A. By the time you are at 9 to 3 microns, you will notice that your mirror is translucent but not transparent. If you wet your mirror, then you can read text through it, but the surface is still not as smooth as, uh, glass. In order to get your mirror optically smooth, you will need to change from fine grinding to polishing with a soft lap which holds the abrasive in place, instead of allowing the abrasive to roll between the two pieces of glass. Leon Foucault describes using paper laps for this purpose; others have used honeycomb foundation, shower curtains, and even road or roofing tar. There are numerous types of optical pitch made by various companies out of of tree sap, petroleum, or some secret proprietary ingredients.
B. An innovation developed at the Delmarva ATM group is to save time by using ophthalmological polishing pads (links here, or here, or here) for the initial polishing. It cuts the number of hours of polishing roughly in half, which is great, because polishing with a conventional pitch lap can take about one hour per linear inch of diameter, or more. (So, roughly six hours for a six-inch mirror.) The amount of time required to fully polish a mirror is greatly dependent on the amount of force applied by you, the mirror-maker.
C. To use the polishing pads, you clean the tool very thoroughly with isopropyl alcohol or denatured ethanol or acetone to remove all traces of fingerprint oil. Wash your hands thoroughly. Consider using latex gloves and tweezers. Get scissors and a clean, sharp single-edge razor blade. Carefully peel a pad from its roll, trying to touch as little of the adhesive side as possible. Press it down onto your tool slightly off-center. Then continue applying pads with one big caveat: NO PADS MAY OVERLAP OR TOUCH. It is OK to trim the pads before applying them, or afterwards, and it’s OK to be artistic about it. Do NOT strive for symmetry here – it will cause problems with your mirror, believe it or not. Don’t make pieces that are very small; the size of your smallest fingernail is about the smallest piece you would want. Once they are all in place, use a clean artist’s J-roller to press them all down, firmly.
D. You will now need a polishing stand with cleats to hold the mirror in place, because you are going to be pushing quite hard on your mirror or lap. Without the cleats, your project will end up on the floor, broken. In the DC ATM class, we use lazy-Susan turntables that rotate around a fixed pivot point to make it easier for you to rotate mirror and the lap in a regular manner. You should also cut some shelf-liner material to fit underneath your mirror and tool, to prevent irregularities in the wood substrate from deforming your mirror.
E. You will use a slurry of Cerium Oxide (formula CeO2 also known as cerox)* mixed in distilled or purified water. Don’t use tap water because it might contain particles that come from the pipes and thus might scratch your mirror. It is totally non-toxic. Mix it up fairly thick, something like heavy cream. Do not apply very much liquid to the pads; if you do, the petals of your polishing pads will come off of the glass. Do polish hard, and polish long. Rotate everything as usual, and alternate tool on top with mirror on top. Do NOT let the cerox or even individual droplets of water air-dry on your mirror: they will etch the surface somewhat, and then you will have to polish some more!
F. When are you done? Simple: you are done when it’s fully polished out. How can you tell if you are polished out? Here are two simple tests:
G. Use the highest magnification and best illumination you can on your microscope, and carefully inspect the mirror surface for any remaining pits or scratches. If you see anything at all, it’s not done. (If you see something that looks like a white caterpillar, relax: it’s probably a bit of lint from a paper towel! Wipe it off and look again!)
H. The laser test uses a red or green laser shining into and onto the surface of the mirror from about a 45-degree angle. Ideally, the light should pass through the surface leaving behind almost no trace of its passage. If you see a bright spot where the laser hits the surface of the mirror, then you need to keep polishing. Or else the mirror is dirty.
* Note: there are several other commonly-used polishing compounds, such as zirconium oxide, red rouge, black rouge, and even Barnesite. Compounds that are labeled ‘cerium oxide’ often have a fairly substantial proportion of other ‘rare earth’ oxides in, since they are so difficult to separate chemically and probably act in the same way. Thus one cerium oxide preparation might look white as talcum powder, while others will appear pink or brown. Red rouge gives a very fine polish, but it’s slow-acting and extremely messy, staining your hands and clothes — the latter, often permanently. Black rouge is even finer and messier. Zirconium oxide acts faster than red rouge but slower than cerium oxide. I haven’t used Barnesite much, but it’s hard to get, messy, and almost as slow as red rouge. Professionals working on the finest telescopes in the world have abandoned red rouge because of its mess. These professionals can also afford extremely fine proprietary mostly-CeO2 slurries that cost a LOT, such as Hastillite. We can’t afford that. So we tend to use cerium oxide at first and switch to rouge when doing the final figuring.