I have written a lot about focus motor and temperature compensating.
Then we need some numbers of have much focus precision we need.
When shall the motor change the focus when temperature changes.
The depth of focus depends mainly on three things:
The seeing, i It could be from 1" to 10",
a seeing of 2" (2 arc second) is very good.
The wavelength you observing at. The resolution of the
telescope are different at H-alpha compare to the blue wavelengths.
Normally when talking about optics in the visually wavelengths,
about 380 (blue) nm to 650 (red) nm we use the green / yellow wavelength 550 nm as a reference.
If you only take photos of the H-alpha red glow you maybe should use 656 nm instead.
If doing photographing in the UV spectrum, then maybe 350 nm is better to use as the reference.
More to read about wavelength and electromagnetic spectrum: Wavelength, Wiki
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There can always be a mistake in my equations behind the calculations, check that the result is correct.
Let me know if you find something wrong and I try to correct it.
Some calculations are very simple done and not correct in the small details.
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