Why worry about PEC?

[Geoff45 (Geoff)]

Is PEC in today's mounts really important? It seems to me that most people autoguide, so if the mount is halway reasonable and the software does its job properly, PE should be guided out. I would have thought that PEC would only have been useful for manual guiding, which not many people still do.
Or am I missing something?
Geoff

[citivolus (Ric)]

PEC reduces the larger mechanical, predictable errors caused by gears not being perfectly round. Guiding can then concentrate on the smaller residual random errors caused by slop, vibration, mount shift, etc.

[leon]

Geoff, I have to agree with you, I have a G11 Mount and have never bothered with PEC, and my short exposures were quite OK.

My set up is very accurately aligned, and now that i have learned auto guiding recently it hums along spot on with very long exposures, without any help from PEC,

Leon

[rogerg (Roger)]

Guiding is re-active, PEC is pro-active.

It makes a real difference for me, I don't think I would've been able to achieve the sharpness of image and depth of exposures at the image scale I'm at if I didn't have PEC.

For short exposures or short to medium focal lengths it might not matter.

[tempestwizz (Brian)]

I have a few questions...

GetOutOfJailFreeCard
With an Alt/Az mount exposure times are critical, and in my experience would suggest keeping them below 10 Seconds. I trust we are not talking about Alt/Az mounts in this thread.

1. PEC is important for non-guided photo sessions. Guiding, if effective, takes over from that. Where is the cut-over? 10 seconds, 30 seconds, 180 seconds,300 seconds, more?
2. Where does the quality of the mount come into play, (how can Takahashi, AP and other high-priced mounts justify the expense?) and where does electronic feedback take over?
3. Surely, well machined mounts have minimal (or at least predictable, and therefore correctable PEC), and if properly aligned, there should be little need for guiding.... or is there?

4. In a permanent, perfectly polar-aligned world, is not the only issue PEC, or tracking rate? (I acknowledge that portable , and less-permanent (aligned)installations introduce DEC issues, and guiding may then be of benefit)

[In all of this, I also recognise the diffraction issues about near-horizon tracking. I am focussing on things more near our current Zenith as it progresses]

P'ing into the wind for me, since I have recently committed to one of the better mount systems. I'm hoping the native tracking results justify the ecpense.
As Murphy insists, I'm not getting a chance to play with my new toy for at least several weeks now.

Retirement is looking better by the day!

Interested in comments...

vbr

[g__day (Matthew)]

If you want the best you can do - reduce every error you can. That's it in a nutshell. Periodic errors can be spotted and removed - so why not do that - means less work and fewer corrections for the guider.

No one solution is perfect - but the better you can tweak all your gear and processes - the closer to terrific results you'll achieve.

[citivolus (Ric)]

While looking into this a bit further, I ran into a thread on a mailing list from 10 years ago, which brings up an interesting point:

http://astro.umsystem.edu/apml/ARCHI.../msg00523.html

Do modern mounts over-ride PEC when receiving guiding input, or do they use it to augment PEC replay? If the behaviour has not changed, PEC may well still be mooted when autoguiding.

On a side note, I once bought a scope from a man who thought he had to program his PEC every night. Yes, he sat there for 30 minutes every time he went out, manually training & checking his PEC. I think he was a masochist.

[Dennis]

Originally Posted by citivolus

>snip
On a side note, I once bought a scope from a man who thought he had to program his PEC every night. Yes, he sat there for 30 minutes every time he went out, manually training & checking his PEC. I think he was a masochist.

Sounds like a pretty onerous task! I think some early models (maybe Celestron) did not have PEC retained in memory. So, when you powered off the mount, the PEC settings were lost. Meade then introduced PPEC (Permanent/Programmable PEC) which was retained when power was removed from the mount.

On the wonders of modern electronics eh.

Cheers

Dennis

[AndrewJ]

Just a bit of info on the Meades

Do modern mounts over-ride PEC when receiving guiding input, or do they use it to augment PEC replay?

In the Meades, they have a std base tracking rate based on sidereal
When PEC is on, the system adjusts this rate once every 2.4seconds ( approx ) when polar. This new base rate is the std rate plus PEC adjust.
All handbox or guider adjusts then use this new rate as their base rate,
so the PEC isnt "overridden" when a guide request is sent.
Thus, for scopes with really bad PE, having a good PEC model can greatly reduced the load on a guider.

Eg, there is one user who has a PE of approx 120arcsec.
Looking at his data, this 120arcsec occurs over approx 168 secondstime )
Sooo, without a PEC model, the guider will always be playing catchup to the tune of 0.7arcsec per second.

The Meade gears are straight cut, and have a very wide variation in production tolerances, hence the data from each scope is different.
Best answer is try it and see.

Andrew

[g__day (Matthew)]

I guess if you're doing long duration shots - you hope for minimal gudie corrections. If corrections are very minor and more than 5 seconds apart things are probably pleasing for you. If corrections occur erratically and are major - forget autoguiding everything out - maybe re-think your imaging set-up and environment.

[JohnH]

I have a Vixen SXW mount and PEC has been a recent addition to that mounts capabilities. For me the low smooth PE of this mount was one of the major resons for purchase, PE guides out well. I hope PEC would improve things further - for my setup it did not. I do not worry about PEC anymore. For unguided/manually guided shots it would be useful...

[[1ponders] (Paul)]

I'm in the same boat as JohnH on this one. I have the Digital Drive on my Losmandy and with a PE of only 15 arcsec I found the PE created more dramas than it solved. I autoguiding fights more with seeing than it does with PE.

Now my GM8...That's another story