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What effects focal length in your scope?
1. Temperature differences in the atmosphere (seeing)
2. Surface errors on the objective.
3. Errors on the secondary mirror.
4. Aberrations in the eyepiece.
5. Imperfections in your own eye.
6. Errors in collimation of the optical parts.
If each effected the on the incoming light ray in the same way and by the same amount the effects would be additive. We could tolerate a max of only 1/24 of a wavelength of sodium light in each (1/4 wave). We can do better, of course, and 2,3 & 6 can be kept to a minimum, but there is not much we can do about the others.
Errors that might not be obvious:
Wedging error in the mirror. If the mirror is thicker at the edge at one point than another there is more tilt to be made during collimation.
If the curve generated is off centre by a couple of millimetres then placing a central dot doesn't help as it is not "on-axis".
So we get it as close as possible and hope that all is well.
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