Kevin_A On my fast scopes 1s is ok but not my f5.6 scopes.
It is a consequence of how ZWO "autofocuses." It uses a fixed exposure value (1 sec, in your example, at some fixed gain value that you have chosen).
It then defocuses to get HFD around 6 to 15 range. When you do that, the photons from a star are spread over many pixels, like 100 or more pixels. I.e, if is like exposing for 1/100 seconds. If the brightest stars are too dim at 10 millisecond exposure, then ASIAIR fails on the very first attempt to get a point on the curve.
This is why narrowband filters also need longer exposures.
ZWO should have changed exposure value as the star HFD gets larger and smaller (i.e., as the same number of photons that are spread over different number of pixels change). Again, it shows the complete uneducated approach from the folks at ZWO (yes, I know because I exchange email with them; that is exactly why I would not touch their mounts with a 10 foot pole).
The other consequence of ZWO's approach is that to keep a star unsaturated (to compute HFD) when the exposure value is kept constant, is that they have to use different stars at different EAF (thus HFD). Just imagine what that does to an OTA that does not have perfect field flatness. Remember the method I use to backfocus? Based on the fact that focus is different for different parts of the frame when the image plane is not flat. And that is what ZWO's autofocus is doing -- it is picking different stars and thus different HFD as it goes through autofocus.
Here is a secret (I don't think anyone else will tell you)-- ASIAIR autofocus is actually quite accurate when the field is flat! Just excruciatingly, and unnessarily slow. When the backfocus is not perfect (i.e. HFD is more than 10% different across the frame), ASIAIR fails miserably since can't get an accurate curve -- the points on the curve comes from different stars and they have different focus when the field is not flat. But that is also where a high quality Bahtinov mask will show different focus across the frame too.
It is just that when we use the Bahtinov mask, we focus on one fixed star! (Usually 1/3 of the way out from optical axis.) If ZWO had used different exposure values by adjusting exposure time and gain, and also use a single fixed star to make its curve, it would be just as accurate as a Bahtinov mask, if not more so (without additional tools, it is not easy to find the perfect focus of a Bahtinov spike, either). I have even had the Sigma 40 and the Samyang 135 produce better ASIAIR autofocus than simply casually judging the Bahtinov mask without using rulers, etc to measure the spikes. But the backfocus must be right. The different stars must be perfectly round.
Personally, I would pick the brightest star in the field that is between 25% to 50% away from the optical axis, and then change exposure values on it to keep the fixed star from saturating as it gets in and out of focus. We know that no star (except the Sun) will saturate a sensor at 1 milliisecond type exposures, while picking the brightest one will allow the star to have enough SNR when it is way out for focus, without having to resort to exposures that are longer than 2 or 3 seconds. But science and critical thinking is not ZWO's strong point.
So, for now, unless you have a perfectly flat field, (that is why the Askar backfocus adjuster is an essential tool), a Bahtinov mask easily beats out the ASIAIR. Again, pity the people in this hobby who depend on Easy as 1,2,3 instead of doing some critical thinking. I have always said that an experienced person will not have problems with the ASIAIR. It is the TikTokers who have problems with the ASIAIR. The experienced user knows what part of ASIAIR (and when) does not work, and know what work-arounds to use (like using a Bahtinov mask when their OTA is not flat). Note that TikTokers who depend on "55mm backfocus" will also not have correct backfocus, and therefore poor autofocus. BTW, this is why the SeeStar images are so out of focus; they advertise "APO," but don't tell you that the glass lacks a flattener. The gullible will fall for the techincal word salad everytime. I'll bet 99% of the TikTokers don't know the difference between "APO" and flatness of the image plane. Not sure 99% of ZWO developers know either.
Chen
P.S. 10:40am: the machined parts for the Nikon tilt adjustment is here.
