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idahoman
professor emeritus
Reged: 11/25/05
Loc: Idaho, USA
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Re: Autoguided PE -- how good is good?
[Re: freestar8n]
#2988589 - 03/17/09 10:15 AM
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Some more info to throw into the stew! A friend of mine images at a site 50 miles from my house. He is a bit higher and darker. Anyway, he reports that he usually sees FWHM of 1.5 to 2.5 on a "steady night", but 2 - 3 is more typical. His opinion is that the atmosphere around here is too unstable for anything over 1500mm focal length. I'm imaging at around 1440mm or so.
I don't have CCDInspector, but when focusing Maxim reports around 4 or 5 as my FWHM (if I remember correctly). So, my guess is that the best I can achieve is 3 to 4 and thus I have some room for improvement.
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dickbill
professor emeritus
Reged: 09/30/08
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Re: Autoguided PE -- how good is good?
[Re: freestar8n]
#2988659 - 03/17/09 11:02 AM
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On the topic of fwhm - in order to get the smallest fwhm in the image, all errors will contribute - and I find there is enough error in the guide star centroiding that there is benefit in doing things differently. So focus, guide tuning, latency, etc. all need to be addressed - and even then the centroiding algorithm can be improved, which is why I wrote MetaGuide.
Thanks,
Frank
Which brings us back to the mount accuracy. Since the purpose of guiding is to correct tracking errors AFTER the errors are made, good mounts will produce less errors that won't need to be corrected and so the centroid produced by an average mount, even in perfect focus and seing conditions, cannot be as small as a good mount with small PE.
If I understand well however, Permanent Periodic Error Correction, such as implemented in the new CGEM or CGE-pro, is supposed to prevent tracking errors before they happen and so the pictures obtain with these mount should show an improved resolution...But, in practice, I suspect that the correction made under PPEC, being automatic, can be also inaccurate and to a certain point it remains to be seen how a middle range mount like a CGEM really needs PPEC when it is also autoguiding.
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Strgazr27
Reged: 10/04/04
Loc: StonyHill Observatory
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Re: Autoguided PE -- how good is good?
[Re: dickbill]
#2988690 - 03/17/09 11:21 AM
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Although PEC can and does help, depending on the situation and the mount there is still as you say an error to it. Wether by eye or by camera, the errors will be corrected for either as they happen or after they happen. In the case of my G11, until I get the new worm block I have an issue with the 76 sec error. This makes using PEC a non issue as the error occurs at a different point in each worm cycle. Other mounts do show a marked improvement with PEC. The NXGPS, CPC, and CGE models have always had PPEC.
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idahoman
professor emeritus
Reged: 11/25/05
Loc: Idaho, USA
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Re: Autoguided PE -- how good is good?
[Re: Strgazr27]
#2988741 - 03/17/09 12:00 PM
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My Nexstar GPS does not learn PEC for some reason. That was why I got the AO-8. I had more than 80" periodic error, and there was no way to program the PEC into the mount. Apparently this was a problem with the Nexstar's (at least, the one I have). So, I run with NO PEC. I think it is amazing that the AO-8 improves my mount the way it does.
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Miguel Lopes
scholastic sledgehammer
Reged: 01/04/07
Loc: Portugal
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Re: Autoguided PE -- how good is good?
[Re: dickbill]
#2988757 - 03/17/09 12:06 PM
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Quote:
Quote:
On the topic of fwhm - in order to get the smallest fwhm in the image, all errors will contribute - and I find there is enough error in the guide star centroiding that there is benefit in doing things differently. So focus, guide tuning, latency, etc. all need to be addressed - and even then the centroiding algorithm can be improved, which is why I wrote MetaGuide.
Thanks,
Frank
Which brings us back to the mount accuracy. Since the purpose of guiding is to correct tracking errors AFTER the errors are made, good mounts will produce less errors that won't need to be corrected and so the centroid produced by an average mount, even in perfect focus and seing conditions, cannot be as small as a good mount with small PE.
If I understand well however, Permanent Periodic Error Correction, such as implemented in the new CGEM or CGE-pro, is supposed to prevent tracking errors before they happen and so the pictures obtain with these mount should show an improved resolution...But, in practice, I suspect that the correction made under PPEC, being automatic, can be also inaccurate and to a certain point it remains to be seen how a middle range mount like a CGEM really needs PPEC when it is also autoguiding.
+/- true. If you SBIG examples, they do make a test on a Paramount and there is a slight improvement...
Also... Paramount = 14.000€, AO = 850€ - 1800€
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Autoguided PE -- how good is good?
[Re: Strgazr27]
#2988794 - 03/17/09 12:28 PM
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lthough PEC can and does help, depending on the situation and the mount there is still as you say an error to it. Wether by eye or by camera, the errors will be corrected for either as they happen or after they happen. In the case of my G11, until I get the new worm block I have an issue with the 76 sec error. This makes using PEC a non issue as the error occurs at a different point in each worm cycle.
Hi-
My CGE is like most mid-range mounts and also has noise that isn't commensurate with the worm, so it can't be removed by PEC. But PEC does remove the fundamental and harmonics effectively, and has no downside that I can see, so it does remove a lot of the work for the autoguider and helps reduce the fwhm. But there is still a range sources from 1s to 50s that I deal with by low latency corrections and short guide exposures.
Although autoguiders tend to be reactive, MetaGuide is I think unique in that it can lock onto an error frequency and proactively correct it - independent of PEC. This was largely done as an experiment, but it sounds like a 76 second G11 would be a good candidate. You just enter 76 seconds as the period and begin autoguiding. It will sense that frequency in its corrections and determine its phase and amplitude - then begin applying it proactively rather than reactively. Normal autoguiding continues on to correct remaining errors.
I never saw a real benefit of this with my cge so I haven't used it much - but it's there.
Frank
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idahoman
professor emeritus
Reged: 11/25/05
Loc: Idaho, USA
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Re: Autoguided PE -- how good is good?
[Re: freestar8n]
#2988842 - 03/17/09 12:58 PM
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Miguel brings up a good point. The new Paramount is $14500. The AO is under $1000. The AO, however, does occupy a significant amount of backfocus. Especially the AO-7. And, adds weight. But, it sure improved my life! Before the AO (I had an AO-7 before my AO-8), I could not take subs longer than a couple of minutes since I had to throw so many away.
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Jaxdialation
Post Laureate
Reged: 04/01/07
Loc: Northeast, FL
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Re: Autoguided PE -- how good is good?
[Re: idahoman]
#2989176 - 03/17/09 04:32 PM
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You can get a trial version of CCDInspector.
It might also find some optical issues with your system which are affecting your FWHM.
Quote:
Some more info to throw into the stew! A friend of mine images at a site 50 miles from my house. He is a bit higher and darker. Anyway, he reports that he usually sees FWHM of 1.5 to 2.5 on a "steady night", but 2 - 3 is more typical. His opinion is that the atmosphere around here is too unstable for anything over 1500mm focal length. I'm imaging at around 1440mm or so.
I don't have CCDInspector, but when focusing Maxim reports around 4 or 5 as my FWHM (if I remember correctly). So, my guess is that the best I can achieve is 3 to 4 and thus I have some room for improvement.
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