#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?
#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.
Montpelier, VA USA
#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.
Montpelier, VA USA