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Hi Steve,
By insuring that your focusing by moving the mirror
up, your using the weight of the mirror to preload the internal focusing screw
and focusing arm of the mirror mount. This keeps the _upward_ facing surface of
the threads loaded with the weight throughout the exposure. On any Celestron and
Meade SCT's that I've seen, this is done by turning the focus knob
CCW.
If you focus by turning the knob CW, your using the
_lower_ facing surface of the threads to pull the mirror down in the tube.
After achieving focus this way, any slop in the threads or focus arm link can
continue to allow the mirror to settle backwards in the tube, until the mirror's
weight causes the focusing mechanism to settle onto the thread's upward facing
surfaces. Any slop in the focus arm link will also settle out with time then.
Meade 12 and 16" SCT's can be an exemption to this, as
they have a spring loaded mirror. The spring pushes the mirror "UP" in the
tube. This spring is stronger than the force of gravity, and so it constantly
preloads the lower facing surfaces of the threads, and so many users focus
these scopes CW. This is what I do with my Meade 12" SCT, although some people
still report that they get good results CCW.
Hi, I'm a beginner and I thought that you should use gravity to your advantage when focusing and final adjustment should be done when the mirror falls down. This way any movement from motor tracking, etc. would be less apt to happen. Have I been using the wrong technique? Thanks, Steve |