Browndog Were you able to carry out any other tests?
Yes, I was able to get a pretty clear night with low (less than 5 mph) winds yesterday evening.
The stars do look quite dim on the guide window, but I think that is just an artifact of ZWO displaying the raw data at a different brightness/contrast than before in the ASIAIR release.
At times, autoguiding would even give me 1-minute long stretches of better than 0.2" total RMS guiding, with the error distributed very evenly between RA error and Declination error (i.e., about 0.14" error each). Most of the time, it is between 0.3" and 0.4" with occasional wind that brings it above 0.5". (The "total RMS" in this case is just the square root of the sum of the squares of the RA and declination errors.)
Although the guide window display is dim, the auto guiding does appear to be working. It also appears to reject saturated stars (a couple of stars that are "brighter" on the guide window), and the primary star that it picks, although very dim, appears to have an (8-bit) ADU in the high 100's to low 200s.
This (rejecting the brighter stars) is the intended condition that I normally have my guiding gain set, so that I can get a more even distribution of star brightness from the stars that are selected in multi-star guiding. Each star magnitude (change of 2.51 in linear gain) gives about 3 times more visible stars, so high gains will reject (due to saturation) the brighter stars, and provide remaining stars with more even distribution of brightness (since the ASIAIR weighs the centroid errors by using Signal to noise).
What I had been doing is to find a gain that gives me 11 or 12 stars, and then add 10 dB (a linear gain boost of 3.16) to that gain for guiding. With ZWO gamersa, that 10 dB of gain is achieved by adding "100" to the gain. For example, if I see 11 stars at gain of 170 on a ZWO camera, I would increase the gain to 270. This will avoid having a bright star hogging the centroid averaging and the rest of the stars will have less effect, and thus you are reduced to the atmospheric turbulence (you will be chasing "seeing") from that bright star. Mult-star guiding depends on many stars to average out the variance since every star will be affected differently by atmospheic turbulence -- multi-star guiding depends on the centroid errors to average out. This might affect item (2) in your post.
I do not use an OAG, so I am not star starved compared to many OAG situations. Check how much gain is needed to get 11 to 12 stars, then add more gain. If you can't even see 12 stars to start with on your OAG, then you will be at a disadvantage. And it is advantageous to be able to apply 10 dB more gain after you have achieved 12 stars.
That being said, my guide scope (a FMA180) is not large (only 40mm aperture), but I made sure it has a reducer to keep the guide camera "flat" -- that way the average centroid remains moderately constant as SNR changes the weighing of the stars when each one is dimmed through scintillation. I also picked a monochrome sensor that has low readout noise (currently a ASI678MM when using ASIAIR, but I may switch to an IMX676 sensor in the future to get a larger sensor) to compensate for the small aperture (although I am happy enough with my guide results already). If you see distorted stars with different shapes with your OAG, that can affect autoguiding too, as the SNR weighing shifts among the stars that are picked.
But for now, I think you can be pretty certain that the ASIAIR release is not buggy with guiding (except for the dimmed display). So, perhaps concentrate on getting the prism stem as low as possible without vignetting the main camera sensor, and that will also reduce the aberrations with the off axis stars. Try to see if you can get 11 to 12 stars, and add even more gain.
Since you are using a decent worm geared mount that can use longer guide exposures (like 3 seconds), perhaps the thing that is better for you is to manually select a guide star in ASIAIR, so that it uses single star guiding. Just pick a star that gives you an ADU of between 150 to 220 or so. See if that is better. Albeit, trying to pick a star in the dimmed display is a pain :-).
Chen
