It didn't take long to find the cause: the mirror cell was designed with clips to hold the primary mirror in place, and these clips covered the top of the primary mirror by about 1/4" . That's 1/4" ON THE MIRROR...and those nice sharp edges on the mirror clips act just like the sharp lines in a spider, causing diffraction. There was my culprit! Additionally, with mirror clips that hold the mirror down from the top can be pulled down too hard against the mirror, causing astigmatism when the mirror is slighly warped out of shape by the pressure. I had also noted some astigmatism in the images I saw...clearly, those mirror clips had to go!
Here's a detail of what the cell clips looked like before the "clipectomy" surgery:
The modification detailed here will remove the top part of each clip (there are three) that overhangs the top of the primary mirror to get rid of the unwanted diffraction spikes and to reduce the chance for astigmatism from clips that are too tight. To hold the mirror in place and keep it from falling off of the mirror cell, which is what the clips do in their original form, silicone cement is added at the bottom of the mirror where it contacts the mirror support block, and on the sides of the clips to keep it from shifting laterally and provide additional support. The modified clips and mirror cell will look like this when finished:
Making the cell modification step-by-step:
Start by setting the telescope upright, corrector plate down, on a stable work surface. Put a piece of tape on the side of the tube, so that it crosses both the side of the mirror cell and the white part of the tube -- this will serve as an index mark so that you put the mirror cell back in the same position when you're done! Cut through the tape where the joint between the cell and the tube is so you can remove the cell. There are three small allen screws that hold the mirror cell in place, around the edge of the mirror cell. Remove these three screws, set them aside where you'll find them again, and GENTLY pull the mirror cell out of the tube. It may be a somewhat tight fit, so you may have to lift one side of the cell then the other, working it back and forth. Go slowly and carefully, you don't want to scratch the mirror or bend the tube!
Once you have the mirror cell out, look at it closely. The stock cell has three rubber vertical risers that are in an inverted-L shape (see diagram above) with a metal clip on top of them. Each is held to the main mirror cell by two allen nuts. The L-shape goes on top of the primary mirror and holds it in place, with tension adjusted by the allen bolts. The mirror sits on three small mirror support blocks, with cork pads on the sides and bottom to cushion the mirror. All in all not a horrible mirror cell, except for the part where the clips cross in front of the mirror!
You can see all of this in the picture above. Before you do anything with the clips, use a piece of tape to index the mirror just like you did on the mirror cell and the outside of the tube. You want to put the mirror back in exactly the same orientation as it was when you remove it...Meade matches corrector plates and mirrors for the best optical quality, and this match is made on the basis of a particular orientation of the mirror and corrector plate. Don't rotate the mirror, or you may pay the price with reduced optical performance.
After you've indexed the mirror to the cell, remove all three rubber mirror clips by removing the six allen bolts, then lift the mirror from the cell and set it aside (covered with a soft tissue, of course, to keep dust and debris off it!). Below you can see, with one clip removed, the mirror support block and the cork pads. The removed clip sits off to the left.
Start by getting a nice sharp knife (a sharp steak knife or Xacto knife will work). Remove the allen bolts from one of the rubber clips, and pull of the little metal plate on the top. Using the knife, slice off the part of the L shape of the clip that extends on top of the primary mirror. You'll cut off a little rubber slice, and be left with what you see below:
Next, turn the clip on its side, and drill a small hole right in the middle of the two bolt holes as shown below:
This hole will be used to squirt silicone through for the mirror side supports later. The size of the hole is up to you, but I used a 3/16" drill. Be sure to run the drill bit through the hole back and forth several times, as the hard rubber used for the clips tends to want to close up the hole, and you need enough room to squirt the silicone through when you're done.
Next, take on the mirror cell:
The view above shows the mirror cell with the mirror and all the clips removed. You can see the cork mirror pads in place on two of the support blocks, with the cork removed from one of them. Go ahead and remove the cork pads from all three support blocks, making sure to get ALL of the adhesive off, and removing the underlying paint on the block to get to bare metal. The silicone glue will stick MUCH better to bare metal than it will to painted metal, and adhesion is important here! You an also see the particular brand of silicone I used, available at Home Depot in the U.S. for about $4 a tube. Note also the tape acting as index marks on both the cell and the mirror in the background...
Once you have all of the cork removed, and all of the holes drilled in the rubber side blocks, you're ready to re-assemble the cell and mirror using the silicone to hold everything in place. If you have any reservations about the holding power of the silicone, ask any ATMer who's built a scope or two -- this stuff holds. I personally built a telescope with a 12.5" mirror, held in place only by 9 little pads of silicone, and it hasn't shifted positions at all in over 2 years (see here and click on the Papa Joe link).
Open up the tube of silicone, and place three GENEROUS globs of silicone on the mirror support blocks where the cork pads used to go. You want a big enough glob of silicone to go down to the bottom of the mirror cell, and also to squish up the side of the support block a bit, as shown above. Once you have the silicone in place for the bottom support blocks, place the mirror back into the cell, BEING CAREFUL TO ALIGN YOUR REGISTRATION MARKS, and center the mirror carefully in the cell. Then set the whole thing aside, covered with a tissue, to cure for at least 24 hours. DO NOT reinstall the side supports (the hard rubber you drilled holes in) until the bottom portion has cured! You don't want the side supports carrying any of the mirror's weight, and they might do so if you install the side supports before the bottom is dry.
The next day, re-install what used to be the clips (and are now side supports) by screwing the allen bolts directly into the rubber supports, WITHOUT the metal plates on top. Install all three side supports, and tighten them down fairly securely (you don't have to worry about astigmatism from them being too tight now, they don't touch the mirror top anymore!). Once they're in place, squirt some silicone into the holes you drilled in them, so that it flows through the hole and makes a nice little blob between the rubber side support and the mirror itself. Refer to the diagram at the top of the page to get an idea how big the blob should be. Squirt all three supports, then set the whole thing aside to dry for another 24 hours.
Incidentally, while you've got the mirror cell off, take a look into the back end of the scope:
On the 6" model that I did this work on, you can see how much the focuer intrudes into the image space when it's nearly fully racked in. This causes light loss and more diffraction effects, though the diffraction isn't as pronouced as the mirror clips cause. With my color filter wheel and CCD camera setup, I originally had to have the focuser this far in to reach focus...I shortened up the optical train of the CCD setup a bit, but I also used a bit more silicone at the bottom of my mirror cell, pushing the primary up about 1/8" from its original position. This has the effect of moving the focus point a bit further out the side of the tube -- with that modification and my shorter image train, I can now reach focus without the focuser sticking so far into image space. This is less of a problem on the 8" and 10" Schmit-Newts, since their tubes are nearly 2" larger than the primary mirrors, where on the 6" the tube is only 1.1" larger than the mirror.
Once everything is cured, you can carefully re-install the mirror cell into the tube, lining up your index marks. Then re-collimate it, and take it out for a test drive! My own scope showed a vast improvement after this *surgery* was done on it...compare the two star images below:
The image on the left is a normal magnification image of a fairly bright star from a CCD image I took the first night I had the 6" SN. The image on the right was taken after the clipectomy, and is a 400% enlargement of a star nearly as bright as the one on the left. The diffraction pattern seen on the left is COMPLETELY gone, and the star shapes looks much better in the second image. I expect the scope will now, with its smooth optics, flat field, and lack of color aberration, provide images close to what a 6" APO refractor can give, with of course a bit of contrast loss due to the SN's central obstruction. I haven't been able to take any long detailed shots yet, but here's what this scope can do BEFORE the modification...
Full-sized M16 image from 5-14-02
half-sized M16 image from 5-14-02
If you've read this far, congratulations on your patience. The mirror cell modification is not hard to do, and pays off with big benefits for imaging and visual observing. If you're at all squeamish about opening up that brand new tube and voiding your warranty, then DON'T DO IT! At the very least get an experienced mirror-cell maker to help. You should also read Gary Seronik's article in the May 2002 Sky and Telescope, "A Simple Double-Plate Cell for your Telescope's Mirror", which covers many of the same techniques I covered here, but for home-built mirror cells.
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