I also found some more information here
Its a bit long but should be OK. Rodstar may need to take note as well.
http://www.isomedia.com/homes/cvedel...mirrorlock.htm
Subject: Mirror Stabilization Bolt --part 1 of 3
From: Chris Vedeler
As I'm sure most of you know, a SCT focuses by moving the primary mirror up and down inside the OTA. The mirror is mounted inside an aluminum or steel mirror cell that rides on the central tube (where the light comes through) inside your scope which you can easily see. This mirror cell is moved by screwing up or down on the focus knob. As far as I know, there are only 2 points of contact with the mirror cell, the central tube and the focus screw. Since the central tube is a nicely machined match for the mirror cell hole, and they use special grease the mirror cell doesn't move from side to side much at all, but it still moves a tiny bit (giving a slight image shift). There is an inherent slop in the focus screw that can only be improved upon 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 teeth in the mirror cell or visa versa depending on the direction of the focus. If you have ever played with a bolt and a nut from the 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 SCT.
When your 10" or 12" SCT first arrived from Meade, the mirror cell was locked down to avoid problems during shipping. This is 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 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.
I got my idea for my mirror stabilizer from Randy Rubis:
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http://www.look-inc.com/rrubis/mir_loc1.jpg>
All I really changed from his design was adding a compression spring in between the wing nut and the spacer. In theory what you are doing is locking down the mirror cell just like you would for shipping, except the mirror is not moved all the way down to the bottom of the OTA. This requires a longer screw. It takes about a 2" or 2.5" screw just to reach the mirror from this hole. The reason you need to use a 6" screw is so you can clear the focus knob (i.e. about 3 .5" outside the OTA). It is necessary to have a nice tall spacer so that you can turn the wing nut above the end of the focus knob (imagine trying to screw a wing nut between the focus knob and the back of the OTA.)
I've tried to remove the backlash per Meade's instructions as well, and it really didn't do anything either. Unless it is really, really bad, I wouldn't bother. I put pictures on my webpage:
<
http://www.isomedia.com/homes/cvedel...mirrorlock.htm>
Clicking this link should evoke a new browser window.
Subject: Mirror Stabilization Bolt --part 2
>Okay, I have some pretty simple questions. Sorry if these are rehashed questions, I just need some clarification.
>To make my mirror stabilized, I need the following?
>1) one (1) six inch long, 1/4 inch thick, 20 thread bolt.
>2) one (1) wing nut for the 1/4 inch bolt.
>3) two (2) one inch tall nylon spacers.
>4) one (1) three inch long, 1/4 inch internal diameter compression spring.
>
>Then, I assemble them like this?
>1) Take 6" bolt and thread on wing nut.
>2) Sleeve on one nylon spacer
>3) Sleeve on the compression spring
>4) Sleeve on the second nylon spacer
>5) Screw the entire assembly into the focus locking bolt hole one to
> two and a half turns. Is this correct?
You almost have it. One thing I did different. The spring goes between the wing nut and the nylon spacers, but I guess it would work your way too as long as you clear the focus knob. I've used just one 1" spacer with some success as the spring is anywhere from 3" to an inch depending on how compressed it is. I got the other spacer just in case. Ultimately the spacers are just a way to clear the wing nut past the focus knob.
> Okay, I'll assume this is correct. Now what?
> Do I focus as usual and then, when I've gotten as close as I can get,
> I screw in the bolt assembly inorder to stabilize and then make minor djustments?
Once it is installed (remember, don't thread it in more than a turn or two into the mirror cell!) with the spring uncompressed you focus as normal. Once you get close to focus, screw down the wing nut until you compress the spring to about one-half it's original size. This puts a gentle and controlled pressure on the mirror cell that keeps in place. Then do the final focus. You will find the focus behaves much more predictably, smoothly and stops on a dime (the way it should from Meade, but doesn't). If you move the focus a whole lot (add a barlow, different camera adapter, add focal reducer etc.), make sure you watch the spring, as you don't want to compress it all the way or you could damage your focus mechanism (just like you don't want to try and move the focus when the shipping screw is installed). The spring makes this a lot safer, but you still want to be careful.
> Or, do I just begin the night by screwing in the bolt in 1 to 2.5
> turns and let the spring take the tension away from the mirror when I focus?
If you aren't changing focus drastically, you can just leave it in all night. One thing though, I do remove it when I am packing the scope up for the night. It does stick out a ways (3" or 4" from the back of the OTA), and is directly connected to the mirror cell inside the OTA. I'd hate to bump it and do damage to the mirror or mirror cell!
> This brings me to my final question. Does this bolt thread into the
> locking bolt hole, or does it just slide into the hole?
The locking bolt hole is not threaded, it is just an access hole to get to the mirror cell female threads inside the OTA.
Subject: Mirror Stabilization Bolt --part 3 of 3 (Mirror Locking Bolt's Effect on Collimation)
From: Bruce Dickson <bdicksonmweb.co.za> Date: Sep 2003
I did some experimenting with a mirror locking bolt (as originally proposed by Chris Vedeler).
I've been doing some planetary imaging using a webcam, and based on comments by Thierry Legault, I decided to check my collimation just before I started up the camera.
I was quite surprised - although the mirror lock bolt kept my mirror static, it twisted so much that the collimation was rubbish. I found that I achieved far better planetary images without the "stabilised mirror". Although I tried, I wasn't able to achieve acceptable collimation with the bolt in place.
I've also found that the mirror "flop" was sufficiently repeatable that provided I _always_ focus in a clockwise direction, the object will return to the centre of my webcam's field of view, even at f/45. (The direction might is different for the 7", 8"and 10" LX200s.)
That's not to say stabilising isn't useful for _long_ exposures, it's just that for high resolution imagery on low contrast objects, perfect collimation is far more critical.
