Schmidt-Cassegrain Mirror
Stabilizer
By Chris Vedeler
Copyright 1997
1- Introduction
Schmidt-Cassegrain (SCT) telescopes focus by moving the primary mirror up and down inside the optical tube assembly (OTA). The mirror is mounted inside an aluminum or steel mirror cell that supports the mirror and holds it precisely in place. Screwing up or down with the focus knob moves the mirror cell up or down. As far as I know there are only two points of contact for the mirror cell, the central baffle tube and the focus screw. The central baffle tube is nicely machined to match the hole in the middle of the mirror cell, and a special grease is used to make the mirror cell move smoothly over the baffle tube. However when one is observing under high magnification it is still possible to see the image shift as one moves the focus. This happens because there is still a little play in-between the mirror cell and the baffle tube.
There is also an inherent slop in the focus screw that can only be improved by machining a better focus screw. When you change direction with the focus knob, you have to make up a tiny bit of slop in the screw before the top of the teeth of the screw engage with the bottom of the teeth in the mirror cell or visa versa depending on the direction of the focus. If you have ever played with a bolt and matching nut from a hardware store, you know that there is a tiny amount of play when the nut is screwed onto the bolt. The same thing is happening inside your Schmidt-Cassegrain.
When your Schmidt-Cassegrain first arrived from Meade, the mirror cell was locked down to avoid problems during shipping. This was done by moving the mirror all the way to
the back of the OTA and screwing in a 1/4" screw through the hole right next to
the focus knob. When this is done, the mirror cell can not move at all (which is
what you want when shipping in case UPS drops it). When you are focused using
normal eyepieces and cameras, the mirror cell is actually about another inch or two
deeper inside the OTA. The original shipping screw that came with your scope is
not long enough to reach the mirror cell now. The mirror (inside the aluminum or
steel mirror cell) is now "floating" on the central baffle tube and is adjusted by a screw
mechanism that is controlled by the focus knob. Given the design of this type of
focusing mechanism, a little movement is inherent and unavoidable.
2- The $1.50 fix
What you need:
1) You need a 6 inch long 1/4 inch, 20 thread (standard) bolt from a hardware store. Make sure it is threaded the entire length of the bolt.
2) A 1/4 inch wing nut for the bolt.
3) A 1 inch tall, 1/4" inside diameter nylon spacer.
4) A 3 inch long, 1/4 inch inside diameter compression spring. There are several different kinds. Find one that takes a little effort to squeeze together with your fingers. Maybe a 5 pound load.
Step 1: Thread the wing nut all the way to the end of the 6 inch bolt so that the wings are facing the end you are threading towards.
Step 2: Slide on the compression spring (if using it for visual).
Step 3: Slide on the nylon spacer (or 3 spacers for astrophotography or
CCD imaging).
Step 4: Remove the dust cover for the little hole right next to the focus knob on the LX200. (The LX50's have the same hole I am told).
Step 5: Carefully slide the bolt assembly into the hole. Once the end hits the back of the mirror cell start turning the bolt
(not the wing-nut) clockwise to begin threading it into the mirror cell.
Step 6: DO NOT thread this too far! I'd say two
or three complete turns is plenty. If you thread it too far you will hit the back of the mirror itself! If the bolt makes contact with the mirror it will cause astigmatism to the optics (only while the bolt is in contact with the mirror), or worse, cause a stress fracture in the glass itself if severely over tightened. It is better not to touch the glass at all with the end of the bolt!
Step 7: Focus as normal. Once you have a star focused pretty close, tighten down on the wing nut until the spring is compressed about 1/2 way.
Step 8: Do your final focus.
When doing large changes in focus (adding a barlow or focal reducer), make sure you do not compress the spring all the way. Once that spring is fully compressed the mirror cell is locked down (in the direction of travel) and will not move further without damaging something. Always keep the spring about half way compressed. If it becomes too slack you will loose the benefit of the system. If it becomes too tight, you run the risk of running out of slack and damaging your scope.
3- Real World Impressions
I have found this little gizmo to work wonders with the feel and precision of my focusing. It is about the best, cheapest and easiest improvement I have ever made to my LX200. I no longer need to worry about backing the focuser counter-clockwise in order for the image to remain in focus. The focuser feels smoother, more precise and stops right where I put it.
For the demanding requirements of astrophotography and CCD imaging, I have
found it necessary to lock the mirror down completely as the spring still permits significant mirror slop.
With the mirror locked down, the focus is less likely to shift during the
exposure and the mirror is less likely to "flop". This greatly
improves the quality of images possible with the LX200.
This is not something you want to leave on the scope when not in use. It sticks out about 4 inches from the back of the OTA
(see images) and could easily be bumped or hit while trying to pack up the scope. I would also not recommend using it when lots of people and especially kids will be looking through the scope. They my grab it or bump it.
Keep in mind that this little bolt is millimeters away from your primary mirror. Treat it with great respect.
I'd like to thank Randy Rubis for the idea for this device, and my friends on MAPUG for all their wonderful questions that made these instructions possible.
Comments: ckvedeler@access4less.net
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