heyjp What is the Duration analogy?
Greetings from a fellow EE (long past "sell by" date; took my first control theory class in about 1967 :-).
The first thing to grok is the relationship between time and motor angle.
The reason it is colloquially called "pulse guiding" is that to move the mount by x arc seconds, the computer would issue a slew command (yep, hobby astronomy is that primitive). Then t seconds later, it would issue a stop slew command. t is the "pulse duration."
The time scale (relationship between x and t) is computed during calibration time. The camera angle is also computed during calibration, so that the x-y corrections that are needed can be related to the RA-declination slews on the mount. (FWIW, I try to adjust my guide camera angle so that the RA-dec vectors are orthogonal to the x-y camera angle in the Calibration Data picture. This way, the rotation matrix has very small cross terms, to completely prevent any numerical errors.)
I do not know the internals of ASIAIR, but my believe is that the MAX value is not any part of the loop gain. I think that it is simply the value where the feedback "voltage" of a loop is clipped (i.e., where a linear control turns into a "bang-bang" control (to use control theory analogy). My guess is that it is there just to prevent a large instantaneous turbulence or wind error to produce a large unnecessary feedback. I have been estimating the periodic error of my mount (very large for a RainbowAstro RST-135), and set the MAX to to where the MAX pulse is sufficient to overcome the slope of the periodic error.
So your intuition is perfectly correct. With mounts that have small and sinusoidal periodic error, something like 100 milliseconds is sufficient as a MAX guide pulse duration. With sloppy gears, you may need to increase it so that you can keep track with deviations from smooth sinusoids (you often see people measure the harmonics of the periodic error).
That being said, with multi-star centroiding, we now get fewer wrong corections, so a larger MAX value is no longer as harmful.
Aggressiveness is closer to the analogy of the loop gain. (Again, with multi-star centroiding you can tweak the loop gain up higher -- in the past, a mistake in estimating the centroid will make the mount swing unnecessarily, requiring a pulse almost immediately in the opposite direction, and starting an oscillatory motion).
The guide rate, on the other hand, contributes pretty directly to the loop gain. This is why you need to recalibrate when you change the guide rate. I.e., angular movement = guide rate x pulse duration.
Mounts that have large periodic error slope requires a larger guide rate to keep up with the rate of change of the periodic error. However, using a large guide rate also makes any latency in the pulse timing more critical.
As an example, a 9600 baud serial link takes about 1 millisecond to send a single byte of the command to the mount. A typical command is 5 to 10 bytes. So, you potentially have a 10ms inaccuracy (unless it is taken out during calibration). This 10ms inaccuracy translates to a larger angle inaccuracy for larger guide rates. So, use as small a guide rate as you can get aways with. This is why ST-4 guiding can be much more precise than pulse guiding. I still marvel at how smoothly the Hinode Solar Guider guides my mount using ST-4. I still use that with my etalon since ASIAIR is all but useless for daytime guiding.
Chen
