Chapter 3 - Maintaining a Telescope
Jack Kramer
1. Collimation
Newtonian Reflectors
"Collimation" refers to the process of aligning the primary and secondary mirrors so that when you look down the tube with an eyepiece, you're looking at the center of the mirrors. Lack of collimation means you're only getting a portion of the light from the object observed. If you de-focus ("rack out") the image of a bright star in your eyepiece, the large, fuzzy, out-of-focus image should be circular. If it's oval, that's an indication that your optics may not be collimated. Newtonian reflectors tend to need re-collimation more often than refractors; it's a good idea to become familiar with the process, because you should check this regularly. The following steps refer primarily to a Newtonian.
1. Using a compass draw a circle on photograph mounting board the exact size as your primary mirror.
2. Cut out the circle to use as a template over your primary and enlarge the center hole made by the compass point.
3. Remove the primary from its cell and place the template over the primary. Then using a waterproof marker pen, place a dot in the center of the mirror through the center hole in the template. You may want to use a Dennison gummed hole reinforcer to surround the dot as a way to highlight it for easier alignment. Replace the primary in its holder. (Don't worry, on a Newtonian the center of the primary mirror is not used to gather light, since it lies in the shadow of the secondary mirror.)
4. A dot on the secondary mirror will be a help in aligning the components, but it's not absolutely necessary. In fact, very few observers place a dot on the secondary. If you choose to perform this step, remove the secondary from its holder. Place graph paper over your secondary and use finger pressure to make a crease outline of your eliptical and then find the intersection of the major and minor axes. Perforate the graph paper with a compass point at the intersection. Then realign the graph paper template over your secondary and place a dot on the mirror at the intersection hole.
5. To make sure your eye is perfectly centered as you line up the optics, you should have a collimating eyepiece. (It's not really an eyepiece - there are no lenses in it.) Make a collimating eyepiece out of a 35mm Kodak film cannister by cutting off the bottom of the cannister and making a very small hole in the exact center of the cannister cap. The exact center is already marked on the inside of the cap during the manufacturing process. You can use a hot compass point to melt your way through or use a small awl. (You'll note that the outside diameter of Kodak film canisters is exactly 11/4" - there probably are some others that also will fit.) You can also use an uncut film canister to plug up the eyepiece end of the telescope to keep dust out of the inside of the tube when it's not being used.
6. Reassemble your scope. Put your collimating eyepiece in the eyepiece focuser. Adjust your secondary until the dot on it is in the center of your eyepiece. (Vertical adjustment.) Then rotate the secondary about its axis until you can see your entire primary mirror. If you can't then you will have to adjust your secondary until you can see all of it.
7. Using the three screws on the mirror cell, adjust the primary mirror so that the dot on your secondary lies over the dot on your primary. If there is no dot on your secondary, simply make sure the dot on the primary appears centered on the secondary mirror.
As you look down the focusing tube, you'll see the secondary mirror with an image of the primary mirror, in which is an image of the secondary mirror. The secondary mirror image should appear exactly in the center of the primary mirror.
Refractors
One thing that'll make a refractor perform better is perfect alignment of the optical elements. This means that the tailpiece (the part that holds the focusing mount and eyepiece) should be square with the objective lens. On a refractor, all adjustments should be made to the tailpiece, never to the cell containing the objective lens. This is because the objective lens cell must be very precisely aligned in the main tube; only an experienced optician should realign the primary lens in a refractor. Remove the right-angle adaptor and eyepiece, then take a look down the focusing tube toward the objective lens. Does it look as though you're aimed at the center of the objective lens? To help you check this more precisely, use a collimating eyepiece made from an ordinary 35mm film canister as mentioned previously. This will ensure that you position your eye directly in the center of the focusing tube as you check for proper alignment.
To square up the optical elements, you may simply need to loosen the screws holding the tailpiece, then move it around until you're lined up. Finally, tighten the screws securely so that the telescope optics will stay where you set them. If a refractor is handled with considerable care, it may never have to be re-collimated.
Schmidt-Cassegrains
Collimation of an SCT employs principles similar to those for a Newtonian; however, it's somewhat trickier due to the design of the optical elements. Since adjustment points are unique to the various instruments from different manufacturers, we won't describe the procedures here. The owner's manual that comes with each SCT contains instructions on optical alignment.
2. Cleaning Mirrors and Lenses
It's important to master the fine art of cleaning telescope mirrors, lenses and corrector plates. Optical surfaces do get dirty from dust, dew, and airborne grime. A dirty mirror or lens results in less light being transmitted or reflected, and the objects you see may have a halo of glare around them.
First, the Materials - The optical surfaces in telescopes are very precise and are coated both to protect them and to increase light transmission or reflection. They're also more delicate than what you may be used to, so don't try to clean them like you'd clean some other optical surfaces. Above all, avoid "lens tissue" like the plague...it's usually too abrasive and may do more harm than good! The best cleaning solution is just a bit of mild dishwashing detergent in warm water, and under certain circumstances, plain distilled water may do the trick. Above all, tap water should only be used in a soapy solution; whenever straight water is used, it should be distilled water. (Minerals in the tap water will leave spots.) We've also heard of using a cleaning solution of 50:50 Windex and water for lenses. If the optics become dewed up while you're out observing, you can gently swab them off with a cotton ball. (After all, dew is another form of distilled water.) Another form of distilled water is your breath which, together with gentle use of a Q-tip, is a respectable way to clean eyepiece lenses. Cotton balls (100% real, sterile, surgical-quality cotton) are the best materials with which to wipe the surfaces. But most cotton balls will leave little "hairs" on your optics. Some drug stores carry a type of cotton ball that is lint and fuzz free; though they cost somewhat more than the regular variety, they're well worth it. Change cotton balls several times so as not to spread around the same dirt you just cleaned off, and always hold the cotton ball by the same spot so that your skin oils aren't transferred to the optics. A good way to get a speck off your mirror or lens is to use a very fine camel's hair brush that's specially made for cleaning lenses, or use a Q-tip...or just blow it off. Speaking of blowing off the dust, it's not wise to use the aerosol "canned" air sold in camera stores for this purpose. This air may contain traces of the propellant chemical.
Lenses and Corrector Plates - Both these items are cleaned in pretty much the same way. If all you want to do is remove dust, they may be gently swabbed with cotton balls and a small amount of distilled water. Make sure the lens or corrector plate is pointed slightly down toward the ground so that the water runs off rather than seeping into the telescope body from around the lens. If the optics are really dusty, it's advisable to use a liberal rinse of distilled water before swabbing with a cotton. That way, most of the dust will be floated away rather than moved around with the cotton ball to possibly scratch the lens coating. If your lens harbors a coating of grime, liberally swab it off with a soap solution, then rinse with distilled water. Unless you're very experienced in optical techniques, avoid removing the refractor lens cell from the telescope tube - recollimation of a refractor is extremely difficult. Good refractor lenses are made as carefully matched sets of individual lenses; even rotating these individual lenses in the cell can destroy the ability of the system to bring objects to best focus. So never remove lenses from the cell. If some water happens to seep into the cell, leave the telescope with the lens uncovered in a warm room until it is dry.
What!! Remove the Corrector Plate? Maybe.Most Schmidt-Cassegrain telescopes will just need the outside cleaned. If there is dust or a film on the insideof your corrector plate, or you want to get rid of stains on the enclosed optical elements of your Schmidt-Cassegrain, then the corrector plate must be removed. It's not a good idea to remove the corrector plate unless you definitely need to clean the internal surfaces. For those brave souls who want to clean the inside, here's how to remove a corrector:
1. Place the telescope with the plate facing up so the name on the secondary holder is perpendicular to the top/bottom axis of the plate.
2. Carefully remove the screws and retaining ring.
3. Mark the positions of the shims around the plate with a permanent-type marker. (A small drop of colored fingernail polish at the position of each shim works fine.)
4. Very carefully and gently pry (yes, pry) the plate from its rubber/felt mounting with a very small screwdriver.
5. Keeping track of the shims, gently remove the plate to your work surface.
6. After cleaning, reassemble in reverse order. Make sure the shims are in their correct spots.
Mirrors - To do an effective job, you will have to remove the mirror from the telescope. If the mirror is very dusty, it's wise to give it a preliminary rinsing with distilled water. To clean off the grime, bathe the mirror liberally with the soap solution, drawing the cotton ball over the entire surface to remove stubborn spots - use practically no pressure at all on the cotton ball. Then rinse the mirror by flooding it with distilled water. Now soak another cotton ball in distilled water and gently wipe the entire surface again. (You may note some tiny bubbles; that's an indication of residual soap film that this second swabbing will remove.) Then immediately flood the surface with distilled water again and hold the mirror on edge to let all the droplets run off. There should be virtually no droplets left on the surface, but if there are any, gently blow them off using a soda straw to concentrate and aim your breath. (The soda straw trick also helps when cleaning a lens or corrector.)
When Do My Optics Need a Cleaning? One indication that they need a cleaning is when normally bright objects look unusually dull in a good sky. Another indication is when bright stars have a persistent glow around them. This is caused by light from the object that's scattered by the dust and grime on the lens or mirror. Obviously, another indication is whether the optics simply look dirty. Some SCT owners report that after a good cleaning of both sides of their corrector plates, there is less of a tendency for the plates to fog up on humid nights. SCTs are closed systems, so their inside surfaces are well protected, but these scopes aren't air-tight. That means that there will come a time when the inside surfaces will have to be cleaned. Finally, spots that will not come off a mirror when it's thoroughly cleaned may indicate a breakdown of the coating...if so, it's time for a re-aluminizing.
3. How to Star Test Your Telescope
When people grind their our own mirrors, at some point they say, "This is good enough - I'm tired of beating my head against a wall trying to make it perfect!" There's always that feeling that if they had spent a bit more time in the polishing stage, they might have gotten an even better figure. It all boils down to a willingness to accept some imperfection in exchange for finally being able to use the mirror. Since we know what it takes to make good optics, it's possible to look through most any Newtonian reflector and see the defects. These defects may be the result of an inexperienced amateur mirror maker or a commercial firm that wants to pump out a lot of mirrors as cheaply as possible. But not all defects are really serious, and just about every mirror exhibits some defect. It's helpful to know which ones you can live with and which are "killer" defects. Even if you haven't the foggiest idea how to do a Foucault test, there are a variety of ways to easily check your own telescope next time you're out observing. In fact, the following tests are actually more stringent than the Foucault test and were once used exclusively by makers of telescope optics. Some of the same principles also apply to refractors.
These tests require that you sight in on a bright star. (Polaris is a convenient one to use because those of us without clock drives don't have to keep moving the telescope.) Wait for good, steady sky conditions so the atmosphere doesn't induce defects in the images. Use an eyepiece that gives at least 6x to 10x per inch of aperture on your scope, then rack-out the eyepiece so that the image swells to a fuzzy blob that shows concentric dark rings. Before conducting any of these tests at the eyepiece, make sure your telescope is collimated as accurately as possible. An uncollimated scope will make a racked-out image oblong, rather than perfectly circular - the rings will be "bunched up" on one side. If the telescope is collimated but the image is oval, that may mean the mirror has astigmatism, which results when light rays from the same zone of a mirror do not come to a focus in the same plane. If star images have little tails on them as you bring them into focus, and these tails shift 90o as you pass through focus, that's a sign that your mirror has astigmatism, which is generally a killer defect.
Coma can look a lot like misalignment (lack of collimation), but the bunched-up rings always point toward the optical axis. Coma is a tail of stray light that occurs because the outer zones of the mirror have a slightly longer focal length than the center.
In the racked-out image, examine the concentric dark rings in the blob. The rings should all be about the same thickness, spaced roughly about the same distance apart, and they should be circular. Irregular spacing of the rings could mean there are zones on the mirror - high or low spots. Now rack-in the eyepiece about the same distance. The fuzzy blob you see should look the same as it did in the racked-out position. I've seldom seen a mirror that looks exactly the same in each position, but you want as much similarity as possible. In the case of over-correction, the inside of focus image will show the central rings to be bright and will drop off in intensity as you near the outside (circumference) of the image. The outside of focus image will have brighter outside rings and become fainter near the center. In the case of under-correction, just the opposite of the above will be true. Over-correction means that more light rays are converging from the outer edges of the mirror, while under-correction indicates more of the light rays are converging from the center portion of the mirror.
Now let's try another test. Look at the circumference of the racked-out image. Notice that the edge is "fuzzy", with lots of little spikes sticking out. This fuzziness should be uniform all the way around the image. If longer on one side, this means that there might be a turned down edge on the mirror. This scatters light and results in a light background when bright objects are in the field of view. Ideally, you want as little fuzziness around the edge as possible, and certainly don't want any prominent spikes sticking up.
A poorly supported mirror or too tight a mirror cell can cause the rings to look like triangles. Too much surface roughness caused by rapid machine polishing of the mirror can make the rings hard to see. Bad seeing can have the same effect, but atmospheric turbulence moves, surface roughness doesn't.
When we look at images racked in and out we've been taking a cross-sectional slice of the image before and after it gets to focus and examining the configuration of the light rays. In order to be sure that you're really seeing the effects of the primary mirror, rotate the eyepiece. If the defects seem to rotate as well, then you may be seeing defects induced by the eyepiece. For these tests, I prefer using an eyepiece of fairly simple design, such as an orthoscopic, so as not to induce too many other variables from all the lenses in the high-tech eyepieces. Also, you might want to reposition your eye and note whether anything changes; some apparent defects could result from such things as astigmatism in your own eye.
Of course, the bottom line is whether your optics present a pleasing in-focus view. Almost all mirrors exhibit some defect, even mirrors or lenses that perform excellently. But if defects seem too pronounced or too numerous, the telescope will perform poorly. Again, I can't stress enough the need for well-collimated optics. Any in-focus image of a bright star at high magnification is going to be fuzzy - that's the nature of the optics. But at high power, faint stars should still be pinpoints. The better the optics, the more power you can use before bright star images become "mushy".
Extended objects, such as planets, should be crisp under steady atmospheric conditions. Again, the more power you can use on planets, the better your mirror. But spend some time watching the images. Moments when the image gets very sharp indicate the passing of a cell in the atmosphere that has steadied the image. Larger telescopes are more affected by poor seeing conditions because they're looking through thicker columns of air. As a general rule, most telescopes give decent images under low magnification and images deteriorate as you increase magnification. If you can never get good high-power images when sky conditions are right, then your telescope probably has too many optical defects. Finally, as the opportunity presents itself, it never hurts to look through other peoples' telescopes to see how your images compare with theirs.
4. Is It Time to Re-Aluminize?
All users of telescopes that employ mirrors eventually face the need for re-aluminizing the optical surfaces. Those of us with scopes of the Newtonian design have to face this more frequently because our mirror surfaces are more exposed to the elements than are those in a closed Schmidt-Cassegrain system. While today's coatings are surprisingly rugged, they will deteriorate from the scratches that occur with handling, the abrasive nature of dust that inevitably settles on the mirror, and the wear of repeated washings of the mirror. In addition, any dew or condensation that forms on the mirror also contains some acids that gradually deteriorate the coating. By careful cleaning of your mirrors at reasonable intervals, you can prolong the life of the coating; I found that I have to clean my mirrors about twice a year. Most experts say it's best to tolerate a small amount of dust on the mirror rather than to wipe it away, because the abrasive nature of some dust particles will cause more scratching as you try to remove them. Blowing the dust off with your breath is probably a better idea. Wash the mirror when the dust gets really noticeable.
When is it time to re-aluminize? Despite our best efforts to care for the mirror, there comes a time when the reflective property of the coating has deteriorated to the point that performance of the telescope is seriously compromised. But generally this is such a gradual process that you're not really aware of it. How can you tell when "it's time"?
My current mirror went seven years before re-aluminizing. Other than for a few minor scratches at the edge of the mirror, there was no obvious deterioration of the coating. There were no blotches or hazy spots; after a washing it always looked bright and clean. Yet other observers were usually surprised that I had been able to go such a long time without re-coating, and this was just the usual commercial coating. But there were tipoffs that things could be better. Bright stars always had a halo around them. Most often, this is the result of light scattered by dust on the mirror, but this occurred even after washing the mirror. Also, when I compared objects seen in my scope with the same objects seen in other scopes of the same size, they seemed a bit duller in my scope. Surprisingly, I was still able to catch objects fainter than 14th magnitude on exceptional nights, so the deterioration hadn't yet become severe. But logic seemed to say that re-aluminizing might improve the view nonetheless. Obviously, it also makes sense to have the secondary mirror recoated at the same time.
What kind of coating? There are a number of different firms that do aluminizing work; some also provide "enhanced" aluminum coatings which typically have reflectivities of 95-96% versus 86-89% for normal aluminum coatings. Enhanced coatings also cost more than twice as much. When I began thinking about the eventual need to recoat, I asked other experienced observers their opinions about enhanced coatings. The consensus was that the improvement due to enhanced coating is somewhat noticeable, but it's questionable whether this improvement is worth the additional cost. The clincher was the general observation that enhanced coatings deteriorate faster than the regular coatings - within a relatively short time, you're no better off with an enhanced coating. The following is a quote from issue #46 of Telescope Making magazine.
Enhanced coatings consist of a base layer of aluminum or silver overcoated with several layers of materials with varying refractive indices and thicknesses whose combination causes constructive interference of the reflected light, raising the reflectivity of the original metallic film. ...Some coatings enhance only a specific portion of the spectrum. ...Silver-based enhanced coatings tend to tarnish even if they're overcoated because overcoatings tend to be porous. ...Furthermore, enhanced coatings tend to be expensive and must be protected from acid dew that will degrade reflectivity and cause much scattering. ...Is the extra 10% light worth the added expense? It's hard to say, particularly considering that smog, acid dew and adverse atmospheric conditions may force you to recoat every several years.
Regular aluminized mirrors are generally overcoated to protect the soft aluminum from scratching, especially the very fine scratches ("sleeks") that are an almost unavoidable result of cleaning. Most overcoatings are silicon monoxide (SiO). Magnesium fluoride (MgF2) is sometimes used, but it isn't as mar-resistant as SiO. (The Hubble Space Telescope's mirrors were overcoated with MgF2 for better transmission in the ultraviolet end of the spectrum, rather than for its value as an overcoat.)
Doing it! After checking around, it seems we have one of the best places right in our back yard, so to speak. The P. A. Clausing firm has long touted its Beral coatings...and they're located in Skokie, Illinois. Moreover, a number of commercial telescope makers use Clausing for their coatings. (When I went there, they were coating several very large mirrors for Galaxy Optics.) For convenience sake, this certainly was better than boxing up the mirrors and shipping them across the country. The Beral coating is an alloy that's harder than aluminum, so Clausing does not overcoat the mirrors. They indicated that overcoating would provide a somewhat harder finish than Beral by itself, but that with appropriate care, the mirror should not have to be recoated more frequently than an aluminized and overcoated mirror. The experience of others indicates it's suitably durable, and in fact, it has a slightly higher reflectivity (90%) than regular aluminum. Incidentally, one point brought out in Telescope Making was that it's impossible to guarantee an exact reflectivity; conditions vary slightly in each coating run, so shops advertise a range of reflectivities. One way you can improve the odds of getting a better job is to remove any paint or adhesive on your mirrors before sending them for recoating. Residue from such chemicals will cause outgassing during the process, resulting in a less than optimum deposition of coating.
Results Was it worth it? It's hard to tell the difference simply based on how the different objects appear - no two nights are exactly the same and it's impossible to test the "before" and "after" side-by-side. But my general impression is that stars now appear somewhat more "sparkling", and bright stars definitely have less of the scattered light glow around them. It was worth having the mirror re-coated.
