FRONT SURFACE MIRRORS and LENSES
(With comments on the air-spaced objective)
Keeping ones optics clean is a problem that every owner of a reflecting telescope faces; a problem, in fact, far more daunting and scary than owners of refracting telescopes must face. It's that front surface coating. We suspect that it is delicate and soft and liable to scratching. And, we fear touching it in any way. This fear often results in people not wanting to clean their optics at all. Occasionally one can see mirrors coated with years of accumulated grime, their owners in fear of touching the surface. Actually, it's not really as much of a problem as people have been led to suspect. And the coating is not really all that soft. In fact, silicon monoxide coated aluminum is really just about as hard as glass. Well, almost.
Basically, amateur front surface telescope mirrors are coated with two types of reflective material; protected aluminum and Beral. Protected aluminum is simply a thin layer of aluminum that has been evaporated onto the substrate material with a thin coat of quartz evaporated over this. The coatings are extremely thin, but in reality, really quite indestructible. (I've actually tried to destroy coatings on mirrors sent to me for refiguring and have been amazed at how much abrasive force I can apply before anything truly serious happens to the surface in the way of distruction.) Beral coating is a proprietary concoction of H.L. Clausing and, like aluminum, is deposited by an evaporation process. But unlike aluminum it is not over-coated. Beral is much harder than unprotected aluminum, about five times harder, about his hard as chrome. And, although less likely to resist scratching than a silicon monoxide over-coating, still quite durable and resistant to abrasion.
It goes without saying that to keep any optical surface in top operating condition it should be cleaned when excessive dirt appears noticeable on the surface. Having said this, one should not clean mirrors or other optics with great frequency. Obsessive cleaning merely abrades and wears the surface. The best bet is to keep all optics covered when not in use and avoid getting them dirty. Small amounts of dirt and dust on the surface of an optical element will not really significantly impact the image quality. But when they get really cruddy, it's time for a bath.
The following process will work is well for lenses, both uncoated and coated, and front surface mirrors coated with Beral or protected aluminum. Mirrors with enhanced coatings that are coated with silicon monoxide should suffer no ill effects, but mirrors coated with bare aluminum or other exotic materials that are not over-coated may well be too sensitive for ordinary cleaning methods. In these cases one should contact the coater for specific mirror cleaning instructions.
The mirror should be placed in a plastic tub filled with warm water and a few drops of liquid dishwashing detergent in solution and allowed to soak for 1/2 hour. The surface is then lightly swabbed with a wad of several cotton balls while immersed face up. The mirror is then removed from the bath and thoroughly rinsed under the faucet and finally rinsed with steam distilled water to remove any minerals in the tap water. The mirror is then placed on its edge on a mat of paper towels (chocked with a couple of erasers so it will not roll away) and allowed to drip dry. Lingering droplets should be absorbed with the tip of a piece of plain unscented bathroom tissue. Do not attempt to rub off the drops. Though some very minor stains may remain from the droplets and very light sleeks appear after several cleanings, these are of no consequence. The important thing is that the mirror surface remain clean. Diagonals should be treated in the same manner. One must remember that there is a difference between cosmetically clean and optically clean. A few spots may not be cosmetically attractive but are completely inconsequential optically.
Lenses are treated in a similar manner to mirrors and may be cleaned using the instructions as given above with the following exceptions. In no case should cemented achromatic lenses be immersed in warm or hot water. Cemented achromats are made in sizes up to including about three inches in diameter and the larger the lens the more likely it is to suffer injury by warm water immersion. Sudden unequal expansion between the differing types of glass may cause fractures and even separation, particularly with older lenses. Cemented achromatic lenses should be cleaned only in room temperature water.
The Air-Spaced Objective
The cleaning of air spaced objectives requires special instructions. There are basically two groups of sad telescope owners who have attempted to clean air-spaced achromatic objectives: those who have chipped them trying to remove them from their cells and those who have reassembled them out of order and have no way of knowing what the correct order now is. Many fine objectives have been seriously chipped when owners in an attempt to remove them simply unscrew the retaining ring and turn the cell over and dump the lens out onto their hand, or onto a folded cloth. There's a special procedure to observe when removing objective lenses from cells. The following procedure will usually result in a successful operation.
Find a food can with a diameter somewhat smaller than the clear aperture of the objective within its cell. It is best if the can still contains its contents as this will make it heavier and more stable. The top of the can is covered with a wad of folded paper towel not to exceed the clear aperture of the objective lens within it's cell. (Viva paper towel at present appears to be the softest towel available.) Two sheets of paper towel are set aside on the table to receive the individual objective lenses once they are separated from each other.
The retaining ring is removed from the objective cell. The objective and cell are now raised above the can and slowly lowered down over the can. Take care and do this very slowly. You'll notice that the objective will begin to rise up out of it's cell once it begins to be supported by the top of the paper towel covered can. If the lens begins to bind within the cell, stop the process and slowly raise the cell up until the lens re-sets itself; then begin again, jiggling the cell slightly to keep the lens loose and free. Lower the objective cell until it rests on the table.
You may exhale, now.
Carefully examine the edge of the objective and see if there are any "witness marks". These are usually pencil marks made on the side of the lenses one above the other to mark the relative position of the lenses. The relative position of the lenses must always be preserved. On occasion, the optician who made the lens will rotate the two lenses in relation to each other until any apparent astigmatism is reduced to its smallest amount, so the relative position of the lenses may well be important. If no such marks exist they should be made at this time either by using a pencil or a fine felt marker. I also recommend that additional marks should be made in the form of arrow heads pointing toward each other so that the individual will know how the lenses should face each other upon reassembly.
It should also be noticed as to how the two lenses are separated from each other. Often, makers use three small pieces of aluminum or tin foil. Also, check to see if the lenses are also held together by small pieces of tape along the sides. If such tape exists, it should be carefully removed. If the tape is old and difficult then the use of a new single edge razor blade is suggested to carefully scrape material away from the sides of the lenses. The upper lens should now be carefully removed and set on one of the sheets of paper towel reserved for this purpose. The three separators should be removed and set-aside. Finally, the lower lens is removed and set on its piece of paper towel.
At this point the lenses may be cleaned in the manner described above. An additional touch with lenses, after the soap and water cleaning, is to clean the surfaces with denatured alcohol. A soft paper towel is soaked with alcohol and the surface of the lenses gently rubbed. This apparently works as well with coated as well as uncoated lenses. Great caution should be exercised in using harsher solvents upon coated lenses. Alcohol is known to be an extremely mild cleaner, but the same cannot be said of some of the other solvents available on the market. Anyhow, alcohol is plenty sufficient for this purpose.
Once the lenses have been cleaned reassembly may begin. Reassembly consists of performing the dissassembly in exactly reverse order. Begin with placing the rear lens upon the padded can, carefully replacing the spacers (a tiny piece of tape is sometimes useful for holding them against the side of the lens so they don't repeatedly drift away and drive you crazy), and replacing the leading element and aligning the witness marks. The cell, which has remained on the table surrounding the can, is now slowly raised up and over the lens combination. Once this is accomplished, the retaining ring can be screwed back in. When screwing in a retaining ring two things must be considered. Always begin threading a retaining ring by placing the ring in the cell and turning the ring counterclockwise until the ring can be felt to firmly engage itself in the female threads of the cell. Then, the ring may be very gently and slowly turned clockwise. One should immediately feel the ring thread easily. Any tendency to thread hard should be viewed with caution and the threading stopped. Hard or difficult threading is likely to be what is known as cross-threading and will result in destroying the cell. If care is taken, all will go well. But, be careful.
And, lastly, do not screw the retaining ring down tightly. My usual procedure is to screw the retaining ring down until it just touches the lens ever so slightly, and then back the retaining ring off about an eighth of a turn. I have also stuck three pieces of Scotch tape on the underside of the retaining ring and trimmed off the excess, so that just the front side of the tape touches the lens and avoids metal to glass contact.
Occasionally, amateurs come into the possession of old achromatic objectives. These are frequently the work of old master craftsmen who have a tendency to amuse themselves by finishing their brass cells with extremely fine and delicate threads. A cautionary note, brass as well as aluminum as a tendency to gall or bind if threading is not done with great care.