Mirror Removal/Installing/Recoating

       
 

Subject: LX200 Mirror Removal/Installing   

From: Rob Roy <rroyexeculink.com>

Turn focuser counterclockwise as far as it will go. Remove the 3 screws holding focuser assembly plate to the scope. Loosen the 2 screws on either side of the focuser knob. Hold the brass piece which is uppermost on the threaded rod and unscrew the focuser knob until it and the plate with 3 screw holes come off. Be careful to keep spacers intact and in their correct order. The brass piece left on the threaded rod will come off but it is not necessary to remove it.

Tilt the main tube up slightly from horizontal and remove the corrector plate retaining ring. Paint a pair of alignment marks on the corrector plate and tube if none exist and lift out the corrector plate. It's ~1/4" thick, so don't be afraid to lift up or tug on it a little bit if it's stuck.

Remove the wire keeper ring at the very end of the cylinder on which the mirror mount slides. At this point moving the scope to a low stool will make it easier to reach into the optical tube, which should be vertical for the next step. The threaded rod has to be removed with the mirror. Once the mirror is free of the center cylinder, it has to be tilted ~60 degrees to clear the 1" openings on either side of the main tube. Watch that the threaded rod doesn't fall off or get in the way of moving the mirror through this narrow opening at the top of the tube. Once the mirror is out, the threaded rod can be slipped off.

I used a small sponge brush with a 1/8" notch cut into the side to apply the flat black paint. This notch made it virtually impossible to paint deeper towards the center than the desired 1/8" depth. The mirror was centered on a lazy Susan to better control its circular motion under the brush.

Reassembly is just the reverse, with a couple of extra steps. Before reassembling the focuser, use the threaded rod to move the mirror up and down on its the cylinder 10-12 times to redistribute the grease evenly. After the focuser is completely assembled, turn it clockwise until it stops. Loosen the two screws on either side and turn the focuser knob clockwise until it stops once again. Back off very slightly to just unbind it and retighten the two side screws. These last two steps are to remove any backlash in the focuser and any possible image shift.

Subject: Corrector Plate Removal Procedure --Part 1 of 5 

From: Ron Kunk

To remove the corrector plate:

1. Point the Optical Tube above the horizontal, so it is easy to get at the front end of the OT and so corrector plate cannot fall out when it is loose.
2. Take out the six Allen head screws around inside perimeter of corrector plate's collar. This loosens a flat plastic ring. The ring is only about 1/2 inch wide and less than 1/8 thick.
3. Lift off the flat plastic ring.
4. Find the orientation marks that align the corrector plate to the Optical Tube Assembly (OTA). Mine has two little white dots, one on the corrector plate and the other immediately opposite it on the OTA.
5. Lift out the corrector plate by gently pulling on the secondary mirror holder. The corrector plate should come out without too much trouble. If it won't move, it has become stuck to the cork spacers. See suggestions below.
   If the secondary mirror holder tends to turn, put some alignment marks on it and the corrector so you can put it back in the same orientation. In fact after you lift out the corrector plate you can now tighten the secondary to the corrector by simply twisting the inside cylinder clockwise relative to the outer housing.
6. Re-assemble in the components in the reverse direction, being sure to position the corrector to match up the alignment marks.

Subject: Corrector Plate Removal Procedure --Part 2 

From: John Rostoni

The first time I removed my corrector, it was also stuck pretty tight. I pointed the scope down, and padded the space between the corrector and the fork base with a pillow, leaving just enough room to get my hand under there. I removed the visual back and stuck the output hose of my small shop-vac over the hole. I put a piece of felt between the hose and the hole, just to keep extra dust out.

Five seconds of this, and pop...out it came.

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Subject: Corrector Plate Removal Procedure --Part 3

From: Craig Tupper

I did try to take the whole front end off, but I found that loosening the 6 outer bolts did not allow me to take the whole front end off. I suspect it is a pretty tight press-fit job, and I didn't want to try hammering it off. So I went back to square one, and tried the following:

1. I took a small x-acto type blade, and carefully worked it in between the corrector and the 8 pieces of cork, to see if that was the source of the binding. No dice.
2. I reconciled myself to applying some kind of lateral force ("prying") on the corrector plate. I had tried using my hands on the secondary holder, but that hadn't freed it up. Realizing that using something hard and pointed (like a screwdriver!) had the potential of chipping or cracking the plate, I decided to use wood shims to distribute the force and compress a bit. Whenever I am woodworking, I always keep whatever thin strips of 3/4" thick wood I happen to trim off; it comes in handy for lots of jobs. I found one that was just thick enough on the end to work gently but firmly into the gap, rocking it back and forth slowly. I ended up sticking one piece in there and leaving it, then working another piece into the gap on wither side of that piece, 45 degrees away. After doing that, I was able to remove the shims and finally pull out the plate, although it still took a lot of effort and it only came slowly. I learned that there is a thin felt-like ring behind the plate, which was partially adhered to my corrector almost all the way around.

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Subject: Corrector Plate Removal Procedure -- Part 4  

From: Michael

----- Original Message -----
From: Al Brockman <alanbcscoms.com>
> I am looking for a way to get the corrector lens off my 10" LX200. It
> won't budge even after unscrewing the cover ring and removing the cork
> shims. Any advice appreciated.

A safe suggestion is to point the corrector end down to allow corrector glass to free fall safely on a soft material such as bubble wrap, pillow, or sponge.

Use a blow dryer on the higher heat setting and heat the outer portion of the OTA nearest the corrector. The idea is to soften the seal and most likely the gasket/ seal is sticking from dew moisture.

A soft tapping action with your hand would "help" at the same time as heating.

PS: note the indexing mark / orientation of corrector glass so you can return it to same location.

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Subject: Corrector Plate Removal Procedure -- Part 5 of 5  

From: Robert Haler <rhalerlymax.com>

Percussive Maintenance--

Take a soft hammer...and with the scope pointed straight up…*gently* tap the outside of the casting all around the tube at the level of the corrector plate. Force is not what is important -- sudden impact is. I do this all the time to loosen corrector plates on old scopes. Works like a charm.

Editor's Note: I have been successful by gently prizing around the corrector with a small wooden stick, like a ice cream stick, until the corrector broke loose from the gasket.

Subject: Corrector Plate Replacement  

From: Bill Dougherty <bd5572yahoo.com> Date: Nov 2003

I had also heard that the optics of the LX200 were a matched set. But a while back someone who had to replace the corrector on their LX200 reported that Meade told him that the corrector plate was not matched to the primary and secondary. They are in fact rather generic parts. So he had no trouble getting just a replacement corrector plate. Apparently, it is only the primary and secondary that are matched carefully. So it seems that the real reason for not rotating the corrector upon replacement is to maintain the factory setting of the secondary to the primary, not the orientation of the corrector plate to the primary.

Remember that Meade makes literally tons of these optics. They are very good at it. As long as the primary and secondary are truly rotationally symmetric to their mounting points, even their rotation with respect to each other will not matter. Still, to be on the safe side, you might want to try to maintain the rotation of the secondary to the primary when you replace the corrector.

Subject: LX200: Primary Mirror Recoating    

From: Michael Hart

> My question to the group is this, has anyone ever had their primary

> recoated and if so, how did it go, and were you happy with the results.

> A scan of the MAPUG archives revealed no history of this question

I have not needed to have a SCT mirror recoated. They seem quite robust. I believe they can be recoated much as other mirrors with a few extra precautions.

You must scribe a mark on the primary mirror position on slider if needed because the primary will need to be removed from the slider before re-coating. Scribing involves scratching the mirror back and the slider with a diamond (possibly carbide) scribe to index their position. Some correctors are not marked and may need to be scribed as well. The old coating is typically removed chemically by the re-coater without any changes to the mirror's figure. The mirror is placed in a high vacuum chamber and cleaned quite well before coating, possibly using an ion bombardment to the surface while pumping down the chamber.

Small riders of pure aluminum wire are suspended on coiled tungsten filaments inside the vacuum chamber and heated to over the 600 degrees C. melting point of aluminum forming small drops of liquid aluminum on the tungsten. On the hottest portions of the tungsten element (1200 degrees C.), the aluminum evaporates and the resulting molecules radiate to the glass surface quite rapidly. Other evaporators inside the vacuum chamber are used to apply silicone monoxide coatings, for example.

The mirror coating process will not effect the figure of your mirror, so if you etched an index mark on the back of your mirror, you will want to line it up with the index mark on your slider. Don't forget to install any gaskets (often cork).

You may have several choices in coating reflectivity. The high reflectivity enhanced coatings usually loose reflectivity with a bit of time and may actually attenuate the wavelength of interest. Ask and your recoater will explain details. I believe the re-coater can confirm the suitability of recoating your SCT mirror with the information in this post.

Subject: Baffle Tube Off-Center   

From: Scott <ScottXRAyaol.com> Date: Sep 2003

Rvan wrote: my baffle tube measures 4mm off-center, is this a problem?

I also have a 7-year old 10" with an offset baffle tube. The problem is caused if the exterior shell of the Optical tube was not seated fully at the time of manufacture. It is a pressed fitting that should be completely even at the rear of the scope where the rear plate meets the rear cell assembly. A gap on one side, or the other, of the assembly will cause the baffle to be offset within the optical tube towards the side opposite of the gap. If you do not see this gap at the rear cell carefully remeasure the offset using a caliper. A tape measure is not adequate for this.

On mine there is a .75mm gap on the bottom right side of the rear cell, resulting in about 2.75 mm offset to the upper left of the OTA. I have spoken to Meade about mine, and unfortunately the only fix is either OTA replacement or return to Meade for disassembly and realignment. This CANNOT be repaired in the field, as the OTA is glued to the rear cell, then screwed.

If the gap is large enough it CAN lead to problems with collimation at DIFFERENT focus positions, as the corrector plate/secondary is no longer parallel to the primary mirror. There will be a slight angle of offset, and the secondary "sweet spot" will move slightly as the primary mirror slides up and down the baffle. Using different accessories, such as a two inch visual back, vs. the 1-1/4" would show this effect. When properly collimated I also had a lobing effect to one side of my images , due to the offset Primary corrector, I think. Of course the mirrors in the visual backs must also match collimation. If you do not see this effect then STOP...do not do anything. Your scope will be fine as is, although not mechanically perfect.

You CAN correct this by two methods.
1. If the gap at the rear of the scope is say 1mm at the bottom of the scope, place cork or other shims behind the corrector plate at the front of the scope of 1mm thickness at the top , and 1/2 mm about 1/3 of the way around the OTA towards the bottom, etc. etc. to make the corrector plate parallel to the primary mirror. Before moving the secondary make sure the plate is indexed to the OTA with a mark of some sort. (small paint mark). When reinstalling, make sure you don't over tighten the retaining ring, or you will have a pinched optics effect. Making the shims can be difficult as they have to continuously vary in thickness. I used electrical tape cut to small pieces overlaying the support base in the front ring of the OTA.

After completing this try recollimating and using your scope. Step two may not be needed.

2. The secondary usually has about 2.5 mm of play within the corrector plate cutout. Try to index the secondary to the corrector plate before removing it, by placing a scratch in the front of the secondary carrier at the same place as your corrector plates index mark. Shim the secondary using small pieces of tape wrapped around the base to offset the secondary to the same side as the offset of the baffle tube. Without doing step 1, this will not be effective. If you truly have 4 mm of offset this may be impossible to fully correct

Precise collimation is of course necessary for the best imaging. If you can collimate your scope, as is, correctly or if you do not use the scope for multiple purposes (photo vs viewing) and don't routinely change your optical train much, the previous procedure is probably not necessary, and you probably won't see any difference in the results. Even if you follow the procedure you will still have to collimate with changes in the optical train. However you won't have to make LARGE changes in collimation, which is what I was trying to accomplish.

You will have to recollimate the scope following this procedure. If carefully done it will correct most if not all of your problem. There are detailed collimation procedures here in the Topical Archives.