Advanced VX & PEC - How does it work?

[Scorpius]

I’ve owned a Celestron AVX for a while now and have thought about delving into PEC but never took the time considering all the other things necessary to get setup and imaging with a tripod. I did add guiding (OAG) a while back - which has performed well - but would still like to further improve tracking performance so wondered if programming PEC might help. Now that I’ve constructed a permanent pier, thought I would ask if anyone could provide some details on how to measure PEC on the AVX and how to program the mount to null it out.

Guess my main question is - can PEC be used in conjunction with guiding or is guiding just another form of PEC that will override any PEC programmed into the mount? In other words, can guiding and PEC work together at the same time for improved tracking performance or is it necessary to choose one method over the other?

Any info at all regarding the AVX and PEC would be greatly appreciated...

[cjdawson]

Everything that I've read and heard about PEC and Guiding is that the do not cancel each other out.   In fact if you have PEC you shouldn't need Guiding... AS MUCH.

Periodic Error is a mechanical error that occurs in the drive train of all telescopes.  It doesn't matter how well engineered the scopes are, there will be wear and tear that will alter the tracking characteristics of the scope over time.

There are two forms of PEC.  First, is the hardware version (Meade call this PPEC, Permenant Periodic Error Correction) and a software version (Meade call this Periodic Error Correction)

The software version is programmed by running the scope as it tracks a star.  On the Meade mounts (LX-90 for example) you can program this by starting the tracking, then guiding the scope yourself for about 10 minutes.  The guiding can be done manually, or with an autoguider.  What that will do is allow the software to calculate the error, and compensate as the gears turn.  Once programmed, this information can be retained by parking the scope, but the moment you release the clutches, everything will go wrong.

The hardware version uses encoders on the scope's axels to tell the software when the scope as moved a predefined amount.  Pretty much the way that an old mouse with a ball works, as the wheel turns, the signals are read and the motors are driven to keep the motion constant.

Either way, once PEC is included in your setup, provided that you have good alignment, there should be much lower need to guide the scope from a mechanical point of view.  This means that in reality the guider won't have to work quite as much as it'll be concentrating more on the astmospheric conditions alone.

I'd still go on using Auto-guiding, but PEC should help.

[Thalestris24]

Hi

Since periodic error is a slowly changing tracking error then autoguiding should easily compensate for it. Also, as autoguiders don't get tired, there doesn't seem to be any point in PEC unless you are doing unguided exposures. I'd have thought it would be more useful to ensure proper balance for imaging and to tune out the AVX backlash.

Correct me if I'm wrong!

Louise

[Scorpius]

Everything that I've read and heard about PEC and Guiding is that the do not cancel each other out.   In fact if you have PEC you shouldn't need Guiding... AS MUCH.

Periodic Error is a mechanical error that occurs in the drive train of all telescopes.  It doesn't matter how well engineered the scopes are, there will be wear and tear that will alter the tracking characteristics of the scope over time.

There are two forms of PEC.  First, is the hardware version (Meade call this PPEC, Permenant Periodic Error Correction) and a software version (Meade call this Periodic Error Correction)

The software version is programmed by running the scope as it tracks a star.  On the Meade mounts (LX-90 for example) you can program this by starting the tracking, then guiding the scope yourself for about 10 minutes.  The guiding can be done manually, or with an autoguider.  What that will do is allow the software to calculate the error, and compensate as the gears turn.  Once programmed, this information can be retained by parking the scope, but the moment you release the clutches, everything will go wrong.

The hardware version uses encoders on the scope's axels to tell the software when the scope as moved a predefined amount.  Pretty much the way that an old mouse with a ball works, as the wheel turns, the signals are read and the motors are driven to keep the motion constant.

Either way, once PEC is included in your setup, provided that you have good alignment, there should be much lower need to guide the scope from a mechanical point of view.  This means that in reality the guider won't have to work quite as much as it'll be concentrating more on the astmospheric conditions alone.

I'd still go on using Auto-guiding, but PEC should help.

Thanks for the great explaination of PEC and I didn't realize there were two different types. Also didn't realize releasing the clutches would require PEC to be re-programmed. Did some more reading in the manual and it sounds like the recording process on the AVX takes about 10 mins and then you can "play it back" to compensate for mechanical errors which sounds straightforward enough. Thought I'd need some type of software to set PEC but the manual makes it sound like it's all done within the mount's onboard electronics.

One thing I noticed is Celestron calls it Permanent Periodic Error Correction which is somewhat misleading since it doesn't sound like the permanent hardware solution you described.

Anyway, I'll probably give it a try in conjunction with guiding just to see if it makes a difference on the PHD graph.

Thanks again for the great info...

Regards,

Scorpius

[Scorpius]

Hi

Since periodic error is a slowly changing tracking error then autoguiding should easily compensate for it. Also, as autoguiders don't get tired, there doesn't seem to be any point in PEC unless you are doing unguided exposures. I'd have thought it would be more useful to ensure proper balance for imaging and to tune out the AVX backlash.

Correct me if I'm wrong!

Louise

Hi Louise, yes - I always try and balance the scope but probably do need to check those backlash settings Maybe PEC isn't really necessary when guiding but I'm curious about how it works and if I might see a slight improvement in tracking.

I noticed you have an AVX as well and wondered if you've tried any of the more obscure settings like RA/DEC index and cone value. If so, maybe you'd be willing to explain because I don't understand what they do. Just curious, what backlash settings do you use on your AVX?

Thanks,

Scorpius

[Gazabone]

Thanks, I found that useful. Still think I might need to invest in an auto guider at some point.

[Thalestris24]

Hi

I'm afraid I've not gotten round to fiddling with the settings even though I know I have some backlash. The first thing to do would be to adjust the motors. But that means taking the scope off and removing the covers. I should really take the trouble and have a go...

One of these days...

Louise

[Thalestris24]

Here's a useful video

[cjdawson]

Thanks for the great explaination of PEC and I didn't realize there were two different types. Also didn't realize releasing the clutches would require PEC to be re-programmed. Did some more reading in the manual and it sounds like the recording process on the AVX takes about 10 mins and then you can "play it back" to compensate for mechanical errors which sounds straightforward enough. Thought I'd need some type of software to set PEC but the manual makes it sound like it's all done within the mount's onboard electronics.

One thing I noticed is Celestron calls it Permanent Periodic Error Correction which is somewhat misleading since it doesn't sound like the permanent hardware solution you described.

Anyway, I'll probably give it a try in conjunction with guiding just to see if it makes a difference on the PHD graph.

Thanks again for the great info...

Regards,

Scorpius

That sounds like the Celetron marketing people have been at work.  The important thing about PEC is, the manual says that it'll keep the settings even if you turn off the scope, and unlock the clutches.  If it's programmed, then it's more likely a software routine that's playing a memory map back, instead of actually running some observable feedback loop.  They might be calling it permanent because it can be programmed once then remain in place, but it'll still be a software routine.   I suspect that it fits with my description of software based PEC rather than hardware based.   The point is that as it's not actually observing something, errors can creep in and make things worse, not better.

[Scorpius]

That sounds like the Celetron marketing people have been at work.  The important thing about PEC is, the manual says that it'll keep the settings even if you turn off the scope, and unlock the clutches.  If it's programmed, then it's more likely a software routine that's playing a memory map back, instead of actually running some observable feedback loop.  They might be calling it permanent because it can be programmed once then remain in place, but it'll still be a software routine.   I suspect that it fits with my description of software based PEC rather than hardware based.   The point is that as it's not actually observing something, errors can creep in and make things worse, not better.

Went back & checked & the manual actually calls it - Permanently Programmable Periodic Error Correction - I left out the word programmable before which confirms it's a software solution just like you said