How to Fix a Loose Secondary Cell in an LX200 Telescope

I purchased a 10" LX200R Meade telescope back in 2007 and have been really happy with it, until....recently. The main symptom had been around for a while - unstable collimation - but since I was getting quite proficient at it, I kept doing it and things seemed to work out fine.However, I recently started imaging Mars, and noticed I had to re-do the collimation each night, and on one occasion, even twice the same night! Then one night I accidentally used too much lateral pressure during collimation, and shifted the entire secondary mirror cell! It had turned before, so I knew things were not perhaps ideal, but this was the first time it had actually shifted laterally. And so, my quest for a solution began, culminating in this presentation which I have put together for the possible benefit of others in my position.

Solution: Tighten the two cylinders that sandwich the corrector lens.

Summary: It is possible to do this without removing the center cell, but my cylinders were too difficult to screw tighter. As it turned out, the problem was that the paint coating on the cylinder threads was so fresh, it made it extremely difficult to screw the cylinders any tighter. Consequently, I ended up separating the center cell from the corrector lens to get access to them independently so that I could screw the two cylinders comprising the cell tighter to remove and shave off the fresh paint coat. This then made it easier to tighten the cylinders later when I reassembled the OTA.

 

Step 1: Angle OTA and remove the corrector lens plastic ring.


First, angle the OTA at about 30 degrees relative to the ground, so that the main mirror is lower than the corrector lens. This will prevent the corrector lens and secondary cell from falling out once the plastic ring is removed.

Unscrew the 8 philips screws on the periphery of the corrector lens, being very careful NOT to allow them to drop onto the corrector lens as you remove them.

Remove the ring.

Note the white factory marks on both the corrector lens and the surrounding lip of the OTA.

I would hope every scope has such marks. If they aren't there, or are very faint, I would suggest making your own marks so that you will always know the correct orientation of the corrector lens with respect to your main mirror.

Click on any of the pictures for a larger view.

 

Step 2: Mark all components to insure proper reassembly.


Mark points on the center cell and OTA perimeter opposite the factory mark forming a line along the diameter of the OTA. See the picture directly to the left.

These marks will insure that you will properly reassemble the secondary cell relative to the corrector lens.

Next, mark the two "rings" shown in the second image on their outer rim.

These marks will insure that you know the correct positioning of the cylinders relative to the corrector lens, with the secondary mirror cell removed.

Click on any of the pictures for a larger view.

 

Step 3: Compare the schematic to the picture of the secondary cell.


I found a set of schematics for an 8" LX200GPS on this website: LX200GPS.CRCM.NET

Compare this schematic with the second image in step 2 to get a good idea of the various components and how they are held together.

Of course, the schematic shows the secondary mirror assembly detached from its cell. DO NOT DO THIS. The only way it can be detached is by removing all three collimation screws, which you should NOT do under ANY circumstances.

Click on the schematic for a larger view.

 

Step 4: Prepare for removal of the corrector lens


Instead of trying to manipulate all of the components with the corrector still in the OTA, I decided to remove the corrector entirely and place it on a piece of plywood with a hole cut out of it.

This enabled me to work with the center assembly safely away from the main mirror. I also decided this would be the perfect opportunity to clean my corrector lens at the same time.

First, the plastic ring was used as a template to cut out the hole for the corrector lens support. In this way, the corrector lens could be placed on the plywood with only the periphery touching something solid (as it does within the OTA).

Cotton pads were placed around the edge of the hole as shown in the image to the left.

The whole board was then placed securely over a bucket, supported by two pillars (in this case, my "pillars" were a stool and an ottoman:).

With the center assembly removed (of course), this setup enabled me to spray cleaner solution and distilled water directly onto the corrector lens without worrying about getting liquid in other parts of the telescope. If the board is perfectly level, the cleaning solution can also be left on the corrector for a minute or so, to allow the solution to loosen any dirt on the corrector prior to cleaning. I do NOT go into cleaning details for the corrector. You can look them up yourself on the internet.

Click on any of the pictures for a larger view.

 

Step 5: Remove the corrector lens


Put on some rubber gloves to minimize oil transfer from your fingers to the corrector lens.

Firmly grasp the corrector lens assembly by the two "rings" of the center assembly.

Pull the center assembly and corrector lens out of the OTA and place it on the cotten pads of the plywood. Here's some advice: DON'T DROP IT.

Click on any of the pictures for a larger view.

 

Step 6: Remove the secondary mirror.


Unscrew the three OUTER screws. DO NOT unscrew the collimation screws.

Firmly grasp the ring of the assembly containing the secondary mirror (see closeup and schematic of the center assembly above).

Slowly, pull the mirror assembly out of its cell being careful not to hit it on the side, and place it in a safe place, away from the rest of your setup. Here's more advice: DON'T DROP IT.

Click on the picture for a larger view.

 

Step 7: Unscrew the secondary assembly cylinders.


First, make a mark on the ring as shown to the left, in a line on the diameter connecting the factory mark and the previously marked location. This will allow you to replace the cylinders in the correct location, so that the secondary mirror can be correctly returned to its place.

Now the tricky part: from the top, place fingers inside and thumb on the ring edge to get a good grip of the external(upper) cylinder.

Reach with the other hand underneath the corrector lens and grip the lower cylinder.

Now, slowly apply torque to turn the lower cylinder in the direction shown in the image, and the upper cylinder in the opposite direction.

Important: be careful not to push up or down on the corrector lens as you turn the cylinders. For my telescope, this required a bit of force, approximately the same about of force as you would use to unscrew a jar of pickles.

Click on the picture for a larger view.

 

Step 8: Inspect the cylinders and gaskets.


Once the cylinders are unscrewed, you should notice two gaskets. On my telescope, one was loose and on the side of the lower cylinder (see picture left). The other was firmly adhered to the upper cylinder (and not detachable).

At first, I considered adding a thicker gasket. However, I decided to try to screw the two cylinders together, now that I had them away from the corrector and could experiment with as much force as I needed.

On doing this, and as I mentioned in the summary, I discovered that I *could* tighten them together, but only after the threads were "worked in". This resulted in some of the thread coating "shaving" off, which I could clean away with impunity, being away from both the corrector lens and the main mirror.

Doing this guaranteed that I could then easily tighten the cylinders when reassembling them, obviating the need for a thicker gasket.

Click on any of the pictures for a larger view.

 

Step 8: Clean, tighten and reassemble


The rest of this is described in text, because I was too busy to take pictures!

First, I blew the dust off my corrector lens, blowing it off it with a plastic bulb blower.

Next, I took advantage of the bucket and hole in the center of the corrector to thoroughly clean the corrector using suggested methods.

Once the corrector was clean, I began the process of reassembling the OTA.

I used a caliper to measure the outside diameter of the center cell and the diameter of the corrector lens. This let me calculate a required distance from the edge of the corrector to the outer edge of the assembly, so that I could properly center the assembly in the corrector hole. I then set the caliper to the computed distance, which approximately corresponds to the distance shown in the picture on the left (except here I show it after I've reassembled the OTA, whereas the actual distance was measured with the corrector lens out of the OTA). I used that distance to properly center the assembly as I screwed the two cylinders back together. This time, however, I could tighten them more to result in a nice snug fit for the center assembly.

Note: of course, the main assumption is that both the main mirror and the secodary mirrors are centered within their respective cells.

And that's about it! Just remember to line up all the marks when reassembling, don't dirty up the corrector lens, and most of all, DON'T DROP ANYTHING!

I'd like to give a special thanks to NeoDinian for his tips and moral support!

 

So now, you be the judge!


I took these two pictures of Mars on two different nights. These are typical raw single frames from a web cam video.

The top image was taken before my procedure. I had not actually collimated that night, but had done it previously and my star images were reasonable in a low power eyepiece.

The lower image was taken after my procedure. In this case, I did collimate.

Obviously, the top image shows more features because of atmospheric clarity and/or the precise features visible on Mars that night. However, the bottom image is clearly more focussed and the planet edge much sharper than before. You can also see a well defined polar cap in this particular snapshot.

So, needless to say, I am very happy with my results, and can't wait to take some more pictures!

 

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