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Autoguided PE -- how good is good?

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#1 idahoman

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Posted 06 March 2009 - 08:36 PM

I have a Celestron 9.25 Nexstar GPS mounted on a wedge. I image at f6.1 and so my scale is 1.06"/pixel (so, roughly 1 to 1). I use an AO-8 with a ST2000XCM. I have been thinking about deforking and going with a GEM like a CGE. But, I question whether I'll see any improvement. Using Maxim and the AO-8, binning my guider 2x2, my guiding errors are typically 0.4 pixel rms or less as reported by Maxim. Occasionally, I might get a 0.6 rms. I must rely upon the digital display of guide error since Maxim will not graph guide errors when using an AO-8. Anyway, it doesn't matter how long the sub exposure -- 5 min, 10 min, etc. But, I wonder if my stars could be made smaller by better guiding. I've attached an example image.

Some people tell me "Get a GEM". Others tell me that I won't improve over that. I sure don't want to redo my pier to mount a GEM just to match what I am already getting. Plus, there's the issue of meridian flip.

Any advice or comments would sure help me make my decision.

Thanks.

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#2 LLEEGE

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Posted 06 March 2009 - 09:16 PM

I don't think you will improve much over that.

#3 Jared

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Posted 06 March 2009 - 10:22 PM

You might get a little improvement. If you are binning 2x2, then your 0.4 pixels is approximately 0.8 arc seconds RMS, yes?

When imaging with my 1800mm fl scope on a very good GEM (Mach1 GTO), I typically see RMS guide errors around 0.3 to 0.4 arc seconds on a night of good seeing. That is a bit better than you are reporting, but it could just be that I have better average seeing conditions than you.

What is the typical FWHM (in arc seconds) you are getting on non-saturated stars? If it is seeing limited, then you are not going to see an improvement in results with a GEM.

#4 idahoman

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Posted 06 March 2009 - 11:21 PM

Thanks for the replies. From memory (I'm still figuring all this stuff out -- ha, ha) I think my FWHM's are about 4 to 5 arcseconds. What does that mean? Is that an indication of seing?

#5 idahoman

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Posted 06 March 2009 - 11:22 PM

Oh, concerning your question about binning and guiding rms -- I would like to know the answer to that as well. That's why I mentioned that I am binning 2x2.

#6 Jaxdialation

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Posted 06 March 2009 - 11:50 PM

Seeing, guiding and anything else that affects the image, like the as/pixel of your system and focus.

Since the guiding error is measured by your guider camera is may have a different scale than your main camera.

Like the others, I think your guiding errors are pretty good.

I just checked a bunch of images I took early this week using CCDInspector. They average FWHM of 3 and I image at .98 as/pixel. I would estimate my guiding error RMS is in excess of .4 most nights. And my guide camera is running at about 1.8 as/pixel through my OAG.

#7 Miguel Lopes

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Posted 07 March 2009 - 06:48 AM

Overall, IMO, you will not see an improvement, as you are already using an AO. An AO is allways faster correcting errors than any GEM. The real question is: is the poor CPC RA motor generating more errors than the AO can handle at your guide rate?
What refresh rate are you getting? An easy way to improve guide error is to guide faster. Guiding with AO should be done over 2Hz, ideally over 5Hz.
Of course that requires a bright guide star...

Also, how are you focusing? I bought a Moonlite focuser with a stepper motor for my C9.25 and it improved my focus a lot.



#8 idahoman

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Posted 07 March 2009 - 09:54 AM

I am guiding at from 2 Hz to 0.5 Hz, depending upon how bright the guide star is. At my focal length, guide stars can cometimes be few and far between.

I am focusing using a Robofocus unit and the builtin focusing algorithm in Maxim. I could use FocusMax, but haven't.

My best guiding has been at the fastest refreshrate. But, I can't always achieve that.

Thanks.

#9 Jaxdialation

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Posted 07 March 2009 - 11:11 AM

Maybe Focusmax can get you a touch better results. The newest version has an enhancement which takes multiple exposures at each point in the V curve.

In my opinion you are a pretty capable imager. Can't blame you at all for wanting to squeeze all the performance you can out of your system.

Have you used CCDInspector to measure your images "in mass" ? It allows you to graph your image FWHM vs several variables like time of night, position of the object, filter etc. perhaps using the trial version on a batch of images would point out a trend you could make some conclusions about.

#10 idahoman

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Posted 07 March 2009 - 12:14 PM

Thanks, John. I've been wanting to shift over to Focusmax. In general, people seem to think it does a better job than Maxim.

Other people have assessed my images with CCDInspector. I don't have a copy. They've basically checked my curvature and collimation, both of which seem to be pretty good. I was concerned because I have a bit more backfocus than ideal (fr 6.1 versus 6.3).

#11 Miguel Lopes

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Posted 07 March 2009 - 02:10 PM

I am guiding at from 2 Hz to 0.5 Hz, depending upon how bright the guide star is. At my focal length, guide stars can cometimes be few and far between.

I am focusing using a Robofocus unit and the builtin focusing algorithm in Maxim. I could use FocusMax, but haven't.

My best guiding has been at the fastest refreshrate. But, I can't always achieve that.

Thanks.

2Hz seems too low. Don't settle for the first stars that come up on the guide CCD. Plan your imaging session so you can find the best guide star for each target.
Anything equal or brighter than mag 11 should give you more than 2Hz in your C9.25.
<2Hz means you are using some +400ms for each exposure. That is way too much. I've nailed a mag 9 mag star with 40ms. A rule of thumb is to multiply that time by 2.5 for each increase in magnitude.
That would give 250ms of exposure for a mag 11 star, which would yeld >3Hz.

Make an experiment. Pick a target close to a bright star, maybe mag 5-6. Use several different exposures for the guide star and look at the guide error.

#12 Jared

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Posted 07 March 2009 - 02:18 PM


Since the guiding error is measured by your guider camera is may have a different scale than your main camera.


As it happens, the camera in question has the same pitch for both the main chip and the guide chip.

#13 Jared

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Posted 07 March 2009 - 02:27 PM

I am guiding at from 2 Hz to 0.5 Hz, depending upon how bright the guide star is. At my focal length, guide stars can cometimes be few and far between.

I am focusing using a Robofocus unit and the builtin focusing algorithm in Maxim. I could use FocusMax, but haven't.

My best guiding has been at the fastest refreshrate. But, I can't always achieve that.

Thanks.


You mentioned FWHM in the 4-5 range. Obviously, I don't know anything about your seeing conditions, but I suspect that your scope is capable of a bit better than that (though you're not off by much).

The real advantage you will get if you decide to switch to a GEM is not needing to find a bright, nearby guide star. Since you won't be trying to take 0.5s exposures for your AO unit, you will be able to get similar (or even slightly better) results with ordinary guiding.

Personally, it seems to me the rig you are currently using is working pretty well. I wouldn't mess with it unless I was willing to jump to something significantly more expensive--like an AP-900 class mount (or perhaps the newly announced CGE-Pro). If you don't have that kind of budget, then I would just follow Miguel's advice and be careful to plan out your imaging sessions so that you know where the brightest potential guide star is located. If you can get up to 3-4 Hz you will probably see results that are about as good as your scope is capable of.

#14 idahoman

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Posted 07 March 2009 - 06:06 PM

Thanks, everyone. What I'm hearing is:

1.) Up my AO-8 guide rate by using better guide stars
2.) Fine tune my focusing
3.) Keep what I got until Jared gives me his Mach1 GOTO. Ha, ha.

All sounds like good advice. Is anyone trying to do what I am doing with a fork and AO unit? I'd be curious how well they guide as a comparison.

#15 Miguel Lopes

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Posted 07 March 2009 - 06:22 PM

Is anyone trying to do what I am doing with a fork and AO unit? I'd be curious how well they guide as a comparison.

A fork with a wedge is +/- the same as a GEM. High-end observatories work like that. The issue is that most cheap fork mounts are not created for AP, so the RA motor and gears are not the best...
But one of the main reasons to buy an AO unit is to avoid buying a Paramount-like mount! So your ideia to ditch the fork to buy a GEM...

#16 idahoman

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Posted 08 March 2009 - 12:42 AM

Miguel -- I take it you think I should keep my setup? The AO is definitely a miracle for fork-mount-imaging.

#17 Jaxdialation

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Posted 08 March 2009 - 02:43 AM

Luke has an AP900GTO that he is trying to give away I heard.

3.) Keep what I got until Jared gives me his Mach1 GOTO. Ha, ha.



#18 Miguel Lopes

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Posted 08 March 2009 - 02:36 PM

Miguel -- I take it you think I should keep my setup? The AO is definitely a miracle for fork-mount-imaging.

Ask JPM! He has made amazing images with a fork-wedge 12" scope:
http://astroanarchy.blogspot.com/

#19 DeanS

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Posted 08 March 2009 - 05:53 PM

I think you are getting very good results, however a good quality GEM will amaze you at how much easier it can be overall. Plus you will have the option of mounting different OTA's.

I struggled with forks a long time, of course without AO's, and don't see myself ever going back. Only downside is the flip, which is not too big of a deal.

#20 Jared

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Posted 08 March 2009 - 11:12 PM

...
3.) Keep what I got until Jared gives me his Mach1 GOTO. Ha, ha.


Sorry, I still owe my wife two to three hundred favors before the Mach1 is fully paid off.

By the way, while I have never tried imaging with a fork mount (or at least not recently), I used to use an AO-7 with a Losmandy GM-8 that had periodic errors that were too fast to be guided out very well. In practice, I never got quite as much resolution with the AO-7 and mediocre mount/tracking as I have been able to get with the Mach1. It certainly came pretty close, though... And it was dramatically less expensive.

As one or two others have said, I think the real advantage to the high end mounts is not that you will necessarily get better results, but that you will get them with less effort. With the Losmandy and AO-7 (admittedly loaded to 25 pounds) I would need to make sure worm was adjusted perfectly, balance was perfect (a few ounces East heavy), backlash set correctly, guide star was bright enough for a high sampling rate... With the better mount I can skip all that. Is the balance vaguely correct? Cool. Just turn on the autoguider, and I can be assured that I am limited only by seeing.

#21 freestar8n

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Posted 09 March 2009 - 12:34 PM

Thanks for the replies. From memory (I'm still figuring all this stuff out -- ha, ha) I think my FWHM's are about 4 to 5 arcseconds. What does that mean? Is that an indication of seing?



Hi-

For guiding quality the fwhm of raw frames of decent length - say 5-20 minutes - is a key thing to go by. It is affected by seeing, focus, guiding, and pixel scale to some extent.

If seeing is decent and guiding is good, I think fwhm's should be down around 2.5". If you have AO and a brightish guidestar, it should go to 2.0" and below.

I don't know of a fundamental difference between GEM and fork - I would think it mainly depends on the quality of the bearings and gears.

I am in the northeast U.S. not known for great seeing, and I guide a CGE and C11 to get regularly 2.5" and below fwhm - sometimes even to 1.5" - and that is without AO.

Examples are at:

MetaGuide C11 CGE images

Another thing is - guiding errors can be misleading and much smaller than the true error seen at the imaging camera. So fwhm's always tell the full story.

I think you should be able to get smaller fwhm's with AO, especially on a night of good seeing. But if seeing is really bad - maybe it is a fundamental limit for you. Even then - watch for nights of steady air.

Frank

#22 Jaxdialation

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Posted 10 March 2009 - 09:52 AM

I think there are too many variables, which are unknown to give 2" as an expected outcome.

If seeing is decent and guiding is good, I think fwhm's should be down around 2.5". If you have AO and a brightish guidestar, it should go to 2.0" and below.

I don't know of a fundamental difference between GEM and fork - I would think it mainly depends on the quality of the bearings and gears.



#23 freestar8n

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Posted 10 March 2009 - 11:48 AM

I think there are too many variables, which are unknown to give 2" as an expected outcome.


One thing to look at is if someone nearby with an AP1200 is getting below 2" fwhm, then seeing isn't a fundamental limit. And a medium grade mount plus AO with a bright guidestar should make the mount less of an issue if things are tuned right.

I guess another question is, what are the fwhm's without AO? If they are also around 4", then the AO may not be operating correctly.

A basic thing is just to make sure it is well focused in the first place. What are the fwhm's in a 0.5 second exposure of brightish stars without AO?

Frank

#24 TxStars

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Posted 12 March 2009 - 09:11 AM

The problem is not your mount, it is the scope.
If you want smaller stars get a Refractor...

#25 Jaxdialation

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Posted 12 March 2009 - 09:27 AM

All the guys with RCOSs would probably take exception to your recommendation.

Which refractor would you suggest to replace a 9.25" at f6? Would it drop right into his existing fork mount?

I don't know that there is a "problem".






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