestar8n]
#2999973 - 03/23/09 12:33 PM
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Hump? What hump?
Seriously, getting over he hump seems to me to require a confluence of many factors along the learning curve. A good mount and an understaning of how it works so you're not pushing it beyond its capabilities, learning how to get a good polar alignment, knowledge about your system and where you might incur flexure, working with and understanding how the guiding software works and its limitations, etc. When I started guiding, I tried to guide an 8" LX90 with a ETX90. Not only was it ambitious (stupid) to start my guiding experience at that focal length, but can you say "mirror flop X 2"? Anyway, I backed down after multiple not so good attempts, learned a few things, bought a few things, listened to the experience of those here, and am now sort of over the hump. At least most of the time, I know what I did wrong.
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Tim C
scholastic sledgehammer
Reged: 11/11/07
Loc: Marietta, GA
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Re: Good Guiding - What got you over the hump?
[Re: freestar8n]
#3000350 - 03/23/09 02:58 PM
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Hello Frank - I posted the pictures above from Friday. I haven't attempted capture logs and analyze what it's telling me yet (not sure I would know how to interpret it). May need to start another thread on how to analyze what your guiding software is telling you and diagnosing problems etc. That probably the next thing for me to do - really understand what correction my software is trying to make. I'd like to take a more systematic approach to fixing issues. Also, I'm going to start using my new 4 inch refractor for imaging and see what kind of results I get.
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