astrosatch Despite apparently much lower QE of asi290, people still thinks it is much better choice than asi120.
I thought I had explained it earlier... the ASI290 has significantly better dynamic range.
With the extra dynamic range with single star tracking, if you set the gain up the guide star is close to saturating, say a peak of 200 (8 bit ADU) then that star can dim by an extra magnitude and still not be lost, when compared to the ASI120.
With multi-star guiding, once ASIAIR picks the primary star, it will have many more stars to pick the rest from with that extra dynamic range, since you have almost an extra stellar Magnitude of stars that are above the noise.
Recall that each magnitude of change brings up an extra 291% number of stars. You won't get all 291% increase in stars in practice for autoguiding because of other factors, but you do get a decent improvement when dynamic range is increased.
Notice in my screen shot above that ASIAIR could pick up stars with the ASI290MM that are not even visible on the iPad screen. Those stars are dim, but they are over the noise floor.
This is completely independent of the sensitivity. If a sensor has less sensitivity, you will just need to use more gain. That will bring up dark noise, but also relatively reduce the read noise. When you buy a camera, you need to balance those factors based on the exposure times and the optics you intend to use the camera for. Too many people on Facebook are blindly buying cameras that other people are using -- when the optical situations are completely different.
If you really want to use an OAG (again, I wouldn't do myself), and need a better FOV and sensitivity, to benefit from multi-star guiding you can for example find an OAG that has a larger prism than 9x9 mm, and use an ASI174MM.
That being said, I have been working with a couple of ZWO people on quickly re-picking lost stars, by initially coming up with a larger set of candidate stars at a high dynamic range (i.e., stacking), so star losses for multiple-star should improve in some future release.
I have seen professional telescopes use 14" SCTs as their guide scopes (pictures of old telescopes have these humongously long guide scope). This is the reason how the DONUTS algorithm (2013) got its name -- for being able to multi-guide when the stars are not in perfect focus with an SCT.
https://arxiv.org/abs/1304.2405
I have an ASI174MM (USB3) in the mail (should get there today) to further experiment with multistar guiding (a larger FOV, and sensitivity, but also different plate scale; I may have to add a Barlow to my guide scope, or assemble a guide scope with a longer focal length). I already have an ASI174mini, but it is being used on my mount simulator, and I wanted a camera with a USB3 interface, since ASIAIR is now using synchronous image captures for autoguiding. As you can see in the photos, the ASI290MM I use is already the USB3 version.
Chen