JeffMorgan The Guide Camera module in ASIAir has "presets" for Gain: Low, Medium, and High. But there is also a slider control for settings within (and beyond) the preset values. I'm wondering if the presets are just a marketing consideration (as easy as 1-2-3)?
The ASIAIR presets are not very meaningful for guiding. Low is usually zero analog gain (for highest dynamic range), high is usually some arbitrary value (and usually not really that high for the short exposure case), and often a medium value that could correspond to what is called "unity gain" (that is when one ADU corresponds to one electrophoton).
Gain 0 is to avoid saturation; but as we discussed earlier, you can (and in fact good to) allow brighter stars to saturate and obtain a larger population of stars to do multi-star guiding -- so you most likely want to avoid Low gain -- that setting is for when you don't want any color component of a star to saturate, else you loose star color for that bright star. We don't care about that for guiding -- we will be tossing the saturated bright stars anyway; and we won't be using color filters.
What you want to do is to experiment a bit with the gain slider, perhaps changing 6 dB (60 ZWO gain steps) at a time, and then fine tuning with 2 or 3 dB (20 to 30 ZWO gain steps) to look for a sweet spot. With multi-star guiding, there is a sweet range where you get 12 (in the case of ZWO's multi-star guiding) stars. The sweet spot will depend on the transparency of the sky that night, the seeing of the sky that night, the region of the sky you are autoguiding in, etc. You can't use the same value even on the same night -- this is one of the things where I shake my head when I read of someone asking for a preset number from someone else (be it gain, or exposure time, or calibration steps).
ZWO could have implemented an auto-gain pass to search for the sweet spot for the user, but instead waste their development effort on the "look, shiny!" stuff to please the Facebook selfie takers. As I have mentioned many times earlier, the mantra "Simple as 1,2,3" is the result of not including useful features (especially the missing parameters in autoguiding, like MinMo). If other programs start chopping functionality, they would be "Simple as 1,2,3" too!
In short, forget the L, M, H nonsense, and use the gain slider instead. If you only have the choice of L, M and H, you may not even be able to achieve 12 stars for guiding.
Just remember that there is a sweet spot (or region). If you use too low a gain, the read noise will eat you alive with the short exposures. If you use too high a gain, all the good stars become saturated, and you are left guiding with lower SNR of the really dim stars (i.e., again, SNR of the usable stars degrade). There is a goldilocks region -- once you find the range of gains that give you 12 stars, dial in the lowest gain within that range. (By the way, some of the selected stars in the ZWO user interface could be hiding behind useless ("look, shiny!") panes, or faded behind the guide graph area.)
I had forgotten that I changed my exposure times from 2 seconds to 1 second.
Mounts like the Avalons and the RainbowAstro are even happier with shorter exposure times. I have been using an RST-135 since that model was introduced a couple of years back. My earlier Takahashi EM-11 could take longer exposure times; but I seldom take it outdoors nowadays.
Predominantly, the mounts that benefit from short exposure times (== fast frame rates) have very large inherent periodic error amplitude. But the periodic error is mostly sinusoidal (with very little higher harmonics), and most importantly, they have zero or very low backlash. I had mentioned this a few times in the past -- the fast frame rate requirement has to do with the need to overcome the rate of change (d/dt for you engineers or scientists) of the gear errors so the feedback loop won't be "chasing the error" and actually cause instability. In more extreme cases, short exposures also avoid stars turning into short streaks, which is really detrimental to finding the centroid.
If you don't have a premium mount (say a 10Micron class) that is very well polar aligned, you really want to avoid long guide exposure times that are over 5 seconds.
With my RST135, a frame rate of 2 (thus exposure time of less than 0.5 seconds) easily beats out a frame rate of 1 (1 second exposure) in spite of losing SNR. The SNR deteriorates with shorter exposure times. That is why I plan on experimenting with Kalman filtering and also motion deblurring techniques to let me still use 1 second exposures -- not done with ASIAIR of course -- my experimental software platform is based on INDIGO and runs on macOS; the Mac Studio Ultra has oodles of compute power.
If you are sure that you have low backlash (for example, a Harmonic drive or belt-driven mount), you might try 0.5 second exposures to see if it gets better. Now, I wouldn't do that if you already don't have enough SNR. I use a guide scope with a 55mm objective. You can pretty much forget about using very short exposure time if you are photon starved (small guide scopes or OAG).
Have fun with experimenting -- this is what technical hobbies are all about.
Chen