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Hi Brian,

I was referring to the main image scale, not the guiding image scale. Yes, that is why I pay attention to the arc second number in PHD2, since it the more meaningful number in translating whether my guiding is good enough. Depending on the night and position in the sky, my guiding varies between 0.5 and 0.7" RMS. That's much lower than my image scale for 1.6"/px on my main camera and scope. Whether or not I am sub pixel as measured by my guide camera pixels is interesting but less material.?

For many people that use a separate guide scope, their guide camera image scale is most likely coarser than their main image scale, but not necessarily if you are using an off axis guider. For example, I use an ASI290MM as my guide cam, and an ASI1600 as my main cam. This makes my guide camera scale finer than my main camera scale, since both use my main OTA (the 290MM has 2.9 micron pixels, the 1600 has 3.8 micron pixels).

What makes autoguiding work at a scale smaller than the image scale of your guide camera is that it is possible to calculate the position of the centroid of the guide star to an accuracy much greater than a single pixel. Because of this, star motions under 1 guide camera pixel can be detected and corrected for.

In the case of Aubrey's question - assuming he has changed nothing else (i.e., his guiding camera and guide scope and OTA are the same, seeing and mount is the same), going to the Starlight Express 814 would show guiding and any other imperfections better if he looks closely enough, or uses software to measure things like star eccentricity.

A simple way to think about it is the following: the guide camera, guide scope, responsiveness and quality of your mount, weight of your setup, seeing, etc. determine what kind of deviations from perfection you get. All your main camera does is record these deviations. Going to a finer image scale (smaller pixels) means that you can measure or record these deviations better, which is not always a good thing. Sometimes, it is best to not look too closely!

Arun