Before breaking out the lens cleaner and tissues, let's review how to minimize the need for cleaning. Always keep the corrector cover in place when the scope is not being used. It's also a good idea to leave the cap on when the scope is outside cooling down prior to use. Storing the telescope in a case also tends to minimize contamination. Keep the rear port capped at all times. Dust, bugs, and other contaminants can enter the OTA via that route, and removing them is a huge hassle. A major source of corrector contamination is dew. Dust and pollen and other contaminants will tend to stick to a wet corrector. Dampness can also promote the growth of fungi that will literally eat the lens. Keeping dew from adding to cleanliness problems is easy to do with one of the corrector heaters described in Chapter 5.
So, how to do the cleaning? Deal with the easy stuff first, the loose accumulation of dust on the corrector's surface. The easiest way to get rid of dust particles is with "canned air," compressed air designed for cleaning optics and electronics. If possible, purchase a brand designed expressly for photo/optical use, since that is more likely to be free of contaminants than a brand designed for cleaning computers.
To dust the corrector, move the OTA until it's just above level and lock it. Remove the dust cover and, holding the canned air "gun" level to prevent liquid propellant from being sprayed onto the lens, give the corrector a few blasts to blow the loose dirt away. Don't hold the trigger down; instead, apply 2- to 4-second bursts and move the air stream around. For best results, blast the corrector at an angle from about a foot-and-a-half away. Continue until all the removable dust is gone. Some amateur astronomers fear canned air, having been told by those supposedly in the know to never use it around telescopes. Like many stories passed from amateur to amateur over the Internet, there is a grain of truth there. It is inadvisable to use canned air on a first surface mirror. Depositing liquid propellant on a mirror or blowing a particle of foreign matter onto it at high velocity can do damage. One of the major applications for canned air, however, is cleaning lenses, which is what the corrector plate is, and it works well and safely in that role if the above cautions are observed.
What if some stubborn dust remains after a canned air dusting? Or if, in addition to loose dust, the corrector has smudges and spots—"crud"—on its surface? Proceed to the next "level" of cleaning to remove small blemishes. What you should consider using for these small jobs is a remarkable device called the "Lenspen " (Plate 39) found in camera stores and at astro-dealers. There's a soft extendable brush on one end for dusting the area of interest. When that's done, the cap is removed from the other end of the Lenspen to expose a soft disk impregnated with non-liquid cleaner that is used to wipe away smudges and dirt with a gentle circular motion. Some CAT users wonder about using the Lenspen's tip over and over to clean; all I can say is that I have been using this handy device for years without a single problem. Lenspens are available with tip sizes that range from small, which is useful for cleaning short focal length eyepieces, to larger sizes appropriate for small areas on the corrector and other lenses.
Cleaning the whole corrector with a Lenspen would be a slow and tedious process. If there are large dirty areas, it's time to hit the lens cleaning fluid and lens tissues. Never use tissues designed for eyeglasses lenses. These are often impregnated with a silicone compound that will make a smudged mess of the corrector plate. Lens tissues designed for photographic use are available in camera stores. Don't use cleaning fluid designed for glasses, either. Who knows what's in it? It is as likely to make the lens worse as better. Camera equipment dealers sell cleaning fluid, but if that's not available the formula Meade publishes in its manuals works well: one part pure iso-propyl alcohol, two parts distilled water, and one drop of biodegradable unscented liquid dishwashing soap (Ivory is good) to make a pint of solution.
Whatever kind of fluid is used, always apply it to the lens tissue and not to the surface of the corrector. It's easy to use too much cleaner, which can migrate into the interior of the OTA and leave stains or promote fungal growth as it slowly dries. To clean, dampen a tissue and wipe using gentle linear strokes and working from the secondary holder outward. Rotate the tissue after each stroke to expose its clean surface, and change tissues often. When the smudges and dirt are gone, set the cleaning fluid aside and gently dry the surface with clean tissues.
A less costly solution? Windex glass cleaner and Kleenex tissues. This combination has worked spectacularly well for this author for over thirty years, yielding squeaky clean correctors. There are a few caveats. First, use only the "original" blue Windex . Additives designed to make the stuff smell like a flower garden won't do anything good for a corrector. Be careful about the type of tissues, too. Buy only unscented, lotion-free white ones. The only problem with the Windex/Kleenex combo is that a small amount of lint can be left over after cleaning. That's easily gotten rid of with a Lenspen's brush or some canned air. You can also use Windex on both UHTC and XLT coated correctors as well as with Meade and Celestron coatings: it does not damage them and cleans better than anything else.
Now, let's consider how to clean the inside surface of the corrector. Normally, it should never require cleaning. An SCT that is kept in a case with the rear port capped will likely go many years—perhaps a lifetime—before "inside" problems develop. However, some older SCTs, especially those stored in hot attics and garages, develop a hazy film on the inside surface. This appears to be caused by outgas-sing from materials used in the OTA interior. Primary and secondary mirror s can also develop cleanliness problems. That is even more unlikely than problems with the corrector's inside surface, however. Stuff happens, though, and it is possible (though not recommended) to clean the primary and secondary yourself.
When a problem develops with an interior optical surface of a telescope, the best course of action is to ship the CAT back to the manufacturer for cleaning. Most often, interior cleanliness issues occur in older telescopes that are good candidates for a thorough factory cleaning of both optical and mechanical components and— importantly for older SCTs and moving mirror MCTs—a re-lube of the baffle tube. The problems are that you'll be without your beloved scope for a while, you'll have to trust it to the tender mercies of the UPS and FedEx folks, and the cost of cleaning and freight charges can be substantial.
If all the CAT needs is an inside surface corrector cleaning, the job is fairly easy and safe to perform and is a perfectly reasonable course of action for many amateurs. There are a few warnings that should be understood. Delving into the interior of the telescope's OTA may void its warranty, though it's unlikely the manufacturer will be able to tell the scope's been opened as long as it has been reassembled correctly. More seriously, any big mistake, ranging from accidental misalignment of optics to breaking the corrector plate, will probably cost more in repairs than a cleaning by the maker would have. If, and only if, you accept full responsibility for these possible outcomes, proceed to attempt a cleaning of a telescope's interior optical components as described in the following paragraphs.
After these dire warnings, the actual act of removing the corrector plate is anticli-mactic. There are only a couple of precautions to observe. Most critical is returning the corrector to its factory rotational position when it's replaced. If the corrector is reinstalled in a different position, images will be degraded. Most Meade telescopes have marks on the corrector and the lip it rests on to indicate proper orientation. If there are no obvious marks, scribe both corrector and lip with a soft pencil or marker. Celestron corrector plates are usually not marked, but there will almost always be a small serial number engraved in the outer surface at the edge. The correct rotational position will be with the serial number at the 3:00 o'clock position.
All SCTs have small cork or paper shims along the edge of the corrector that keep it precisely centered (Plate 60c). Try not to disturb the shims, and mark their positions so they can be replaced during reassembly if they go flying when the corrector is removed. They are clearly visible when the retaining ring is removed and before the corrector is pulled out. It's not a bad idea to make notes on disassembly during the process as an aid in putting things back together.
Before pulling the corrector, place the SCT on a solid, clean surface, and prepare a place for the lens to go when it's out. You might lay it on a couple of soft bath towels. Tilt the OTA up at a 45 degree angle and lock it so the corrector won't fall out when the retaining ring is removed. Remove the screws (either Allen or Phillips head) that hold the plastic retaining ring in place (Plate 60a), set the ring aside in a safe place, and mark shim positions with a pencil. Lastly, remove the corrector plate itself by grasping the secondary mount and pulling gently but firmly (Plate 60d). Sometimes the corrector won't want to come out easily. Deal with a sticky one by gently prying along its edge—very gently—with a small flat-blade screwdriver or a wooden tool (manicure sticks work) while continuing to pull steadily on the secondary mount. Don't pull too hard, though, or the corrector could go flying across the room, leaving you standing there like a refugee from a Three Stooges film. When the lens comes free, set it on the towel with its inside surface up (to protect the secondary mirror). Handle the corrector only by its edges or by the secondary mounting.
Clean the corrector's inside surface using canned air, a Lenspen, or tissues and fluid as appropriate. Be careful. The corrector plate is almost window glass thin and fairly easy to break. Replacing it will cost substantially more than replacing a broken window, though! Be sure to keep fingers away from the exposed surface of the secondary mirror . Luckily it is partially protected by the short cone-shaped baffle that extends out from its mounting.
When the corrector's inside is satisfactorily clean, reinstall it. Before setting it into the OTA , replace any dislodged shims in their original positions. If they don't want to stay in place, moisten them with a little saliva. Tilt the corrector on edge on its towel, touching its edges and secondary baffle only, and, holding it in place with one hand, grasp the outer secondary mount firmly. For safety's sake, keep one hand on the corrector's lower edge and guide it into place, observing the proper rotational orientation (the marks on lip and corrector or the serial number). Double check that the corrector is properly positioned before replacing the retainer ring and installing its screws. Don't tighten these screws down too much—finger tight only. If there are now a few fingerprints on the outer surface of the corrector that were put there during the process of replacing it, clean them as before. Done!
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