Kring my setup does have an ASI174mm for the OAG on C11 2800mm focal length, even with reducer it comes in around 1960mm
Yep, I saw that later.
Your 174 on the OAG might be limited by the prism size, have you checked for that (vignetting) possibility? There are some OAG out there with larger prism sizes than what ZWO sells. Teleskop-Optiks has one with a whopping 16mm x 16mm, but depending on how it is positioned, it might end up vignetting your main camera :-).
I still have a couple of OAGs lying around here, but have avoided using them for years now. With a separate guide scope, you just need to watch out for differential flexure; everything else is so much easier.
A lot of the old wives tales (like using longer exposures) no longer applies when the centroid can now be cleanly estimated with multiple stars. The shorter exposures allow for better control of the feedback loop. But with an OAG, you probably still need to expose for longer to gather enough photons -- and that longer exposure length can limit how much improvement you can get from using multiple stars.
As of Build 9 of the 1.6 beta, the disappearing star bug has not yet been fixed. When a star is lost, it never comes back even after seeing has improved back for that particular star. So, even if you start with 4 or 5 stars, you might find that the extra stars would disappear (when the atmosphereic turbulence takes that star out) over time.
Up until Build 9 of the Beta, the only star that comes back is the principal guide star. But there is also a bug where no centroid is computed when that guide star is lost -- all centroid computation ceases until that principal star comes back.
The multi-star works really well when you don't hit one of these conditions.
However, a couple of other bugs were fixed in Build 9 -- I had found that if the Guide Window is opened for a lengthy time (sometimes for over an hour), ASIAIR would disconnect the Guide Camera and the Mount. They finally found the source of the bug after I gave them the precise particulars on how to make that bug appear, and Build 9 apparently fixed that.
I have in the past 2 days found another multi-star centroid bug that occurs when a star crosses a pixel boundary. This could cause the guide pulse to issue a wrong sign to the mount (i.e., exacerbates the error!! :-).
Where (arc seconds wise) it occurs depends on your guide plate scale (since the bug is directly related to the pixel size). With my guide scope, that pixel boundary occurs if the instantaneous guide error is at 2.3 arc seconds. With a longer focal length, that pixel boundary could be less than 0.5 arc second, and would throw self-inflicted guide errors quite often. You may be safe with an ASI174 (big fat pixels) and a reducer with your long focal length telescope. ASIAIR could just be confusing the truncation operation from the rounding function in the arithmetic. I gave them detailed descriptions to how to reproduce it indoors without clear skies.
The pixel boundary problem can cause a sudden momentary spike in error on the guide graph, and then it eventually calms down again after 2 or three guide pulses (and may be oscillatory as the centroid moves back and forth across the pixel boundary). Since you have an OAG, you might see it more often than I see it (I had to use my mount simulator to discover why it actually happened). In my case, since the error seldom ever gets that large with my mount, it was initially hard to find. But an unbalanced mount (that does not use harmonic drive gears like my mount) could easily drift that 1 pixel even if the fluctuations is much less than 1 pixel.
By the way, this pixel boundary bug does not happen to the single star centroid computation.
There are now two, what could be show stoppers with multiple stars autoguiding (you need a stable artificial condition to measure the harm they cause) -- the disappearing-and-not-recovering stars case, and now the pixel boundary centroid error.
Anyway, these things are reasons why there are Betas.
Chen
