PEC button question

[funkysandman]

My telescope (Celestron C8 SCT Deluxe) is equipped with PEC (periodic error control).  The idea is that you manually guide the scope when it goes off course a little in RA (caused by worm gear irregularities).  You're supposed to do this for about 5 minutes or so, but my question is, do I have to do this every time I turn it on or does it remember for next time?  Wouldn't the irregularities be consistent?  I'm trying to get the most out of it before I spend any money on autoguiding gizmos.

[cjdawson]

I don't know if you have to do it every time.  This will really depend on the firmware in the scope.   I know that for my Meade LX-90, I do have to program it every time I want to use it, as it won't remember if I loosen a clutch, or power off the scope without parking it first.

Some more modern scopes will only need programming the once, then will remember the settings.  Others that use encoders, won't need programming at all (Meade LX-200 for example).  This really is down to the individual scope.

Having done a very quick look, it looks to me like you would have to program it each time. Personally, I'd not bother and take a serious look at autoguiding.  That pretty much negates the need for PEC.

That said, 5 mins at the start of a session to program PEC might help you to extend your imaging time a little.  But I think you'll struggle to get much longer than you are already achieving.  The big thing about Autoguiding is that the optical feedback does also take into account astmospheric conditions, so you don't always need that perfectly still night to be able to get an image - close to will suffice. ;-)

[funkysandman]

Thanks for the reply, didn't consider atmospheric variations that an autoguider would address

[D4N]

Guiding also addresses drift due to poor PA, although it won't cure field rotation caused by poor PA.

/Dan

Sent from my iPad using Tapatalk

[Owmuchonomy]

I think this is the difference between PEC and PPEC, the latter standing for permanent PEC, a feature in my new mount. However, after a fair amount of communication with Synta, involving test protocols for a dodgy worm, if you are guiding then PEC should be disabled. Other more experienced campaigners may know better than me. The suggested run time for your PEC is to ensure a full worm rotation is taken into account. The Synta protocol I ran for them covers 3 cycles!

[glowingturnip]

Hi Chris,

I've got the same mount as you, and just wondering what the gist of your conversations was re the PPEC should be turned off ?

I had a sweet run of guiding one night, less than about 0.4" error, so I turned the dithering off for a few runs and let it PPEC-train at the same time, and have been using it since.  I've not really experimented whether I get a better guide with or without it though.

[Owmuchonomy]

Hi Stuart not wishing to hijack this thread but briefly I complained about the obvious worm eccentricity in my mount. As a result Synta sent me their test protocol to prove as such. They state that when guiding any PEC should be disabled. Like you, I can't vouch for whether it makes any difference, yet. My replacement mount will receive its first test tonight.

[cjdawson]

Just to clear up some confusion that's happening here.  There's PEC & PPEC  These are not the same thing at all.  The confusion is that different manufacturers call them different things so here's my definitions.

PEC - Periodic error correction.

This is something that is sofware driven.  It's based on measurements taken from the scope normally be tracking a star for a rotation of the worm gear. Maybe longer.  This information will be saved as a correction map in the software of the mount. It may or may not survive a power cycle depending on the implementation.  This is a software best guess and based on the software knowning the position of the wormgear.

PPEC - Permanent periodic error correction

This is a hardware based implementation, that is completely integrated into the mount.  This uses a feedback loop, so that the mount can measure the actual amount of movement from the scope for a given movement from the worm gear.  This way the scope knows exactly how much the mount has moved compared to the amount it expected it to move.  This method is not a guess.

To explain the difference.  Let's use an analogy.

Stand up, hold a compass in your hand and make sure that it's pointing north, close your eye's, now turn 180° to face the opposite direction - keep your eye's closed. Open your eye's and you'll notice that you didn't turn exactly 180°  you'll be off by an amount.  This is because you had to guess, you could not see.   This is what the software in a mount is doing because it cannot actually measure the distance perfectly.  It's relying on it's own internal information, distance of a step in the stepper motor normally.  But, it's still a guess, it'll be a good guess, much better than your 180° turn, but it's still a guess.

Do the same thing, but this time keep your eye's open.  I'm fairly sure that this time, you'll be looking at the compass to make sure that you did the 180° exactly.  This time, you used something external to measure tell you how much you really turned.  This is the feedback.  You didn't have to program it, or tune it.  Only calibrate once and keep using.  The calibration for this is normally done at the factory.

Manufacturers will call PEC different things - programmable PEC, Temporary PEC, whatever you like.

They'll also call PPEC different things, Meade used to call PPEC PEC before they introduced PEC to the LX-90, They then started calling the PEC in the LX-200 as PPEC.  Confused yet?  ;-)

The LX-200 has PPEC - it uses optical encoders to feedback the movement (a bit like how an old ball mouse worked)

The LX-90 has PEC - it's programmable.

How does this relate to the real world?   If you have PPEC in your mount great, leave it on, it'll be helping the scope to track better.  In fact, you probable can't turn it off.  Your Autoguider shouldn't need to work as much to keep the star centered.

If you have PEC, turn it off, it's just adding complication to your setup.   Your autoguider is actively correcting with a visual feedback loop.  There is a chance that it could be working against your programmed PEC.  So make life easy and remove it from the equation.

Hope this helps to clear things up a bit.