KC_Astro_Mutt I wish I understood how all of this works as well. I'm afraid I'll be of little help in that regard.
Go back and look for a post of mine that shows a sawtooth waveform. Post 43 on this thread:
https://bbs.astronomy-imaging-camera.com/d/15989-getting-the-best-performance-from-my-am5/84
Once you understand that, all will be clear. That sawtooth is the basis (as I discovered after looking into the actual feedback mechanism) of pulse autoguiding. Pulse guiding does not instantaneously move the mount. The mount slews at two different rates throughout autoguiding, and the movement never stops -- the guide star is always moving during an exposure, and the smaller the sawtooth, the smaller will the guide star movement be. (And since the main OTA is sitting on the same mount, the main OTA will also be moving along the same sawtooth.)
There are two slew rates during autoduing. The rate at the leading edge of the sawtooth is caused by the guide pulse (and if you are pedantic, minus the slope of the periodic error). The rate at the trailing edge is (opposite direction) the slope of the mount's periodic error.
You track a star when the amplitude leading edge of the sawtooth is precisely the same as amplitude of the trailing edge of the sawtooth.
In your case, the slope of your periodic error (trailing edge) is small enough that with a pulse that is no more than 100ms, it is sufficient to overcome the trailing edge of the mount.
Remember that the leading edge is quite violent -- at 0.5x sidereal rate, the slope of the leading edge is 7.5 arcsecond/second (minus the slope of the mount), while the trailing edge is only around 0.1 to 0.4 arcsec per second for most mounts. If your mount's PE slope comes in at 0.5 arsec/second, it is best to get rid of it.
This is why you can often (won't work with the crummier ZWO mounts with terrible slopes) use an even milder than 0.5x sidereal rate (with my RST135, I got away with 0.25x sidereal and 160ms max pulse durations -- but again do not blindly copy that; that number won't work for you. Measure your own mount's slope and use that to determine the max pulse duration given a guide rate. Even if you go out and buy an RST-135, you still cannot use my numbers since the strain wave gears perform quite differently even from the same manufacturing batch -- check out that NASA conference paper that I had posted before to see the details of strain wave gears (not specific to guiding).
Yes, it is too bad you need to apply some technical skills. This is a technical hobby. Until the manufacturers themselves start to understand the mounts that they are selling and give you instructions on how to guide their mounts, the unwashed will just have to live with larger guide errors than their mount is capable of.
But I would venture to guess that at least half of the ZWO mounts (before they changed over to 288 second period) can guide below 0.4" total RMS through a whole night. For the mounts that have large slopes, just chalk it up to the ZWO lottery, where their QC did not measure and reject the ones with poor slopes (the geniuses were probably rejecting perfectly auto-guidable mounts that have large PE amplitude, instead of rejecting mounts that have large PE slopes. The 288 second period will make the slope worse, of course.
Chen
