Posted by Andrea Tasselli at 15:10 on 2010 Sep 11
Actually what matters is that the reflections being centered rather than the actual object, in this case the secondary. If the focuser is tilted with respect to the mechanical axis of the tube, assuming that this can be ascertained with any degree of accuracy, the only way is to use shims (well furnished hardware shops di have them) to de-tilt the focuser. One easy way to assess this is to use a "collimated" laser collimator (some of the cheap ones are not). Once the beam is centered on the primary (I assume the primary is center-marked) moving the focuser up and down the whole length of the drawtube would show shifts in the position of the dot, thus revealing the tilt (and also providing a way to check the correct tilt if shims are used). Obviously this assume the focuser being of a decent sort and thus not bending or shifting while moving.Andrea T.