Jhaunton Have you taken a look at the performance of the EC equipped HD monuts
The two are actually quite similar. The "EC" mounts corrections are based on comparing the current geared shaft position relative to a precise optical encoder. With auto-guiding, we are correcting based on (what really is more precise and with infinite resolution), the stars.
One big drawback with autoguiding is that we have to look through atmospheric turbulence. By using centroids of multiple stars that measurement much more accurate (error variances half when you double the number of equal SNR stars). At least accurate enough that the rest of the autoguiding mechanism becomes the tall tent pole.
The other drawback, a much greater one, is that you can sample an optical encoder in a very short duration, thus, freezing the reading (close to an ideal Dirac delta sampling). With autoguiding, the stars are moving as you expose them for the duration of the guide exposure. So, you will always get a star trail, of which you need to find the centroid from, not the most ideal thing.
But this is also where many multiple corrections also come in, you never hit the mount with a large enough pulse to cause the guide star to instantaneously move much, and then you don't allow the star trail for long before you hit the mount with yet another small correction. Instead of a large sawtooth waveform (where the leading edge of the sawtooth is the pulse correction, and traiing edge is the mount's periodic error), you are breaking the large amplitude waveform into two or more shorter and smaller amplitude sawtooths.
This last phenomenon is greatly reduced by using that "guide using guide rate" paradigm that I mention earlier. I.e., you send a single pulse, like today per guide camera exposure -- but you don't change the pulse duration. You use a pulse duration that is as long as the exposure time. Instead of controlling the mount movement by the pulse length, you control it by puse amplitude (i.e., pulse sidereal rate). Essentially, you are sending a correction that is equal to the current slope of the mount's periodic error curve. The slope of the curve itself (i.e., the second drivative of the curve) does not vary by much every 2 or even 5 seconds.
But this does not appear to be the case - even for the Renshaw equipped RST-135E, the residual PE is still +/- 5" or so.
Yep, the "optical encoder in the sky" has much higher resolution.
However, if executed properly (allowing both correction from the optical encoder, and from mount commands), the mounts with high resolution encoders can help a lot in allowing longer guide exposures when autoguiding. Unlike trying to simply reduce the peak-to-peak error from the gears themselves, a high end encoder reduce the error without including the higher harmonics. This is why in an earlier posting, I had replied to ZWO that it is more important to reduce the harmonics, instead of just reducing the amplitude of the strain wave gear.
One other advantage of optical encoders is of course that they are functional in the daytime. For that, I have added a Hutech Hinode [sunrise, in Japanese, and also a NASA mission to the Sun] solar guider that I have been using for many years now. Not cheap (and needs to mount to support ST-4 :-), but worth its weight in gold if you do any daytime work .
https://astrohutech.store/product/hinode-solar-guider/
Chen
