Even when you are finished with grinding your mirror, it will have a faintly milky surface, and it will not be very transparent. The small pits that remain from the last stage of fine grinding will need to be removed before the mirror really acquires that clear look of glass.
This is accomplished by polishing.
The most common polishing agent used by amateur telescope makers is cerium oxide. Cerium oxide is a fine, tan colored powder that is mixed with water to form a polish. Cerium oxide polishes quickly, doesn't stain, and is in most respects the best choice for amateur telescope makers.
In the olden days, it was common to use ferrous oxide or rouge. Rouge is a slower polishing agent which can leave a very smooth surface, a feature which makes it desireable, and it is still used by some amateurs who desire very smooth polishing. The primary disadvantage of rouge is that it STAINS EVERYTHING. Even your mirror will have a halo of red stain around the edge that will not wash off no matter how much soap, water, and scrubbing you use. I've also had more problems with scratching while using rouge, I suspect that most of it is not made to as high a tolerance as good quality cerium oxide (or it could be carelessness on my part, but I'll never admit it!). Rouge has one other advantage, it is cheap.
You can take all the cerium oxide you want, spread it between your grinding tool and the mirror, and work it for days and you won't get anywhere. You also need a pitch lap.
A pitch lap is really just a layer of pitch (refined tree sap) which is melted and poured onto the surface of the tool. Channels are cut into it so that polishing compound may freely flow over the surface of the mirror, and so that the facets are free to deform to meet the surface precisely.
Making a pitch lap is likely to be one of the more frustrating things you'll
do as a telescope maker. If you do it correctly, you can bat out a nice
lap in an hour or two. If you don't, you'll have a pile of messy molten
pitch and a headache. The following procedure is the one I typically use
at our telescope workshop, and it works for me. There are lots of different
ways to make pitch laps though, so if you have trouble with this way, try
reading up on other ways.
At the Chabot Telescope Maker's Workshop, we are lucky, we get lots of pitch donated to us by Tinsley Optical, a very fine local company. The pitch we get has typically been used once and tossed, so we recycle it by remelting it. While this pitch is cheap, it varies significantly in hardness from batch to batch, so it can be frustrating sometimes to get a lap to behave itself. Sometimes we find a lap just doesn't work for us, in which case we just grab a chisel and peel it off from the tool and make another pitch lap.
If you bought a mirror making kit, pitch will be included. If you need to order some, you can order some from Salem Distributing Company. You'll probably want to use Gugolz 73, which is a medium pitch. If the temperature of your polishing room is very cold, you might try using a softer pitch, or if it is warm perhaps a harder pitch.
You will first want to remove every single trace of grit from your work area. You are done with grinding, so get rid of anything that has grit on it. Clean your tool making sure to scrub all the grit which may be clinging to it. I use a toothbrush to make sure I get into all the nooks and between the tiles. Some people will spray another coat of lacquer over their tool at this stage to fuse any tiny bits of grit to the tool.
I usually lay down a layer of newspapers just in case I have a pitch spill. It is tough to clean off of most surfaces, so it's best to avoid the possibility entirely.
Find some masking tape and run a ring of it around the tool. You'll be pouring a 3/8 inch thick layer of pitch onto the tool, and this ring of masking tape will keep it in place until it solidifies.
Mix up a batch of cerium oxide. Normally I tell people the ordinary polishing compound should look like 2 percent milk, but the exact proportions aren't all that vital. You probably want to err on the creamy side for now.
Find an old saucepan that you'll never use again, because once you have pitch in it, you never will be able to use it for anything else ever again. Plop your brick of pitch into it, and place it on an electric burner.
I don't recommend melting pitch over an open flame, as it does contain volatiles and it will burn. Make sure you have plenty of ventilation, because melting pitch will smell.
Pitch should be heated very slowly, and never allowed to boil. Don't stir it briskly, you don't want to introduce lots of air bubbles into it. Continue heating it until it is melted all the way and no lumps remain. Give it a stir and remove it from the heat.
When the pitch begins to thicken slightly (should be relatively thick and syrupy) pour a layer about 3/8 to 1/2 inch thick onto the tool. Try to do it all in one pour, don't worry too much about it if you get it slightly too thick. Make sure the tool is on a level surface so the pitch is nice and level.
Now, you wait. You have to wait until the pitch becomes semisolid. You can test this by poking the masking tape around the edge. When it begins to feel pretty solid, you can strip the tape off. The best way to do this is to lift the tape and pull it straight back along itself. If the tape doesn't come away cleanly and pulls some of the pitch with it, stop and wait another couple of minutes.
If the edge of your pitch lap is a bit ragged, you should be able to push some of it back onto the mirror. While the mirror is in this stage, I like to press some preliminary channels into the mirror.
By now the pitch should begin to be getting fairly stiff. Take your cerium oxide mixture and spread a thick layer onto the mirror, and place it face down over the tool. Press down firmly and evenly. If the channels that you pressed in remain wide press a bit more. You are trying to get all the facets in contact with the mirror. Don't panic of some of the channels close up, especially near the edge. You can cut them open again later. The main point is to get the mirror in contact with the majority of the facets. Don't worry if some pitch oozes over the edge, you'll be trimming that off later anyway.
Once contact is pretty good, ease off the pressure and slide the mirror around a bit. Keep doing that for a couple of minutes until the pitch seems fairly stiff.
When I do this at the workshop (and sometimes at home because I am naturally impatient) I'll then slide the mirror off and then put the pitch lap under cold water until it is really solid. Be careful when you do this, as you'll probably find the pitch is still pretty soft, and you can leave big thumb prints in your lap if you aren't careful. This isn't usually fatal, but it is worth avoiding.
Once the lap is cold, take a good look at the lap. The channels that you pressed in probably closed up alot, you'll want to recut them to about 1/4 of an inch wide. I use single edge razor blades, and cut them into a V shape by holding the razor blade at 45 degrees about 1/8 of an inch away from where you want the bottom of the channel to be, and cutting one side, then turning the mirror around and cutting the other side. It is not uncommon for some chipout to occur, and the first time you do this you'll probably get more of that than you'd like, but don't worry. You'll have plenty of time to press that lap into shape before you are done with it.
I also typically bevel the edge of the lap so the lap is actually smaller than the mirror diameter. When you are pressing the lap, you don't want pitch welling up around the edge of the mirror, and this taper helps a great deal in avoiding that problem.
To make sure that the pitch lap is in good contact with the mirror, you need to press the lap. Because the lap is soft, under pressure it will flow and conform to the shape of the mirror precisely. One can either do hot pressing or cold pressing.
Cold pressing is simply taking your tool, spreading some of your cerium oxide mixture onto it, placing the mirror on top and then placing a weight on top. Under this pressure the mirror should come into good contact after three to five minutes of pressing.
If your lap is not making very good contact and cold pressing isn't helping, you might try hot pressing. Find a bucket that will hold both your mirror and your tool, and put in enough hot water to cover them both. The water should be as hot as you can make it and still be able to put your hand in it. Place the lap and mirror in it for a couple of minutes. If you put your hand in and place your thumb nail against the edge of the lap, you should feel it getting spongy. Quickly remove the lap, smear on some of your cerium oxide mixture and then press the mirror onto it and really lean on it. You should see the contact improve. It may take more than one try at this to get the lap in shape if it wasn't in very good contact.
Once the lap has reached room temperature, you are ready to polish. Shake up your cerium oxide mixture and put a couple of tablespoons onto the surface of the lap. Place the mirror on top, slide it around to help spread out the cerium oxide, and then begin with your normal stroke that you used in fine grinding. You don't want the lap to dry out, so if it begins to look dusty, spray a little water on the lap, or add a bit more of your cerium mixture. You should only need to add more cerium every 10 minutes or so.
One thing that you will notice is that there is a lot more friction between the tool and the mirror than there was in fine grinding. If your lap is well pressed, it should be a thick steady friction that doesn't grab and release. If your lap does feel grabby, consider cold pressing a bit more.
Try going for about a half an hour, then slide your mirror off gently, rinse it carefully and pat it dry with paper towels. Take it somewhere with good light.
If all went well, you will notice the change immediately. The mirror will have lost a great deal of its frosty appearance, and will begin to be clear. It is not uncommon for the mirror to do this unevenly: usually mirrors polish from the center and slowly spread to the edge, and the reverse is also fairly common. Sometimes you'll see the best polish at half way out, with more frostiness in both the center and the edge, which could indicate that your mirror is not making good contact. Give it an extra long cold press.
At this stage, we like to give the mirror a quick Ronchi test. If the lines appear reasonably straight, then we know the mirror is roughly spherical and should polish out reasonably. If the test indicates that the mirror is hyperbolic, then we generally shift to a shorter, center over center stroke and continue polishing. If the mirror is very oblate, we might make a change to a slightly more aggressive W stroke.
Don't fuss to much with the exact figure of the mirror. Test it after every hour of polishing with the Ronchi test just to make sure you aren't getting too far astray. If the mirror's figure is close to a sphere, then you are probably fine. It is probably better to err on the side of the mirror being oblate rather than hyperbolic. If the surface of your mirror appears rough or irregular (a malady called dog biscuit), then make sure your mirror is well pressed, and that you are polishing slowly and deliberately with some actual pressure.
When you are doing your Ronchi test, take the opportunity to test the state of your polish. Take a small MagLight or even better a small laser pointer and aim it at the surface of your mirror. Early in polishing you'll see some hazy grayness reflected from the surface of the mirror. It will probably disappear from the center after only an hour or two of polishing, but it could persist at the edge for a number of hours. The rule of thumb that we normally use is that it takes about an hour of polishing per inch of aperature to get rid of all the pits, although it could be shorter or longer depending on which polishing compound you use, how much pressure and your lap. If the edge is particularly hard to polish out, then I will polish with the tool on top using a fairly short center over center stroke. This concentrates more polishing action at the edge, but remember to keep your strokes pretty short, or you can quickly produce a bad turned down edge. Done properly, tool on top (TOT) polishing can actually help reduce a turned down edge.
Hopefully after your mirror is polished out, your mirror will be fairly smooth and close to a sphere. Pat yourself on the back, you are mostly done, although some of the most finicky work is still ahead.
All materials on this website are Copyright 2001, Mark T. VandeWettering. Permission is granted to reproduce and distribute these files for non-profit, personal use.
Mark T. VandeWettering <markv@telescopemaking.org>