PEC

[CSM]

This may seem a dumb question but in doing PEC training with my Celestron SCT AVX mount presumably I should do it with the focal reducer as otherwise the corrections will be exaggerated?

[jambouk]

I'd imagine you want to do PEC training with the longest possible focal length to exagerate all errors and maximise them being detected and recorded.

But I've only ever tried to do PEC training once, and On that occasion I was just messing around as there was cloud all over the place and used Mars as the target... How not to do PEC training

James

[CSM]

Hi James - if the corrections are 'exaggerated' when recording then when these are played back with the focal reducers will the corrections not be too large?  Or am I missing something?

[jambouk]

CSM, I am not really the right person to be answering questions on this, there will be people who have used this to good effect. My understanding is that if you had say a 50mm lens on a camera on the mount, you wouldn't even notice any periodic error unles sit was absolutely horrendous, in a similar way which poor PA is better tolerated with short focal lengths. As soon as you shove on a 4000mm focal length scope periodic error (or poor PA) soon becomee very obvious. It is likely even at a very long focal length that the corrections needed will be very small; when applying the corrections, the corrections will bee applied the same, whatever the focal length lf your scope, so in my limited understanding no, the corrections will be the corrections whatever.

You might need to repost the question in the mount thread further down, where more advanced mount members will be able to advise.

Good luck.

James

[Oddsocks]

When you do the PEC training it does not matter what focal length the system is set up for.

Assuming you are creating your PEC correction visually then you will be using a reticle eyepiece, starting with the star centred you apply a sufficient correction to the RA drive each time the star moves away from centre to bring it back to centre.

For example you might need a two second push of the RA drive at a guiding speed of 25% sidereal rate to bring the star back to the centre position.

Now change the focal length by adding a barlow that doubles your focal length and what happens?

The star will appear to move a larger ammount in a given time in the eyepiece but the correction you input to bring it back to centre is exactly the same as before, two seconds at 25% sidereal rate brings the star back to centre.

When you build a PEC curve you are correcting for a mechanical repeating error in the worm gear and this error is totally independent of focal length, it will be the same with a 2mtr focal length telescope as it is with a 200mm camera lens, the periodic error only appears larger or smaller in magnitude with changes in focal length but once you build the PEC curve the correction applies equally to any focal length.

Choose a focal length that you are comfortable with and suits the seeing conditions, if the atmosphere is very stable then add a barlow to the reticle eyepiece to increase the focal length, if you find the star is moving too quickly and too far for you to respond to then use the reticle eyepiece alone without a barlow.

The longer the focal length of the system will show more easily smaller periodic error ranges but seeing turbulence and other external non-periodic errors also become more pronounced.

Whatever you are happy with remember that you are correcting a fixed mechanical error only and it is totally independent of the telescope focal length, the PEC correction curve you build will work whatever telescope or focal reducer you use or swap around, you don't need a new PEC curve each time you change telescopes, reducers, flattners, eyepieces etc.

William.

[jambouk]

Wlliam,

I've heard people say, and I might well have repeated what they've said without understanding it, that permenant PEC training is only really valid for that polar alignment, and once you dosmantle the kit, re-assemble and do a new polar alignment, playing back the PPEC will be less "correct" due to the new polar alignment.

This has confused me, as presumably the PPEC playback is just correcting mechanical errors in the gears and things, which is independant of polar alignment. Yes, a new polar alignment may be poor and there may be drift when doing unguided subs, but that drift will be mostly poor PA than PE. Yes?

I am still amazed at how the handset can differential in errors in PA from periodic ones.

I'm tempted to have a proper go at PEC training as I can't be bothered woth guiding and need all the help I can get

James

[Oddsocks]

Hi James,

I think the confusion with PPEC being correct only for a one-time set up stems from the fact that when you create the original PEC file then polar alignment has to be as nearly perfect as possible otherwise you are building a correction curve that is correcting for both periodic error and polar drift. A PEC curve created under bad polar alignment conditions would indeed only be accurate while the mount remains in that particular polar alignment, also, depending how the PEC correction is handled by the mount software a PEC correction that contains both bad polar alignment and periodic error may behave erratically at the end of each worm cycle and cause the mount to "jump" introducing a bigger apparent periodic error than if you hadn't bothered with PEC in the first place.

So you are correct, as long as you have as near perfect polar alignment as possible when you carry out that PEC training run then the resulting correction curve will only contain the mechanical, repeating worm rotation error and when applied it will be equally valid in the future even if polar alignment is off for future mount setups, the only remaining drift in the aquired image will be due to polar misalignment.

When I bought a Gemini G41 mount with PPEC many years ago the mount builder used to ship the mount having tested and created the PEC file under observing conditions so that you could start using the mount out-of-the box and get the best out of it, clearly he wouldn't have bothered if the PEC became invalid if polar alignment was not the same as when he did the training run.

If you are not guiding then PEC is vital to improve your image, just take the time to carry out a good polar alignment first, use the polar alignment handset routine, confirm after alignment by at drift observation (with RA orientaion along one axis of the reticle eye piece polar mis-alignment will always show only in drift of the star at 90 degrees to the RA axis, you ignore any movement along the RA axis since you have not corrected for the periodic error in RA yet), once you are happy with the polar alignment go ahead and carry out the PEC training run and apply it.

For your future imaging sessions apply the stored PEC, get the best polar alignment you have time for and begin imaging. You will be getting the best from your mount under those conditions and will ultimately only achieve better by guiding.

It is worth noting that when using a guide camera for imaging that some mounts do not respond well to having PEC enabled together with correction inputs from the guide camera, it just depends how the PEC has been implemented in the mount software and it is a case of trial and error to find what works best for you. With my old G41 it always gave the best performance using both PPEC and a guide camera to correct the remaining non-periodic error, with my current MX mount it makes no obvious difference when guding via guide cam whether I have PEC enabled or not.

A long reply but I hope useful.

William.

[CSM]

Thanks William - I have just about got my head around it now!

[Oddsocks]

It does seem complicated at first and I think this tends to put people off trying PEC which is a shame since it is a useful tool for improving imaging performance.

One extra tip, when carrying out PEC training make sure you balance the telescope "east side heavy" so that your guide corrections are not influenced by backlash, you want the RA gear pressure against the worm biased in one direction during the training run.

Some people use extra weights added to one side of the OTA but elastic rope is just as effective, costs a few pounds and has the advantage of adding no mass to the mount.

I use 5mm diameter elastic cord, 2mtr long, bought from a chandlers where they call it "shock cord" and one end is tied to the OTA saddle, the other end tied to a leg of the tripod, or something heavy on the ground on the east side, and shortened enough to provide a steady pull equivalent to hanging a 1 to 2 kg weight on the OTA east side.

Just be carefull not to trip over the cord in the dark and to untie it before slewing the telescope to a new direction.

William.

[jambouk]

Very useful. I will certainly have another go at PPEC in the future now you have put to bed my concerns about polar alignment interferring with it.

Great. Yseful thread; thanks.

James