ETX TECHNICAL TIPS Last updated: 26 April 2005

ETX125 focuser shift

Subject:	ETX125 focuser shift
Sent:	Sunday, April 24, 2005 05:40:10
From:	P. Clay Sherrod (drclay@tcworks.net)
Hello Alan...
Focus shift can be caused by MANY issue, but typically is just one...you
need to remove the OTA from the rear cell; unscrew the focuser shaft
about half way (CCW) and remove the knob or electric focuser unit; there
are three very small hex cap screws which hold the OTA into the rear
cell; remove these and save in a tuna can or similar, because they WILL
get lost if you place on the countertop.

Once done, merely grasp the blue OTA in one hand with a rubber glove for
grip and the rear cell/fork arms in the other hand and pull gently
apart....do NOT twist while separating!

You will see where the threaded focus rod inserts into the
OTA....typically binding is causing the shifting, but most of this is
due to poor milling by Meade on some 125 models.  You can improve these
imperfect models greatly, but do not expect to eliminate all focus
shift.  Run the focus rod gently all the way CCW until you feel
pressure...do NOT force or you will damage.  Once there apply a thick
grease, such as pure white Lithium (available in the tool dept. at
Lowes) in a very thin layer, pushing hard into the threads...do NOT
apply too much or you risk getting this stuff on the optics eventually. 
Once done, put the knob on the rod temporarily and run it in and out all
the way slowly about 3-4 times.  Once done, run it HALF-WAY between all
the way and all the way out and stop.  At that position, turn the OTA to
about a 45-degree tilt, with corrector down and put 1-2 DROPS of 3-in-1
oil right where the threaded rod goes into the threaded rear cell of the
scope and QUICKLY run the focuser CLOCKWISE to get that thin bit of oil
into the rear cell threads.

Once done, run the threaded rod slowly all the way CW and then CCW about
2 more times.

This procedure will greatly improve the performance....NOTE:  do NOT put
more than 2 drops of oil on the rod!

Take your time and do this very methodically....if you get in a hurry
you will be worse off than you started and can greatly mess up the
works.  This procedure will NOT affect collimation or GO TO operation at
all with the scope.
 
Good luck!
 
Dr. Clay
--------------------
Arkansas Sky Observatories
Harvard MPC H41 (Petit Jean Mountain)
Harvard MPC H45 (Petit Jean Mountain South)
Harvard MPC H43 (Conway)
Harvard MPC H44 (Cascade Mountain)
http://www.arksky.org/
 
 
----- Original Message -----
From:  Merv 
Hello, please forgive me for mailing you out of the blue but I have
tried to find info & solutions to my problem without success.

Can you give me any info on the focusing mechanism & what if any things
I can do to stop or improve the awful image shifting when focusing, the
image shifts more than half way across the FOV when using, as an
example, the 26mm eyepiece & when using a DSI or PSI of course it shifts
completely out of the FOV before getting to the focus point, the image
then shifts the other way when reversing the focuser. It makes using the
DSI or LPI almost unusable.

If I take the rear end apart will I need to re-collimate ?
 
Thanks for any help or info
 
Alan

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