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Home > Articles > How To > Advanced > What’s inside a Meade LX200 14” SCT

What’s inside a Meade LX200 14” SCT
By Jerry Wise - 1/5/2007

A friend bought a Meade 14” LX200 UHTC Optical Tube Assembly when the original owner removed it from the factory fork arms. When he received it the fork arm bolts had been screwed back into the OTA. Just in case the new owner wanted them. My friend mounted up his new OTA and tried focusing on a star. The focus knob stopped moving and the star still wasn’t in focus. Turning the focus knob just a bit more brought a loud crinkling snap and the focus knob jerked. Looking into the front of the tube he saw pieces of primary mirror lying in the bottom. One of the fork arm bolts had snagged the mirror chipping 5 square inches out.

LX200 14” UHTC Optical Tube Assembly
A short time later I noticed the scope on Astromart. A quick exchange of emails and I had the scope on trade for a small refractor (about a $250.00 trade). Initially I just wanted the OTA to take apart. I first pulled the corrector and, after examining the optics, decided to patch the chipped mirror. A small felt patch was fabricated and cemented over the chipped area. The optics were cleaned and the telescope re-assembled. After mounting and collimating I was surprised to find a very good star test. Collimation was a little dicey with a black spot over a small segment of the diffraction rings but not too difficult to work around. Some test images proved the scope performed well with just a bit less light gathering capacity. See the patches in the following images.



The scope performed so well I began using it to image. It still had one problem. There was a noise when slewing as if something inside the base flopped around and the mirror lock didn’t work. Obviously, these symptoms were related. The mirror also showed considerable flop so something was not right in the base. This article takes you along as I take the scope apart and try to solve the problem. I don’t encourage or recommend anyone doing this. While no better than average mechanical skills are needed this is very sensitive and expensive equipment. A slip can break a very expensive piece of glass. Also, that glass can severely cut if broken. Best to use this article for satisfaction of curiosity instead of a guide to working on a telescope. I remember several years ago looking for an article like this when someone asked how the mirror lock worked so it might be of interest to some.

Originally my thoughts were either pull the base or pull the mirror assembly and see if the problem was field repairable. After looking at how Meade puts the OTA together, pulling the base this was obviously not a good idea. The OTA (Optical Tube Assembly) is pressed into the base and would take special tools to remove. I could find no instructions on pulling the mirror assembly though. Without a mirror lock and reduction of flop this OTA wasn’t going to be a winner. With the money invested vs. the potential benefit I decided to go for it.

The scope was placed on a padded table. Then all external items (finder mounts, mounting plate and piggyback adapter) were removed. Then the OTA was carefully examined. A person posted a link on the Internet saying there was a circular retaining ring under the visual back that should be removed to pull the mirror. On this OTA this ring retains the baffle tube core holding it in the OTA.

The first thing to do was remove the corrector plate and secondary assembly before delving into the innards. Before removing any screws prepare a paper towel or small tray to drop them on or into. Also have a padded chair close by for the corrector plate/secondary. First, remove the small screws holding the corrector plate retaining ring on. Rubber gloves are suggested as they will keep your fingers from marring the mirror or corrector if touched. Gently lift the corrector plate retaining ring out.

Corrector Plate Retaining Ring Removal


Once the corrector plate ring is removed the full corrector plate is exposed. The corrector plate is carefully aligned with the primary mirror rotationally. For best performance, the corrector plate must be replaced in exactly the same position it was in prior to removal. The factory will have marks on the corrector plate and housing indicating this position. There will also be small cork pads under most correctors. I’ve seen them under both Meade and Celestron corrector plates but, surprisingly, there were none under this corrector. There was a padded foam liner though. I have to assume that took the place of the cork.

Alignment Mark


Once the corrector plate is marked grasp the housing for the secondary. It’s the central section of the corrector with the collimation knobs. Try and gently lift the corrector from the OTA housing. Chances are it will not move. Don’t force it, use dental hygiene. Well sort off. Cut the head off a tooth brush and shape the plastic into a flat bladed screwdriver like tip. This will help lift out the corrector without cracking the delicate glass of the corrector plate. Notice how the corrector is gently pried up at two strange cutouts in the corrector plate mounting ring. More on these later and thank goodness they were there. You don’t have to exert a lifting force on the corrector plate, just a gentle nudge to one side then move to the other side and nudge the other way. You just want to break the pressure seal holding the corrector plate. It should free right up.

Toothbrush Tool


Once free lift it out and put it aside in a secure chair as shown. This keeps anything from touching the optics. Note the secondary mirror inside the corrector plate cell.

Corrector Plate After Removal


Next remove the Focus Knob and Mirror Lock Knob Assembly. These are held in by 3 screws in a retaining plate. The knobs are held onto their shafts by setscrews and are removed first.

The Mirror lock knob slips right off once its setscrews are loose. Once the knob is free the retaining plate is taken off by removing the screws and carefully pulling the plate back just a bit. Once this is done the mirror locking gear mechanism can be removed through the base plate hole. Slant it to the outside of the OTA housing and pull it straight back.

The Focus knob unscrews from its shaft after freeing the setscrews. Carefully slip the focus knob bearing and two washers off after removing the knob. Then remove the retaining plate slipping the plate toward the back of the focus knob shaft. You can then look into the focus shaft hole and see the shaft engaging a pin on the focus arm assembly. Move the focus knob shaft toward the outside of the OTA and slip it off the pin. Carefully lay this assembly aside. I did not know this at the time but once the focus knob shaft is free the primary mirror assembly and baffle tube carrier are free. If you turned the OTA upside down after freeing this knob and shaft the Primary Mirror assembly would crash into the front of the telescope. Don’t do it.

Focus and Mirror Lock Assemblies Removed


With the focus shaft free next comes the easy part. Removal of the primary mirror assembly. Place the OTA facing up on a chair. Reach down into the OTA and grasp the baffle tube carriage assembly just above the primary mirror. Be sure and have rubber gloves on here. You will need all the help you can get pulling the mirror up the baffle tube shaft. It may have lubricant on it keeping you from holding on. Gently pull the baffle tube carriage and primary mirror up the baffle tube shaft. If the assembly catches just as it is coming off the baffle tube the mirror lock is on. Ease the assembly back down. Lay the OTA over and reach into the Mirror lock hole. Rotate the large aluminum gear counter-clockwise about half a turn. This should free the mirror lock enough to clear the baffle tube.

Once the mirror assembly has cleared the baffle tube you are now holding the whole weight and it is one large piece of glass. It weighs 20 pounds so be careful. While holding the baffle tube carrier with one hand reach behind the mirror and grasp the mirror cell assembly with the other hand. Tilt the assembly parallel with the base of the OTA and slide the mirror through the two cutouts in the corrector plate retaining ring. It’s tight but it will work. Also, the focus shaft and alignment assembly may snag on the retaining ring of the corrector plate. Just rotate the mirror assembly a bit and they will clear.

Removing The Mirror Assembly

The inset in the lower right shows the mirror in the grove of the retaining ring.


To the left in the above picture is the aligning block for the mirror assembly. It slips over the small white metal track at the base of the OTA tube seen in the picture below.

Looking Down the OTA with No Mirror


Here you can clearly see the cutouts used to remove the primary mirror assembly. Take careful notice of the small shiny block bolted to the side of the OTA halfway between the focus knob hole (top) and the mirror lock hole (bottom). The mirror cell assembly has an arm that must be positioned precisely over this block.

The mirror not locking was the reason for this repair. Once the mirror assembly was out it was obvious. There lying in the back of the OTA was the mirror lock gear. It had unscrewed from the back of the mirror housing. I just picked it up and screwed it back on the mirror housing and the scope was fixed.

With it this far apart though I figured I’d find out where the weight was in a Meade OTA. Why is it so much heavier than a Celestron of similar size? Now was the time to weigh the components.

First the mirror assembly. The mirror is about an inch thick and is pretty heavy. The cell it is mounted on is well made and well designed. Most of the weight has to be in the glass as the cell is aluminum.

Mirror Cell Assembly


Notice the patches I had placed on the mirror. This was done without removing the mirror. Just reaching down inside the OTA….. way down inside. That is one long OTA tube.

With it out the cell was placed on the UPS scale.


The OTA with no glass comes in at 39 pounds. That just 6 pounds under a full up Celestron C-14 OTA with primary mirror and corrector installed. That’s one heavy base plate assembly.

With the weigh in completed the whole process of disassembly was reversed. All went back together very well. The clutch shaft was slipped back on which secured the primary mirror assembly. Then the Mirror Lock knob assembly slipped in. With the mirror lock gear screwed onto the back of the primary mirror cell the locking mechanism works like a champ. With it all buttoned up it was time for the final weigh in. 67 pounds without a dovetail and 74 pounds with a Losmandy Dovetail.

Summary:

This Meade OTA is one fine piece or work. I've gone round after round with Meade (6 LX200Rs, 1 RCX) on mechanical issues. Also Takahashi and Celestron all in the past year. If everything was like this 14" LX200 nothing would ever break on its own.

I've taken Takahashi refractors, Celestron SCTs and a bunch of other stuff apart and put them back together. Often with only a few parts left over. I just haven't seen many things match the engineering in this older Meade OTA. The mirror cell was just right. Had a solid but light feel to it. The mirror looked like a beautifully shaped creation. Not perfect but just right.

Focus shafts out of well machined brass. Bearings on both sides of the keeper. I've never seen a knob threaded and set screwed to a shaft before. The threading exerts pressure and can actually reduce mirror flop. Another thumbs up for the focus shaft.

Mirror lock out of just enough aluminum to work with strength in reserve. That mirror lock was slick. Just a snug and the shaft was locked with a tapered ring. Anywhere on the shaft. The mirror lock shaft had a drum gear (don't see those everyday) with keepers that had a spring allowing the drum to follow the locking gear up and down the baffle tube. Almost no weight to it but more than up to the job. Man that thing deserves a patent (which I'm sure it has).

The carrier tube for the mirror cell fit the baffle tube perfectly. Gently tapered and solid. A lot of effort went into that machining. Everything with the baffle tube and carrier was amazingly solid.

The corrector assembly was ok. Better than the Celestron C-14 I took apart last year. The Celestron secondary housing had a tendency to rotate. I didn't much like the plastic around the Meade secondary but it does the job well. The primary mirror cell end is where the money is in that scope. And a good job at that. Overall, a well made telescope.

Here is an image from the 14" LX200... Patch and all

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