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You also need the pixel size. Recommendation is for the guiding pixel scale to be no more than about 5x the imaging pixel scale, give or take.
from the PHD2 forum:
Rule of thumb: Guider focal length > 0.2*(GP/IP)*IF
Where:
GP=Guider pixl size in um
IP=Imaging pixel size in um
IF=Imaging focal length
e.g. My guide cam and imaging cam are both 3.8um and imaging scope is 1600mm focal length. So guide scope FL should be at least 0.2x1600 or 330mm FL. The ST80 is 400mm so that is ok.
Other literature also places importance on the f/ratio where a low f/ratio on the guide scope is beneficial.
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