What's the point of training your mount to reduce PEC which is effectively taken out when you autoguide anyway?

Some say yes, some say no... I say... back to basics....

PEC stands for “periodic error correction”. PE is “periodic error”. As you say recall from my photos, those big worm wheels (7, 8 or 11 inches in your case!) are turned by a small threaded shaft called a worm gear, with a diameter of about 20-30mm. The worm gear is turned at a constant rate. If the worm gear (the little threaded shaft) is perfect, then the mount will track in RA perfectly.

That threaded shaft is held at each end by two bearings. These bear on a flat part of the shaft. But, however good you are at machining, the flat part of the shaft will never be perfectly concentric with the threaded bit.

Read the tolerance issues here http://www.mini-lathe.org.uk/making-telescope-worm-wheels-gears-mini-lathe.shtml

Now, that slight eccentricity results in the speed of the tracking to go over-speed or under-speed in a cycle.

e.g. http://www.freewebs.com/eq6mod/D1.jpg

If the worm shaft is well made, this error will be smooth, and highly predictable. For a 360 tooth gear it will repeat every 86164/360 = 239.3 seconds. A paramount ME is 576 teeth, so 149.6 seconds.

This is the gross error arising from the gears in the drive chain.

Higher frequency error can also be superimposed over this low frequency cycle due to reductions gears closer to the motor, but ignore that for a moment.

Because this periodic error is supposed to be highly predictable around the entire worm wheel (because it arises from the fast rotating worm gear shaft), the mount can be taught the shape of the curve and correct for this error by speeding up and slowing down the drive motor accordingly. The mount will learn the shape of the curve by watching a star during the cycle of a worm rotation (149s) – actually you normally average many cycles of observation. The mount will also contain a sensor so it knows what position the worm was in at startup. (so it knows what point in the correction table to start from)

Once this gross error is mostly corrected by the periodic error correction (PEC) then the strain on the autoguiding system is much reduced. The guiding only has to cope with the smaller errors which arise from
• Tooth – to – tooth differences on the worm wheel
• Flex in the mount/scopes etc
• Diffraction
• Error in polar alignment
• Transients (E.g. speck of grit in the system)
• Drift in DEC (cause by all of the above)
• Higher frequency periodic errors. These have different periods. Go play with an FFT program if you want to analyse them.

If the periodic error arising from the gross eccentricity of the worm shaft is both smooth and predicable, then periodic error correction will reduce the strain on the autoguiding system and therefore make things better. Think of PEC as correcting tracking errors before they have occurred, unlike autoguiding which can only correct error after it has occurred, the former is obviously preferred. The PEC is also averaging and smoothing the correction over a few seconds instead of the autoguiding making a series of sharp correction every few seconds.

However, if the PEC training is poor, or the periodic error is unpredictable due to poor engineering, the PE Correction will do a poor job. The autoguiding and the Periodic error correction will fight each other and produce an inferior result.

You must always autoguide over longer exposures. The items in the list above dictate this. No mount can correct for transients and flex!

With a good mounty, the PE will be smooth and regular, thus PEC will work well with autoguiding, but to be honest you are only going to visually notice the difference if you are imaging around 1”/pixel instead of your wide field stuff. That said, even in wide field, the better the tracking, the better the signal to noise ratio in the fine detail of nebs and galaxies, even if you can’t detect the drift by eye.

In summary, if the mount is well made, PEC reduces the strain on the autoguiding and ultimately produces better SNR, but if the PE is unpredictable, or the guiding system poor (flexure, seeing etc) then the two will fight and cause a problem.

Practically I would start with the autoguiding and play with PEC when your are bored one night in the full moon.