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Disassembly of an SCT: Child's Play

Posted by Greg Nowell   09/13/2007 07:00AM

Disassembly of an SCT: Child's Play
This article features 10 pictures of a C8's disassembly. Geoffrey, age seven, did almost all of the work himself, but needed help with the clip rings and removal of the primary mirror which is a bit heavy and something of a leveraged reach for a seven year old.

Only rarely will an owner need to disassemble an SCT as completely as shown here. One of the main reasons to "go all the way" is to re-lubricate the main sliding tube (Slick 50 is a good choice of lubricant). Other people get into exotic modifications such as adding cooling fans, custom paint jobs, and so on. Occasionally the screw attaching the threaded focus shaft to the mirror assembly needs tightening and a drop of lock-tite. In order to pull the main mirror out the front, the front casting must be removed. The mirror won't fit through otherwise. Although it is tempting to think that one can remove the front casting with the corrector inside, this won't work, because the screws that hold the casting have nuts on the other side, which must be securely held to re-tighten; you can't do that if the corrector is in place. The same applies if you attempt to remove the rear casting: the nuts must be held in place, so the corrector has to come off.

In these pictures there is an emphasis on using marker tapes, severed at the joint between casting and another removable part, to allow for proper re-alignment of everything on re-assembly. This is because the orientation of the corrector and secondary are crucial the optimum performance of the telescope. One marks the casting orientation to the tube, in order to keep the proper orientation of the corrector in relation to the casting. In the illustration of the corrector marker tape, it is deliberately left askew to show that the tape was cut. The proper position would have both edges lined up. Two pieces of tape are used so that if one should chance to fall off (perhaps during cleaning) you'll have a backup.

One step which is not shown, because it was forgotten, is placement of the shims. Small shims may flutter out. You can mark their position if you are careful and put them back in exactly, but the main idea is just to space the corrector evenly--three shims spaced at 120 degrees or four at 90 degrees. The idea of these shims (usually cork) is to dampen any shocks against the corrector and provide some give for thermal expansion and contraction.

Most typically, people remove their corrector plates in order to clean them. This is something of a "sooner or later everyone has to do it" issue, and it helps to have the confidence needed to do it. If you know the procedure you also need not panic at the thought of a small bug or piece of debris inside the tube, as such foreign matter is easily removed. A corrector plate is also easily removed for travel, and could easily fit in a small package to be put under an airline seat.

Although hundreds of SCTs are shipped every year, the large ones in particular are vulnerable to corrector related damage. This is because the secondary in the middle of the thin corrector glass can put great torques on the glass, no matter how well the rest of the scope is packed. The corrector can crack, and another bad scenario is the secondary can fall off, damaging the primary and destroying the telescope. Separate packaging of the corrector makes a lot of sense from the point of view of stability, particularly of if the corrector is securely wrapped to protect optical integrity, and then supported on both sides by bubble wrap or foam, so that shocks do not stress the glass in the same way. Images of packing this particular C8 with the corrector separately boxed will be provided in a separate contribution.

There are a number of upgrade and "fix-it" operations that can be done while the telescope is apart, including internal flocking, installation of an internal dew heater, tightening of the primary retaining ring to eliminate loss of collimation, and so on. You may also wish to inspect the secondary to make sure that is securely held and that the whole secondary housing is not rotating on the corrector plate. (Detailed presentation of these maintenance issues can be found in the files section of Yahoo! C14 and other places on the web; "How to Clean Your Corrector Plate" gives details on this procedure, which is the most common)

The pictures here show a smooth disassembly, but in about 10 or 20% of cases there is a glitch. Normally the corrector pulls off with moderate force, but sometimes it seems "unreasonably stuck" -- more force than the little shims would seem to exert to keep the corrector in. What is happening is that there is a blackening agent underneath the corrector on the non-optical surface, where the corrector rests against the tube. Sometimes this paint (or marking pen ink, which is what I think it is) bonds to the glass and creates the worrisome impression that you'll have to break the glass to get it out. The cure for this is an eyedropper and some denatured alcohol (NOT rubbing alcohol; oils in rubbing alcohol can streak your glass).

Take an eyedropper with 91% or 95% alcohol and drop some of the solution between in the crack between the corrector plate and the metal of the tube. Do this at various points around the corrector. The solvent will weaken the bond to the ink and you can lift it out. Dry the painted area thoroughly and follow instructions for cleaning the corrector plate (available on Yahoo! C14 and other web sites). Keep the tube horizontal. The alcohol won't damage your optics or your tube and will evaporate readily.

Re-assembling your SCT is a reversal of the process shown here, but care must be taken NOT TO OVER-TIGHTEN THE RETAINING RING at the last step. Snug but not scrunchingly tight is the goal; too tight will crack the glass. The SCT will need to be re-collimated when it is put back together. If your shims have been glued in place (not a bad idea), and great care has been taken to put everything back in the same orientation with which it was taken out, surprisingly little loss of collimation can occur.

It is very easy to disassemble a C8, and owners of C11s and C14s might consider buying a c5 or C8 just to familiarize themselves with the process on a small scope before trying it on a large one. The procedure for Meade SCTs is similar, but there are some reports that the Meade castings are epoxied on to the tube, so the process will not be identical. Check with a Meade discussion group before attempting anything radical.