ETX TECHNICAL TIPS Last updated: 25 June 2001

ETX-90 Image Shift fix

Sent:	Sunday, June 24, 2001 14:41:05
From:	hoyworld@tampabay.rr.com (Michael W. Hoy)
Thanks for your site, it has been very helpfull so...

I thought I would pass on what I found and how I fixed it regarding
excessive image shift when focusing with an early model ETX-90RA. On my
very slightly used 90RA I found when turning the focus knob in or out
slightly the image would shift across the eyepiece (26mm) by 2/3 or so
of the field of view. Some minor image shift is expected but this is
clearly excessive. I first unscrewed the corrector lens from the front
of the Optical Tube Assembly, this won't void any warrantee. I then
racked in and out the focus knob slightly several times and it was
obvious the long central mounting tube that the mirror assembly rides on
was wobbling with each change in direction of the focus knob. The
problem for my ETX as I found out with further disasembly (will
definatly void any warrantee) was a cross threaded central mounting
tube. I suspected cross threading when I unscrewed the tube becouse it
came free in just a turn or two. I confirmed it with a 10X eyeloop when
I saw the very fine threads were damaged at their end. Confirming this
required removing the main mirror wich I do not recommend. Correcting
the problem just involves getting that central mounting tube threaded
into the mirror cell correctly. This can be frustrating as the threads
are very fine and allready damaged. It took me awhile but once the
threads cought well the tube screwed down several turns and firmly
seated, don't torque it like a gorilla, just hand tight is fine. You
have to screw the central tube in working essentially blind becouse the
main mirror with it's backing plate must be in place. I now have
virtually zero image shift in the 26mm eyepiece. This didn't effect my
scopes collimation except possibly to improve it. I must caution people
that this is a nerve wracking fix as the main mirror is exposed at all
times. Also it is possible you may never get the central tube to screw
back in properly. For these reasons please do not attempt this fix
unless the warrantee is up, you are mechanically dextrous and are
willing to sufer the consiquences should something go wrong.

Michael W. Hoy

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