Jump to content


Photo

Good Guiding - What got you over the hump?

  • Please log in to reply
39 replies to this topic

#26 Tim C

Tim C

    Apollo

  • *****
  • Posts: 1085
  • Joined: 11 Nov 2007
  • Loc: Marietta, GA

Posted 19 March 2009 - 09:12 PM

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).

#27 freestar8n

freestar8n

    Soyuz

  • *****
  • Posts: 3907
  • Joined: 12 Oct 2007

Posted 20 March 2009 - 10:16 AM

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

#28 Alph

Alph

    Surveyor 1

  • -----
  • Posts: 1755
  • Joined: 23 Nov 2006
  • Loc: Melmac

Posted 20 March 2009 - 11:23 AM

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.

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.

#29 freestar8n

freestar8n

    Soyuz

  • *****
  • Posts: 3907
  • Joined: 12 Oct 2007

Posted 20 March 2009 - 02:00 PM

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.

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

#30 HunterofPhotons

HunterofPhotons

    Apollo

  • -----
  • Posts: 1062
  • Joined: 26 Apr 2008
  • Loc: Rhode Island, USA

Posted 20 March 2009 - 03:30 PM

....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

#31 Alph

Alph

    Surveyor 1

  • -----
  • Posts: 1755
  • Joined: 23 Nov 2006
  • Loc: Melmac

Posted 20 March 2009 - 05:18 PM

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.

#32 Tim C

Tim C

    Apollo

  • *****
  • Posts: 1085
  • Joined: 11 Nov 2007
  • Loc: Marietta, GA

Posted 20 March 2009 - 05:44 PM

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.

#33 freestar8n

freestar8n

    Soyuz

  • *****
  • Posts: 3907
  • Joined: 12 Oct 2007

Posted 20 March 2009 - 06:26 PM

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

#34 Tim C

Tim C

    Apollo

  • *****
  • Posts: 1085
  • Joined: 11 Nov 2007
  • Loc: Marietta, GA

Posted 22 March 2009 - 06:37 PM

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.zenfo...218953302-5.jpg

M65/66:

http://tcardin.zenfo.../p2803857-5.jpg

#35 waassaabee

waassaabee

    Fly Me to the Moon

  • *****
  • Posts: 5116
  • Joined: 26 Nov 2007
  • Loc: Central California Coast

Posted 22 March 2009 - 07:26 PM

I'm still waiting to get TO the hump... much less over it.

#36 DeanS

DeanS

    Gemini

  • *****
  • Posts: 3258
  • Joined: 12 Jul 2005
  • Loc: Central Kentucky

Posted 22 March 2009 - 08:02 PM

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

#37 Tim C

Tim C

    Apollo

  • *****
  • Posts: 1085
  • Joined: 11 Nov 2007
  • Loc: Marietta, GA

Posted 23 March 2009 - 07:42 AM

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?

#38 freestar8n

freestar8n

    Soyuz

  • *****
  • Posts: 3907
  • Joined: 12 Oct 2007

Posted 23 March 2009 - 08:00 AM

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

#39 wsuriano

wsuriano

    Vostok 1

  • -----
  • Posts: 146
  • Joined: 13 Nov 2007

Posted 23 March 2009 - 11:33 AM

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.

#40 Tim C

Tim C

    Apollo

  • *****
  • Posts: 1085
  • Joined: 11 Nov 2007
  • Loc: Marietta, GA

Posted 23 March 2009 - 01:58 PM

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.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics