What is fundamental with multi-star guiding as far as guide scopes, is to have at least 20 guidable stars available in a guide frame.
What this means is
a good guide scale. Oversampling will reduce the intensity of a star (and not be recognized), while undersampling will case centroids to be completely bogus. So it is more advantageous to oversample, as long as you make up the deficit.
a large sensor. A larger sensor will allow more area of the sky to be imaged, as long as the scope produces a flat enough field to handle the sensor. Just a larger sensor is not useful since any poorly shaped star that are away from the optical axis will do more harm than good. By the way, this has nothing to do with whether the scope is APO or not -- an APO objective converges three wavelengths of light to the same point; but with a refractor, those points will not be on the same sensor plane. You need a field flattener. (The is the biggest joke about the SeeStar. Notice that ZWO claims that it has an APO objective, but there is no flattener! None of the YouTube shills have pointed this out, by the way. ZWO always like making boneheaded design decisions like this -- just like putting emphasis with their mounts have small PE amplitude, while the real problem is the slope of the PE curve.)
at the same time, the sensor must not be so large that the guiding frame rate falls below 2 FPS.
the plate scale plus a large sensor may require a larger objective to get enough guidable stars (the 20+ that I mentioned earlier). You need enough photons.
good focus is imperative (my guide scopes all have an electronic focuser). A large blob of a star will spread the photons over too many pixels. The better focus, the more stars you will see in the guide frame.
Why 20 guidable stars? This has to do with the distrrbution of star intensities, and the fact that ASIAIR uses SNR as the weighting function; including the brightest stars will cause the averaged centroid to just depend on the brightest one or two stars. Even when guiding with 12 stars, you will effectively be guiding using only 2 or 3 stars. So don't get fooled by ASIAIR. You need to increase the gain of the camera so that the first two or three stars are saturated. That will cause ASIAIR to use the remaining stars, which have a more even intensity distribution (thus more even SNR).
Typically, an 10 dB increase in camera gain after attaining 12 stars will allow the brghtest stars to be rejected, but sometimes, you will need an additional 10 dB. ZWO uses a gain scale of 0.1 dB per gain unit, so be prepared to add a gain of 100 to 200 after being able to capture the first 12 stars.
I cannot overemphasize this aspect since picking the first 12 brightest stars may only give you the equivalent of 2 or 3 stars guiding (definitely insufficient for typical "seeing").
Getting a good centroid is only one aspect of the guding. It does make the difference between guiding to 0.35" (total RMS) vs guiding to 0.5". However, if your mount can only guide to 1" to start with, unless you are using ZWO's guide scope/ASI120 combo (really poor choice, I'm sorry for those who fell for it just because it is cheap), then improving your guide scope may not help.
Chen
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