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Looking for Help Understanding PEC with CGEM

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#1 guerillary

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Posted 12 September 2014 - 09:34 PM

I'm a technical person when it comes to digital imaging and an avid photographer, but still a newbie when it comes to astrophotography.

 

Based on what I can tell from my first astrophoto and autoguiding experiences, my CGEM mount has A LOT of periodic error.  I'm interested at this point in using Celestron's PEC utility, PemPro or both... but am really confused about a few things regarding them.

 

Can autoguiders be used with a mount that's running a PEC curve and if so, is it recommended?

 

Is PemPro a much better solution for CGEM PEC than Celestron's own PEC Utility?

 

Does PemPro work on a mac running a Window's emulator like Parallels?  I think it needs ASCOM drivers so those only work on Windows too.

 

Once I successfully upload an accurate PEC curve into the CGEM, can I then not run anything related to PEC until noticing errors again?  For instance, once the CGEM is running a custom curve, can I not run PemPro?

 

About how long do PEC curves last on average before noticeable errors return?

 

Is there a specific manner in which the CGEM mount should be shut down and carried / packed / transported to avoid re-introducing periodic error?

 

THANKS!



#2 rgsalinger

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Posted 12 September 2014 - 11:12 PM

Guiding should be even better with PEC running because it smooths out the transitions- it proactive and guiding is reactive and will therefore need to do less. PEMPRO is the gold standard in terms of calculating the right correction, and while I can't really say if it's "better" it has the ability to capture several cycles of error and average them out so it should be. I can't comment on what works on a MAC - check with the PEMPRO website, but I don't see why a windows emulation package wouldn't work. You should not have PEC running when running PEMPRO as it will give false results and when you load the PEMPRO curve into the mount it will make things worse. I can't imagine needing to run it more than once a year - the one in my Mach1 is now 6 years old and is still within spec. PEC has to do with imperfections in the manufacturing of the gear train and the worm, just be careful with the mount and you'll be fine.



#3 SergeC

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Posted 12 September 2014 - 11:57 PM

You don't necessarily need an external program to train and run PEC on a CGEM. There is a built-in routine accessible from the hand controller that will let you train the mount. You only need Celestron's PECTool program if you want to download the PEC data from your CGEM to your computer.

 

To train your CGEM, first, polar align the mount as accurately as you can (the ASPA routine in the HC should be adequate). Set up your autoguider, but don't actually start guiding yet. Next, start the PEC training routine from the Utilities menu. The mount will slew a slight amount until it detects the gear index. Once the HC display indicates that PEC data are being collected, start your guider and let it run. The PEC training routine will collect and average 8 complete cycles and then store the averaged PE curve. After that, you just start PEC whenever you need it. The PE curve stays in non-volatile memory until you overwrite it with another training session. I only retrain if I do something radical to the mount such as the adjusting the gear mesh or taking it apart to re-grease.


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#4 guerillary

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Posted 13 September 2014 - 04:58 AM

Thanks for the replies, this information is very helpful.

 

Viking: when you say to start auto-guiding when the PEC is collecting data - I'm trying to understand why.  Isn't the pec curve to be developed and applied prior to running any auto-guiding.

 

Thanks again.



#5 Jon Rista

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Posted 13 September 2014 - 07:49 AM

PEC programming determines how much PE there is by using the amount of guided offset sent to the mount every second via guide pulses. This generates a series of single-period plots of the PE for the worm, usually for several (five to eight) worm periods (usually about eight minutes on a mount like the CGEM). After several full worm periods are plotted, they are averaged together to reduce the PEC curve down to just the primary PE, eliminating any small perturbations resulting from seeing and the like. This final curve should generally have a sine-wave like appearance. If you don't guide, then the mount won't know what the periodic error is, and generate a flat curve. Remember, guiding is normally used to counteract periodic error, thus allowing you to track for longer periods of time.

 

Once you have programmed and applied the PEC curve, there will still be smaller sources of error in your mount, from smaller gears, unevenness in the machining of gear teeth, etc. Those small sources of error can still cause enough error to mess up tracking, so you still need to guide...however guiding tends to be more effective as it's not constantly chasing after that really big PE from the worm. So it's best to use both PEC and guiding together, to get the best results.

 

It should be noted that any time the orientation of the worm and ring gear meshing changes, you will need to reprogram PEC. So long as that does not change, you should be fine to reuse a PEC curve.


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#6 SergeC

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Posted 13 September 2014 - 08:00 AM

Thanks for the replies, this information is very helpful.

 

Viking: when you say to start auto-guiding when the PEC is collecting data - I'm trying to understand why.  Isn't the pec curve to be developed and applied prior to running any auto-guiding.

 

Thanks again.

No. Without autoguiding you'd need to enter corrections during training by staring thru a high-power reticle eyepiece and manually pressing the HC direction buttons to keep the guide star centered. While that's possible, an autoguider is more accurate.

 

The idea behind "training" the mount is that the mount knows where the RA gear index is at the start of a training run. As the gear turns, the mount then "learns" how far off from a centered position the scope has become at a given gear position by how much guide compensation needs to be applied at that point to bring the scope back to center. Then, when you later run correction, it uses that information on how much correction needs to be applied for any given gear position in its rotation cycle. Without any sort of guiding, PEC training wouldn't work, and so neither would PEC itself based on that training.

 

BTW, the name's Serge. "Viking1" is just what the discussion board sorftware calls anyone with the number of posts I've made  :grin:


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#7 stellarvuelomo

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Posted 13 September 2014 - 11:02 AM

WRT turning on the autoguider, assuming you use phd, do you let it acquire a star, let it guide a while to settle, and then turn off guiding to start the pec routine and then turn guiding back on?



#8 SergeC

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Posted 13 September 2014 - 11:24 AM

WRT turning on the autoguider, assuming you use phd, do you let it acquire a star, let it guide a while to settle, and then turn off guiding to start the pec routine and then turn guiding back on?

This relates to the CGEM (and other Celestron mounts with permanent PEC): assuming you've already trained the mount, after each time you turn on the mount the first time you invoke PEC the mount will slew slightly to locate the RA gear index. Because of that slew, you don't want to be already guiding in that situation. Anytime after that, though, as long as you haven't turned the mount off, you can start guiding and then invoke PEC. If you've been running PEC when you slew to another target the PEC will automatically stop, so you'll need to invoke it again, but the mount won't need to slew to find the index mark. Letting your guiding settle is always a good idea.


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#9 stellarvuelomo

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Posted 14 September 2014 - 09:10 AM

Thanks for the guidance here on PEC, I employed it last night and it worked like a charm.  Guiding 15-minute subs no problem now!



#10 seitanfingers

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Posted 21 November 2017 - 10:32 PM

Apologies for reviving this old thread, but I haven't been able to find an answer on this re: PEC and NexRemote.

 

If I record cycles through the PEC utility within NexRemote and close/re-open the software, will the data persist? 




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