[APML] Schmidt camera dismanteled

[Robert Reeves <reeves10@swbell.net>]

Greetings fellow Schmidt camera owners,

My adventures in overhauling my 8-inch Schmidt camera continue...slowly.

I have decided to have Clausing apply their 98% reflective Diamondbright
coating to the mirror (unless some one knows a reason why I shouldn't use
this???)  But the $100+ cost of it has me in a hold right now because the
9-to5 daytime job is real slow now for some reason.  So I decided to take
the camera appart anyway, then reassemble it with the existing mirror just
to make sure I can do it right and get it properly collimated and refocused.
This is a training run before I fiddle with a pristine mirror.

After worrying about taking it appart, disassembling the 8-inch Schmidt was
rather easy.  After marking the tube where all the shims and corrector and
mirror orientation went, I pulled the mounting screws out of the collimation
adjuster backets and cut through the silicone with an Exacto knife.  I found
that buried inside each silicone pad glueing the mirror in place was a 1/2
sqaure inch spot of bakalite which acted as a pad between the mirror back
and the collimation adjusting screws.  Once these were out and the silicone
cleaned off the back of the tube, the mirror fell right out.  Wow!  An
8-inch f/1.5 mirror is a deep bowl!  For those who wondered, the mirror is
8.75 inches in diameter and the cell masks it down to 8.5 inches in
diameter.

The corrector plate had four sets of paper shims around the perimeter of the
glass. Each was a stack of three to six thin strips of paper slightly
heavier than typing paper.  I guess Celestron chopped up whatever was handy.
There was printing on the paper, but it was too cut up to understand what it
said.  What really suprized me was how chipped up the edge of the corrector
was.  It is chipped almost all the way around its rim.  This must have
happened at the factory because there are no chips in the holding cell.
None of the chipping is visible once the retaining ring is in place.

I have heard Celestron made the correctors out of ordinary window glass.  I
don't know if this is true, but the thickness of the corrector is very
similar to window pane glass.  Contrary to what I was told, I can tell which
side of the corrector has been ground into the Schmidt corrector shape.
When I looked at the reflection of straight lines that were at an angle to
each other, such as the roof and side of my house as seen out the garage
door, the corrector side showed those straight lines as mushy and bowed
compared to the mormal reflection from the flat side of the corrector.  The
corrector was 8.5 inches in diameter and is masked down to exactly 8 inches
by its retainer.

There was one cork shim about a half inch wide every 120 degrees around the
rim of the mirror centering it in the cell and there was a small 1/4-inch
square spot of cork (very flattened) under the lip of the cell where the
Invar rods meet the face of the mirror.  There is certainly no effort to
make any floatation support for the mirror.  It is simply held into its cell
by being pushed into place with the three bakalite pads under the
collimation jackscrews, then being siliconed in three places.

Now I can see why some Schmidt cameras produce good images while others
produce fantastic images.  It looks like the mirror can be warped slightly
in the cell if it has to be pushed hard on one side to achieve collimation.
I think mine is a "good image" Schmidt :-(   I saw an 8-inch Schmidt image
years ago in S&T that was taken by John Gleason.  His camera clearly makes
better images than mine used to.  After I get through tinkering with it, who
knows?  Maybe I can get this puppy to sing for supper too!

Anyway,  over the next several days I will be adding these adventures and a
few more things to my Schmidt camera guide on my web site.

Robert Reeves                      reeves10@swbell.net
520 Rittiman Rd.                   www.robertreeves.com
San Antonio, Texas 78209    210-828-9036
USA                                     29.484  98.440  200 meters


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