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Very interesting Ken, the old rule of thumb was that the focal length of the guidescope should be around 1/5th of imagescope. But as you have shown, this old rule only applies if the guidecam and imagecam have the same size pixels.
It works against you if the guidecam has larger pixels than the imagecam.
E.g my Lodestar guidecam has 8.2 micron pixels and the QHY12 imagecam has 5.1 pixels.
Bruces' first example
0.2*(8.2/5.1)*1680 = 540mm min focal length for guidescope (about 1/3rd of imaging focal length).
Thanks for clarifying that for us Ken.
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