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Re: PEC and the 76-second error


Jun 3, 2007

 


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#33183 Jun 3, 2007

I spent last night with PEMPro, which shows my G11 as having

+4.6"/-4.7" PE dominated by 76-second error; the power spectrum shows

a 2.42" peak at 76.50 seconds, everything else is 1" or below. Can I

PEC train to lower the integer components of the PE or does the

76-second error mean that PEC doesn't work at all?



Ben.



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#33184 Jun 3, 2007

Ben Ritchie wrote: > I spent last night with PEMPro, which shows my G11 as having

> +4.6"/-4.7" PE dominated by 76-second error; the power spectrum shows

> a 2.42" peak at 76.50 seconds, everything else is 1" or below. Can I

> PEC train to lower the integer components of the PE or does the

> 76-second error mean that PEC doesn't work at all?



Uploading a PEMPro curve reduces the other terms, and PEC works fine.

Prove it to yourself by running another acquisition run with PEC on, and

see what PEMPro reports; you'll be pleased.



Ray once advised me to un-check the 3X fundamental box when creating the

curve, since the 76s error is 3.15X the worm fundamental. You don't want

to include that term in the correction curve because it's not

synchronous with the worm.



The real question is how much the 76s term affects _your_ activities.

Are you taking guided images? Can your autoguider guide through the

2.42" movement due to the 76s error? In other words, the error might not

be a factor at all. Or it could be a major factor, if you're attempting

several-minute unguided exposures at a long focal length.



The 2.42" value seems a bit high, and you might want to consider

fine-tuning the worm bearing blocks (or replacing the bearings) to lower

it. I've read several things here and on various Web sites about the 76s

error, and it's not hard to lower it. PEMPro shows mine is 0.88" - much

lower than before I tweaked the RA worm.



Mike

--



Mike Dodd

Montpelier, VA USA

astronomy.mdodd.com



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#33186 Jun 3, 2007

Ben Ritchie wrote: > If the weather holds i'll try

> a run tonight with PEC uploaded with the 1st, 2nd and 4th

> fundamentals to see what happens - I haven't yet tried uploading a

> PEC curve, just been diagnosing so far.



Definitely do that, and then acquire 4-5 worm cycles with PEC enabled. I

think you'll find the overall error is smaller, the fundamentals are

significantly reduced, and the 76s term is still there. It probably will

be the major contributor to the overall error.

> My aim is arbitrary-length guided exposures at 1040mm focal length

> and an image scale of 1.47"/pixel. I'd hope that isn't a big ask for

> an observatory-mounted G11.



Sounds reasonable to me.



My equipment is a C9.25 SCT with a 0.63 reducer, to yield f/7.5 and an

image scale of 1.05"/pixel. I recently imaged NGC4437 with 10-minute

exposures and guided with a piggyback guider (6.2"/pixel). Stars look

pretty good - my biggest problem is a slight amount of differential

flexure between the guide scope and the main OTA.

> Ideally i'd like to get PEC working, but it seems like

> it shouldn't be the end of the world if I can't.



After I finished the NGC4437 imaging session, I discovered I hadn't even

turned on PEC! I generally try to use short guide exposures (1-2

seconds) so the autoguider corrects the mount frequently.



Mike



--



Mike Dodd

Montpelier, VA USA

astronomy.mdodd.com



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