Sent:	Sunday, August 19, 2012 12:03:14
From:	Paul Smit (2482@nokwi.co.za)
Herewith a long story (about ten year's worth and not yet done) of an
ETX 90 RA.
 
The telescope was bought for my wife. It was well used, neglected - dirt
on flip mirror, no back cap, corrector scratched, slop in mechanics, no
eyepiece holder cap, RA rubbing pads worn away.  Collimation out (So
much for "Set at factory, never needs setting again").  As I go along, I
continue working on it.
 
I made back and eyepiece holder caps.  I  made a steel equatorial wedge
stand (the supplied screw-in tripod legs are impractical and flexible). 
I stripped the base and forks, shimmed up the warped galvanised steel
base plate to get a flat seat for the forks.   The bottom of the top
base bearing surface was flattened on a  flat table with sandpaper (it
was warped about 1 mm).  I inserted 3 teflon pads in the bottom base to
do the job of the worn away factory "pads".  The  3 mm self-threading
main sructure hold together screw was replaced with a thrust bearing in
the bottom base and a long 3 mm cap bolt right through the top base (Was
this base designed by a kindergarten "Meade Design Competition"
winner?).  I machined up a better fitting RA main bearing.  Maybe the
original was not too bad - the problems lay in the overall base design?
 
I opened the corrector lens to do the "secondary baffle fix".  The
secondary mirror shows marks where baffle sat before the fix - it walked
about 3 mm form center - I hope  Meade fixed this on later models.  The
inside of the tube was cleaned.  Collimation was improved by looking
into the front and using the "circles within circles" method. 
(Improved, not perfect - must have been bad if the look-into-front fix
helped.)  I  inserted a foam plug into the opening between the tube and
tube-holder at the bottom, and plugged the two 1/4 x 20 "terrestrial
spotting scope" holes as well. (Do we need more ventilation past the
back of the rear mirror holder plate?  No.  Do we need more open areas
for dust to enter? No.)
  
The RA setting circle tape was cut narrower, with a spring clamp to keep
the RA setting circle tape fixed to the top base between settings - no
more "it slipped/dragged/sticks".  Ideally needs a clamp on the bottom
base, so RA value stays on last setting, eliminating having to re-adjust
before slewing to next object- cannot be done on this base design. 
Thoughtful  design here would have helped.
 
I made screw-on Baader solar filter.  Very handy.
 
As large-as-possible diameter focus, RA adjustment and RA drive lock
knobs were turned up, as well as a close-fitting bearing for RA/AZ
adjuster shaft to eliminate sloppy operation.  Mistake - scope binds due
to assymmetric RA gear wheel - some slop re-fitted.  I cannot remove the
mechanical slop in this base without it getting stuck when tracking.
 
White marks on the Decl/Alt top bearing to indicate center as well as
maximum travel was applied - I can now easily monitor the Decl.
adjustment travel.  The RA and Decl. indicator arrows were also painted
white to enable seeing them when used under night-time star-gazing
conditions (as may have been originally intended?).
 
Collimation was recently improved some more on an artificial star - but
it is still not perfect.
 
The base RA bearing setup was modified extensively:  I inserted a
tapered roller bearing in the top base, with a spacing pillar to the
bottom base, where a roller bearing in the bottom of the bottom base
(where that self-threading screw used to be) takes up the axial load-
this removed almost all stiction - some friction is still left from the
assymmetric RA drive gear plate.  I am still thinking up a better RA
bearing system, the current solution is still flexible (too small
diameter bearings and too small bearing surfaces to sort out the flex
problem).
 
Overall opinion - What was all the hype about?  I was caught by all the
advertisements and the good looks. A long-term lend to a friend (after I
bought his Celestron Super Polaris C8) immediately had him cured of the
portable easy-to-use ETX lure.  Nice compact sized scope to use (when
screwed down onto a solid steady surface), with bad, cheap mechanical
design - A 3 mm self-threading screw to hold the base together and act
as a thrust bearing?? Come on, Meade.   I hope the Questar buyers have a
better deal. We do have a new-in-box ETX 90 RA to "fix" after all the
experimentation is done on the well (ab)used old one, then we can see
about optical quality or lack thereof.  Well, we did see the polar cap
on Mars in 2003 with the new one.  I am rather fond of the ETX 90.  I
bought an ETX 105 EC after GO TO disasters with two 5 inch Maksutovs
(store took both back without a word), and it worked well - I have not
ventured into the Autostar nightmare yet, but at least it has setting
circles and manual RA/Decl locks.  But the old ETX 90 will never let you
down because it's handbox is sick!  I like old-fashioned setting circles
and manual capability backup.  A fully automatic go to scope is an
expensive useless item when the electronics act up.
 
Paul

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