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Subject: [M]: Re: Loose corrector plate on 12" LX200
From: Gary Heath
Reply To: mapug@shore.net
Date: Sat Nov 23 01:41:47 1996

Frank Schiralli wrote:

>Can anyone please advise me on how to correct a loose corrector plate
>on a 12" lx200?? All eight hex head bolts are TIGHT, yet I've been
>hearing (and feeling) a great CLUNK when slewing my scope.
>Upon investigation its the corrector plate dropping from top to bottom
>or side to side 2 - 3mm per shift with a sizeable clunk when it lands
>on the other side. Inspection reveals nothing obviously abnormal.
>When I grab the central secondary housing I can move the plate with
>just a little force.
> Obviously, I couldn't possible have, or keep good collimation with
>this condition either, vibration aside.

Hi Frank,
I don't think your problem is with the corrector plate moving
in the cell, but instead the secondary holder moving in the corrector.
The ring that holds the corrector in should not pinch the corrector
between the corrector cell and the ring, as the recess in the cell is
made so that the thickness of the corrector is at or slightly less than
the forward edge of the corrector recess in the cell.
This is done so that you can't pinch the corrector by overtightening
the ring screws. It does not take a lot to warp the corrector.
One of the unfortunate things about most SCT's is that the gasgets
used to hold the secondary can swell when the corrector is removed for
cleaning and moisture impregnates the gasgets.
This can also happen when there is a large formation of dew on the
corrector, or a scope is put away before it is allowed to completly dry
out.
But it's not the swelling that creates the loseness, it's the
subsequent shrinking of the gasget after it drys out.
The way to fix it is to lay the tube in a slightly upward direction,
and remove the corrector retaining ring, once done you will see the
three pieces of cork that make the corrector stay centered in the cell,
and a white dot on both the corrector and the edge of the cell.
You will grasp the secondary and lift the corrector towards you.
It will be somewhat stuck to the gasket behind it.
Use a steady firm pull, and be ready for the corrector to come lose.
Once you have it out, you will see the baffel for the secondary is a
sort of conical cup which threads to the secondary holder and
compresses two gaskets on either side of the corrector. You must make
sure you don't rotate the secondary holder, but just tighten the
secondary baffel by rotating it in a clock-wise direction, as you look
at it with the secondary in view.
You do not need a lot of rotation to get it tight enough. Although the
propensity to warp the corrector is not as much as it would be if you
could pinch it between the ring, it can still be deformed if you
tighten this up too much. It is much more immune to causing a warping
because it applies a fairly constant force completly around the center
of the corrector as opposed to screws placed at intervals like the ring
has. This is also why the corrector is completly recessed into the
cell.
When I've done this on many different brands of SCT's I've only had to
turn the baffel apx 1/8 of a turn at the most.
Just be glad you have a Meade, the Celestrons place three dabs of glue
on the threads, and as the scope gets older, or the gaskets shrink from
improper storage (without drying out) and/or cleaning *ESPECIALY WITH
ALCOHOL OR ACETONE* they are a real bear to get the glue off so you can
tighten up the secondary holder.
I don't recomend cleaning optics that are gasketed, with anything
other than distilled water with *2* drops of Dawn detergent to a quart
of the distilled water, in a spray bottle. Using sterile cotton, push
the soap/water in circular motions, flush with distilled water
liberaly, dry with the sterile cotton, then buff with your breath and
Klenex brand *Softique non-scented* tisues, by rolling one up and
wrapping another around it to make a pillow.
I'm sure others have their favorite methoods for cleaning, this has
always worked for me.
I do use acetone on mirrors in newt/dob's but they don't have any
gaskets to worry about. I've never had as good luck with alcohol, just
too "sleeky" for my taste.
When you re-install the corrector, make sure to line up the white dot.
install the ring, and cross tighten the screws, a little at a time.
They should be a bit beyond "snug".
Regards,
Gary