ETX TECHNICAL TIPS Last updated: 4 February 2002

Homemade Spanner for the ETX-70AT

Sent:	Monday, February 4, 2002 16:21:39
From:	thomas.henry@magician.org (Thomas Henry)
While observing and star-testing last night, something surprising
happened. At the start of the night the star tests were quite good
(circular images when defocused).  Several hours later I saw the typical
tri-form image caused by pinched optics.  Why the change?  Well, I'm in
Minnesota and last night was quite cold (around 5 degrees or so). You
southerners will never know the joy of touching metal parts that
literally sting like electrical shocks!

Anyway, it's pretty clear the optics were not pinched when warmer, but
the cold air compressed something or other.

I built a spanner in a matter of minutes which lets me loosen up the
retaining ring.  When I used it, I found that the ring was indeed really
cranked down.  So much for Meade's constant reference to "to a firm feel
only"!  Backing it off a bit eliminated the tri-form pattern.

To make the tool, I used 1/2" by 3/4" pine parting strips, and 1/4"
dowels. I clipped the heads off some brads to use as metal pegs.  After
clipping these, I filed them down to remove any burrs and to smooth them
off completely.  Pilot holes in the end of the dowels permit you to tap
them in.  See the attached photo.

To use it, the metal pegs slip into the two index holes on the retaining
ring; just revolve the spanner as desired.  By the way, the spread is
exactly 3" between the centers of the metal pegs, for the ETX-70AT.

Here is a good reference to pinched optics and how to deal with
them:

24.237.160.4/files/astronomy/MyScopes/bresserpulsar/Synta-FAQs.htm

Best wishes,

Thomas Henry

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