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Re: PEC while autoguiding?


Apr 19, 2006

 


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#29507 Apr 19, 2006

I remember discussion from years ago on this topic, should I bother

using the PEC feature on the G11 when I'm autoguiding. I ended up

deciding I didn't need to bother. I use an STV autoguider. However,

someone at the dark sky site brought this up last time I was out, and

said that he did do this with his G11/autoguider, and that it did

improve results.



Anyone actually test this? What were the findings?



Apologies if this is a FAQ ;-)



Thanks,



-Dick Locke

www.dl-digital.com



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#29508 Apr 19, 2006

dtlnew wrote:

>I remember discussion from years ago on this topic, should I bother

>using the PEC feature on the G11 when I'm autoguiding. I ended up

>deciding I didn't need to bother. I use an STV autoguider. However,

>someone at the dark sky site brought this up last time I was out, and

>said that he did do this with his G11/autoguider, and that it did

>improve results.

>

>Anyone actually test this? What were the findings?

>

>Apologies if this is a FAQ ;-)

>

>

>

>

>

Hi Dick,



It is a FAQ ... but there isn't any simple answer to the question. My

personal experience is that PEC does not improve a guided image. In fact

it sometimes makes it worse. But there is a caveat with this and that is

that *if* you have Gemini with the new Level 4 ROM, Pempro and a laptop

then you can improve things considerably. But if you want to keep things

simple, ie: no laptop, then I wouldn't use PEC. Your mileage may

certainly vary.



Regards



Bill



--



William R. Mattil.:.www.celestial-images.com



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#54142 May 5, 2014

Hello,

Does anyone have thoughts about the wisdom of using period error correction while autoguiding? When I purchased my Losmandy G-11 mount, the retailer recommended not using PEC while autoguiding to avoid conflicting messages within the computer. Any thoughts on this? FWIW, I use PHD Guiding.

Thank you for any advice,

Jason



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#54143 May 5, 2014

Jason, I have the same question. Thank you.



Rich



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#54144 May 5, 2014

There's no reason not to use PEC while autoguiding. Properly programmed PEC anticipates and corrects errors before they happen. Autoguider corrects errors after they have occurred, which is already too late since the mount is already away from the desired position at that point.��

If PEC is working properly, there is a whole lot less work for the autoguider to do, resulting in better tracking results.

Regards,

��-Paul



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#54145 May 5, 2014

Thanks for the information, Paul. This rationale makes a whole lot of sense.

Best,Jason



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#54147 May 5, 2014

Another thought on the subject: If you have an adaptive optics guider like SBIG.s AO-8, then it makes a lot of sense to engage Periodic Error Correction when guiding. With adaptive optics guiding, a tip/tilt mirror or prism detects movement of the target on the guiding chip and quickly moves to compensate. It can do this much more quickly than by sending corrections to the mount, because .corrections to the mount must overcome the inertia of the mount. However, adaptive optics can only move so far . they have a limit of travel. When the guide star exceeds this limit, corrections must be sent to move the mount itself. Consequently, guiding in this situation involves first the lightning quick movement of the prism or mirror, followed by the not so fast movement of the mount. If the mount is properly polar aligned, then good PEC can ensure that guiding never exceeds the limits of the adaptive optics device and thus never has to use corrections to the mount. The result is much better guiding and thus a much better image. ��Greg �� �� ��From: Losmandy_users@yahoogroups.com [mailto:Losmandy_users@yahoogroups.com] On Behalf Of jason.tackett@...Sent: Tuesday, 6 May 2014 2:38 AMTo: Losmandy_users@yahoogroups.comSubject: [Losmandy_users] PEC while autoguiding?



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#54153 May 6, 2014

First question is, do you need it? ��If not, then sheer conservatism would say don���t turn it on. If you need it, then try it, by all means. ��The dealers comment is based on an Urban Myth, I believe. You could imagine, if the programmer implemented PEC wrong, that there could be such a conflict. But if they programmed it correctly, keeping track of the index in the PEC table properly, isolating the inner from outer control loops, then there should not be a problem. ��I���ve never been able to find out if there ever really was a case where the programmer messed it up for one mount or another, or if it was just a matter of the Perhaps-A-Tron machine running at full throttle. I suspect the latter. ��Cheers,Mark Christensen



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#54155 May 6, 2014

��There is a lot of old thought floating around about using PEC and auto guiding and I am convinced it is from the old days when creating a PEC table was done looking through an eyepiece and fingers on the control paddle making adjustments. After one work cycle you were (glad to be) done but your eyes are no match for seeing oscillations so PEC would fight auto guiding. I have personal experience with this old method.

�� I use PEC all the time and have always had very good results especially when imaging with a focal length of > 1800mm. When done correct it will definitely reduce the amount of corrections needed while auto guiding. To create an accurate PEC table I use PemPro and run 8-12 worm cycles before creating a profile to load into the Gemini. I put a delay of approximately 1 second backwards to make up for the download/processing cycles. This put the correction period in approximate to the gear errors. PemPro allows that forward/reverse delay.

Try PemPro to program PEC and other neat features it does....compared to what we spend in this hobby it is well spent $$$$$.

Mike Siniscalchihelixgate.net







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#54165 May 7, 2014

O>> First question is, do you need it? >>

>>

>> If not, then sheer conservatism would say don't turn it on. If you need it,

>> then try it, by all means.

>>

>>

>> The dealers comment is based on an Urban Myth, I believe. You could imagine,

>> if the programmer implemented PEC wrong, that there could be such a

>> conflict. But if they programmed it correctly, keeping track of the index in

>> the PEC table properly, isolating the inner from outer control loops, then

>> there should not be a problem.

>>

>>

>> I've never been able to find out if there ever really was a case where the

>> programmer messed it up for one mount or another, or if it was just a matter

>> of the Perhaps-A-Tron machine running at full throttle. I suspect the

>> latter.



Remember guiding is reactive.

There has to be an error, before it can correct.

PEC, is proactive. It acts as an expected error occurs.



Done correctly, 'yes' you do want PEC. Otherwise you are increasing the

errors that the correction needs to compensate for, and degrading the image.



I think there may well have been some cases, where a very badly

programmed PEC table, created more errors, or a specific mount where the

controller could not correctly combine the two controls at once, but on

every mount I have seen in the last 20 odd years, a well programmed PEC,

improves guiding.



Best Wishes



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#54172 May 8, 2014

Thanks for the great information, everyone. PEC seems like a benefit well worth investing the time to learn so I plan to add it to my setup procedure.

Best,Jason



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