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Greg's experience with telescopes confirms what theory predicts. From what I understand, it is easier to focus larger apertures and at slower focal ratios. The most difficult telescopes to focus precisely are fast telescopes with small apertures.
My 105mm CFF definitely requires refocusing every 1 degree Celsius at f/4.5, ideally perhaps every 0.5 degree to maintain perfect focus. So it is best to use temperature compensation at that focal ratio and with this telescope, instead of frequent refocusing.
At native f/6 the same telescope needs refocusing every 1.5 degree change in ambient temperature.
I'm yet to test whether larger aperture telescope also has a larger critical focus zone...
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