LX600 12" Primary Mirror Lock
$174
ScopeStuff

When I began using the Meade 12" f/8 ACF LX600 telescope in 2016 I was surprised by the lack of a "mirror lock" to prevent focus changes when slewing the telescope. The Meade 8" LX200-ACF I previously had in my observatory did have a mirror lock knob to lock/unlock the primary mirror. Meade had designed the 10" and 12" f/8 ACF telescopes so that a mirror lock was not supposed to be required to keep the large heavy mirror from moving while slewing the telescope. Their 14" and 16" f/8 ACF models do have a mirror lock.

Over time I became more and more frustrated by the mirror movement when slewing the telescope as it significantly changed the telescope's focus. This was not a serious problem for visual work, but was a major nuisance when imaging. I would focus the image using a bright star, but found that the image of my selected object was out-of-focus due to the primary mirror movement after slewing the telescope from the star to the object. Even a slew of only 15° (or tracking an object for just one hour) would cause the object to go slightly out-of-focus as the primary mirror moved.

Background

I contacted Meade about the mirror movement and was advised to use a long bolt to lock the mirror. This was a technique that had been used back in the days of the LX200 Classic models. The bolt is screwed into the "travel screw" hole on the rear of the primary mirror cell and then a nut is used to press against the rear plate on the telescope tube. The idea is to provide tension against the bolt and prevent the mirror from moving. This was my "solution" based on Meade's suggestion:

I found that I had to use a small socket wrench to get the bolt tight enough to prevent mirror movement. The only thing that keeps the bolt from sliding back out of the hole as the mirror tries to move during slewing is tension against the side of the bolt hole on the rear plate. I worried that this sideways torque could damage the mirror mounting inside the telescope tube. And of course, it was necessary to remember to use the socket wrench to "unlock" the mirror before turning the focus knob. I was not thrilled with this "solution" to locking the primary mirror but used it anyway. I would still get some occasional focus changes during large slews, so this was not the perfect solution I wanted.

In late 2016 I made a cardboard prototype of a possible better solution. The design provided a plate that the lock bolt could be secured against to prevent the bolt from moving. However, I could not determine a way to unlock the bolt to refocus the image when changing eyepieces or camera. So I did not pursue my design any further and I continued to use the "locking bolt".

In September 2017, I discovered that a possible mirror lock solution was available from ScopeStuff: LX200 Classic 10" and 12" Mirror Stabilizer/Lock. This seemed like a good solution for the 12" LX600. I liked the idea of the springs, which I thought would go a long ways to avoid damaging the mirror mounting and focus mechanism. I contacted ScopeStuff and they sent me one to try out:

Unfortunately, the heavy 12" mirror still moved during slewing. We tried stronger springs, but that did not help.

In late 2017 I shared my cardboard prototype design with ScopeStuff. Using their extensive experience with making useful telescope add-ons they very quickly came up with a potential actual product. Their concept had an excellent answer to the question of how to actually lock the bolt to keep it from moving. I provided them with needed measurements for my 12" f/8 ACF telescope from which they created a fully functional prototype. Using their prototype I did form-and-fit tests on my 12" LX600. We shared ideas on some simple improvements, but the basic design seemed ideal.

I performed slewing tests with the 12" telescope to check that the focus would not change during slewing from the horizon to near the zenith and back to the horizon with the mirror locked. We asked another 12" LX600 owner to test the product and he reported good results.

After many months of development and testing, ScopeStuff now has a product that it can sell to 12" LX600 owners.

Installation of the Primary Mirror Lock

No disassembly of the telescope tube is required to install the ScopeStuff LX600 12" Primary Mirror Lock. The Mirror Lock comes with installation and usage instructions. You first slide the large clamp onto the 3" visual port cover:

You then align the bolt holder using a special alignment tool (provided) and the mirror bolt:

The bolt and alignment tool is removed and the bolt is inserted into the lock knob. Next, you insert the bolt into the bolt hole and thread it into the primary mirror cell. Lastly, you screw the lock knob onto the bolt holder, completing the installation:

With the knob unlocked the bolt slides freely through the bolt hole when focusing (which moves the primary mirror). With the knob rotated finger tight in the lock direction the bolt is held firmly inplace, preventing the bolt and hence the mirror from moving.

Using the Primary Mirror Lock

The Mirror Lock is fully functional for both visual and photographic purposes. I confirmed there was no interference with any of my accessories. Here you can see the Meade supplied 1-1/4" diagonal and 26mm eyepiece:

And a Nikon D7200 DSLR at prime focus using a 2" visual back:

I did notice that the camera would need a slight rotation to clear the lock bolt if the visual back extension was not used, but I rarely image without the extension in place.

Using the Mirror Travel Bolt

Although not part of my original concept, ScopeStuff felt that it should not be necessary to remove the Mirror Lock Clamp to lock the mirror when moving the telescope or putting it into a case. So they include a Mirror Travel Bolt with the Mirror Lock. To use it you first loosen the Mirror Lock knob and then rotate the focus knob clockwise to fully extend the bolt:

Next you remove lock knob and long bolt, then insert and tighten the Travel Bolt:

Now your telescope is safe to move and store. To use the telescope again just reverse the process. There is no need to re-align the bolt holder as was done on initial installation.

Summary

The ScopeStuff LX600 12" Primary Mirror Lock is made from high quality black ABS plastic with stainless steel fasteners and brass clamp parts. The clamp will not mar the surface of the 3" visual port cover or the back plate of the telescope. The Mirror Lock is super easy to install and use. The Mirror Travel Bolt is a nice addition that means you do not have to remove the Mirror Lock to protect the primary mirror mechanism when moving the telescope. This will be very convenient for 12" f/8 telescope users who do have not have a permanently mounted telescope.

Although the ScopeStuff LX600 12" Primary Mirror Lock has been tested with only the 12" f/8 LX600, it should work for all current Meade 12" f/8 ACF telescopes for the LX600, LX850, and other mounts when the optical tube assembly (OTA) is purchased separately. As long as the 3" visual back port is not being used for some 3" accessory then the Mirror Lock can be installed. If a 3" accessory is used at times the Mirror Lock can be easily removed, but then the mirror locking capability will be lost.

Owners of the 10" f/8 ACF OTA who are experiencing focus change should contact ScopeStuff to possibly have the 12" product modified for use with the 10" telescope.

As with any moving primary mirror telescope, focus adjustments must be avoided when the mirror is locked in position. Turning the focus knob with the mirror locked can cause damage to the focusing mechanism. Fortunately, with the ScopeStuff lock engaged the focus knob will be difficult to rotate, which is a good clue that the mirror is locked.

Many thanks to Jim at ScopeStuff for being excited about doing this product and for developing such a great solution. And thanks also to John for testing the final product.

If you, like I was, are frequently frustrated by your 12" f/8 ACF telescope focus changing during slewing or tracking, you will definitely want to add the ScopeStuff LX600 12" Primary Mirror Lock to your telescope. It will be the best gift you can give to yourself and your LX600.

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