Quote:
Originally Posted by bmitchell82
andrew your still missing the point.
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I am not missing any point. In fact, if you read my post, you will see quite clearly that
you missed my point. Mirror spacing has a far greater effect on focus than the tiny focal length that an aluminium focuser has. The focal length of a focuser is maybe 100mm at most, so any contraction that it experiences through a change of temperature is very small. The change of separation between two mirrors in a telescope, however, can have a much larger effect. If the tube is X mm and you get an expansion/contraction that Alex calculated at 45 microns, it will equate to a change in focus at the focal plane of a much higher number. think of a SCT - if you add an external focuser your focus control is very slow compared to how much focus change you get from moving the primary mirror. I don't have the exact math, but a change of seperation of the two mirrors by 45 microns will certainly not equate to a change of 45 microns at the focal plane - because of the secondary magnification it will change the focus point by 5x this amount to 225 microns.