Just a comment here about "chasing the seeing."
If you watch a star scintillate using high frame rates (e.g., video mode at 10 FPS), you will see that the star does not only change brightness, the shape of the star also changes. This is caused by atmospheric turbulence ("seeing").
What this does is to cause the centroid estimate for the star to move around when you use that star as a guide star. (This is also why you cannot allow guide stars to be saturated, since the centroid is wildly wrong).
If you now guide at this frame rate, you will be trying to correct for atmospheric turbulence (which can be done, but that is a different topic in adaptive optics, and ASIAIR customers are not likely to afford them). Trying to guide to the fast frame rate centroids is "chasing the seeing."
Now, the way you get rid of the erratic centroid movements is to for example take two short exposures and average them (or, just take a longer exposure).
Each time you double the guide frame duration (or sum two short duration frames), the root mean square of the estimated centroid improves by a factor of square root of 2. Double again, and the centroid estimate improves by a factor of 2. So, a 2 second guide frame will have 2 times better centroid estimate than a 0.5 second guide frame.
Now, there is this property in probability theory called 'ergodicity," and applies to many practical cases where instead of looking at measuring the star twice, you measure two different stars.
I.e., instead of measuring a single guide star by 4 times longer, you get the same improvement by measuring 4 different stars at the original frame rate.
By using this "multiple star" scheme, you can use 4 stars at 0.5 seconds and end up "chasing the seeing" by the same amount as using 1 star at 2 seconds.
You are basically limited by how many stars you can pick up, and use as high a frame rate the processor is capable of.
Now, there is a bug in ASIAIR that causes even 9 or 10 stars to only improve by 2 stars. I have writting about this before (just do a forum search) and is caused by the distribution of stellar brightness, and ZWO using Signal-to-Noise ratio to weight the centroids.
So, with the ASIAIR, you will typically need 12 stars to even be equivalent to a 3 to 4 star case. But still that is better than nothing. But remember, your guide scope has to.
I have been using used scopes in between the Borg 55FL, the Askar 180 and the Vixen FL55SS for guiding at a frame rate in ASIAIR of 2 FPS using an ASI178M camera, and they work fine. Remember too that the star shapes have to be similar for averaging to work out (averaging two distorted stars and using signal-to-noise ratio weighting is disastrous). So make sure you have have a flattener/reducer on your guide scope too.
Short story -- the days of "chasing the seeing" are over when you use multiple star centroiding.
When do you need fast frame rates -- when you have a strin wave geared mount.
Basically, you need good guide systems (my Vixen FL55 probably cost more tan a typical small imaging telescope) if you want to use a strain wave geared mount. Either pay more for a better mount, or pay more for a better guide scope. There is no cheap way out.
Chen
