Re: SCT question

On Fri, 28 Mar 2008 23:24:04 -0700, David Weinshenker
<daze39@xxxxxxxxxxxxx> wrote:

Yeah, I suspect that the whole "image shift/backlash" problem might
be remedied simply enough by a strategically-placed "preload" spring
so that whatever slack might exist in the mechanism would always be
taken up in the same direction.

In an LX200, there are three sources of focuser backlash. The focus knob
itself may have axial play due to being poorly tensioned. The lead screw
may be sloppy in the drive nut because of poorly matched threads, and
the drive nut may be loose on the hinge pin that attaches it to the
mirror cell. In the two cases I've addressed, the first was by far the
most serious problem. I found that by replacing the nylon thrust
bearings with roller bearings, and then really putting the system under
compression, virtually all backlash was eliminated.

There's another problem that causes something similar to backlash,
although it's a bit different. If you have much mirror shift (which
varies significantly from unit to unit), changing direction on the
focuser rocks the mirror before it actually starts sliding. That's a
kind of backlash, of course, and one that you can't really do anything
about, at least not without major surgery inside the OTA.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.