Making a Pitch Lap to Polish Your Telescope Mirror

Baldwin

Old Way

When grinding your telescope mirror, you had particles tumbling between the mirror and the tool. They were chipping away at both the tool and the mirror grinding them into spherical unity, one concave and one convex. Polishing the mirror is different, the particles aren’t tumbling and they aren’t chipping. There are going to be particles of Cerium Oxide stuck into microfacets on a platform of pitch, and when the pitch lap is rubbed against the glass mirror, it will polish the glass rather than grind (breaking) the glass. For the exact theory on this chemo-mechanical process, I HIGHLY recommend Mel Bartel’s site.

Before we can polish, we need to make a pitch lap. There are as many ways to make a lap as there are ATMers, and we all have our opinions as to how to do this. And, of course, all the other ways are wrong. Just kidding, but what I’ll say in this page will be different than what others will say. That’s OK. Here are a few ways in which I have made pitch laps.

First of all, I buy Gugolz 73 pitch. I use it in hot California weather, and I use 64 when it’s cool. Good stuff. Others will tell you what they like, I like this stuff. You will need to know how much you’ll need. I like my pitch laps to be about ¼” thick. So if I am going to cover a 12” tool with ¼” thick layer of pitch, then I will need pr²h cubic inches of pitch, which would be 3.14 times 6 times 6 times 0.25 = 28 cubic inches of pitch. A can of Gugolz, if I remember, is about 3.5” wide and about 6 inches tall, which would be roughly 60 cubic inches of pitch. If you need more than this, make sure you have two cans ready to use. Always cook up more than you are going to need.

Read this before starting. Ever see those instructions when putting something together? Or are you like me and you start in, and half way through the deal you realize that you needed something, but the process is already underway, and you’re screwed? So, here’s the deal. Read this before starting. Once the pitch is cooking, you will need all your ducks in a row.

Use a pot that you will never use for anything else again. Once melted pitch is cooked in a pot, the pot is useless for anything else, especially food. The pot must be able to be cooked, like a stove top pot with a handle on it. Put your pitch in it and cook it. I use an electric hotplate. I don’t recommend using one that uses flames, such as a propane or city gas burner, since pitch is highly flammable, and has spirits that will flash into flames and burn your house down. Oh, don’t do this inside, it will stink the place up and your wife/mother/etc. will kill you. Garages, driveways, and no flammable items around. Have a fire extinguisher handy.

So, the pitch is in the pot and is cooking and will eventually melt. While you are waiting for this, you can get the other stuff ready. Place your mirror clean side up on the barrel or workbench. It needs to be cleaned. It also helps if it is very level. Place your tool business end up on the workbench or barrel, next to the mirror. Wipe the tool with turpentine (or if you’re in California where everything and its dog is illegal, you can use turpatine, fake turpentine). Squirt the mirror with Simple Green or some other soap. Some ATMers use soap, water and cerium oxide to smear on the mirror. Here’s the deal; the pitch will not stick to soapy stuff very well, and it will adhere well to stuff with turpentine on it. You will want it to stick to the tool, but NOT stick to the mirror, while pouring your lap. Don’t mix the two up. I actually make a slurry out of water, cerium oxide [thick] and soap. It’s like a release agent.

Once the pitch has melted into a clumpless goop, kill the heat. If it is too runny, then let it cool to a clumpless goop. It ought to look like the black puddle that killed Tash Yar on Star Trek: The Next Generation. Pour the pitch slowly onto the middle of the level mirror until it forms a puddle about 2/3 or ¾ the diameter of the mirror. Put the pot down. Hold the tool by the sides and place it onto the pitch puddle. One of two things will happen. Either it will stand on the puddle and you’ll have to push it down, or the puddle will be too runny and it will try to run off the sides of the mirror, and you’ll have to hold the tool to keep it from pushing the pitch away. The idea is to let the tool press itself down onto the goop and push the goop out just to the edge of the tool, but no further. If it is thick enough, you can let go and let it cool. If it is runny, you’ll have to hold its weight back so the pitch won’t run off and over, and hold it until it’ll hold itself without pushing the pitch off, then once it holding you can let go. At this point you now have (from bottom to top) a mirror, black pitch, and a tool, all sandwiched together.

Once the pitch will hold itself, you can flip the whole thing over so the mirror is on the top and the tool is on the bottom. At that point you can hopefully slide the mirror off the pitch. If you can, then slide it off, then place it straight down right back on to the pitch again. The idea is to separate it, then replace it straight down to make sure it keeps the right shape. If you leave it in place without this step, you may get a cemented together sandwich that is really hard to separate. If you do end up with this problem, then let the whole thing cool off, then lean them against a tree in your yard and run cold hose water over it. Eventually they will separate. If they don’t, you can tap on one end of a 2x4 letting the energy travel down through the 2x4 to the other end where it is against the tool eventually knocking the tool off the mirror. It will take the pitch with it. This sounds extreme, but the mirror and the tool with the lap on it will usually survive just fine. You can also put the whole thing in a bag and put it in the freezer. After a couple of hours, take them out and the pitch will break off of everything, and you can start all over again.

OK, read this once or twice more before starting the lap cooking process.

The lap isn’t done, but the cooking part is. Now it is time to facet your lap. You will want to make channels in your lap like a map of a city with north-south streets and east-west streets. The streets will be about ¼” wide and the blocks will be about 1.5” square. I use a razor blade and a ruler. Practice is the only way to make this work well. I usually make two parallel lines with a razor and a straightedge, ¼ apart, then use the razor to clean out the pitch between them. Keep the pitch particles clean and return them to the pot to cook again. Make sure you don’t have an intersection at the center of the lap, and make sure you don’t have the middle of a block in the center. Making these offset is important so you don’t polish zones into your mirror. This is not the case for machine laps though.

Once the grooves are cut into the lap, it needs to be pressed onto the mirror with plastic screen door screen between them. This will not only act as an indicator that all the blocks are in contact by showing the criss-cross pattern on all blocks, but it will give the lap zillions of little microfacets in which the cerium oxide will reside while polishing your mirror.

Sometimes I use a soldering iron to ‘weld’ the pitch to the tool right at the edge of the tool. This may be useless, but it appears to keep the pitch attached on these perimeter blocks.

Another way to make a lap is to pour it onto the tool, which already has turpentine rubbed on it, and then hot-press the mirror onto it until they conform. Then cut the grooves. A hot press is when you soak the tool and lap in hot water, then press the mirror onto it with weights until they have good contact.

New Way

So, I don’t do this anymore because I bought lap mats. They are a silicon like material that works like a template, and the pitch is easier to handle. They const about $40 or $50, but if you do lots and lots of mirrors, it’s worth it. If you’re only doing 1 mirror, either borrow a lap mat or do it that way I described above. Anyways, here’s how it works. You cook the pitch. You lay the mirror face up, clean, not soapy. You lay the lap mat onto the mirror. You pour the pitch onto the lap mat, then lay the tool onto the pitch on the mat. After it has cooled, you slide it off and pull the lap mat off the pitch lap. It’s nearly done, and it’s beautiful. You then chip off the pitch from outside the circumference, and then press it back onto the mirror [with cerium-soap or a screen mesh] to press into perfect shape. No mess, easy, clean, looks professionally done. I love them.

Let’s go to Polishing and get this mirror made.