July 1998 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - July 1998 Issue

The Telescope Nut
by Jeff Baldwin

Pitch Laps and Polishing

I should have had Lloyd Altamirano write this. He's made more pitch laps than most of us. I guess I should tell you why we pour a pitch lap, what a pitch lap is and what to do with it.

The mirror has been fine ground to a smooth curve. It does not reflect light as glass that you are familiar with. It is 'frosted' glass with its fine grind to it. In order to reflect light it must be shiny smooth. The only way to accomplish this is to polish it or melt it. (Theoretically, polished glass is micromelted on the surface--that's another discussion.) Anyway, when you grind, the grits tumble between the tool and the mirror. This chips away at both the tool and the mirror. In order to polish the glass we aren't going to allow it to tumble between the two, but rather slide against the mirror, causing a microscopically small area to melt, resolidify, and hence have the reflection that polished glass has. Here's the ticket.

We have the tool and the mirror side by side, face up. We smear turpentine on the tool and breaking fluid on the mirror. Breaking fluid is a mixture of water, cerium oxide and dishwashing soap. We heat up some pitch (outside in a "never-to-use-again" pot, no flames, the stuff stinks and is gross), which was bought from a company that refines the pitch so that it behaves the way astronomers want it to. When the pitch is pourable, we let it cool a little so that it is very thick but still pourable. We pour a puddle onto the mirror that is about 2/3 the size of the mirror. We then quickly set the tool onto the pitch and either press or hold it back so that it pushes the pitch out to the edge of the tool. If the pitch is too runny, then we hold back. If it is too thick, then we press. The pitch then cools and can be slid off with the tool. We slide the tool sideways off of the mirror. Since there was turpentine on the tool, and since turpentine makes pitch stick, then the pitch stays stuck to the tool. Since there was breaking fluid on the mirror, then the pitch won't stick to the mirror. This is why the pitch slides off with the tool.

The tool with pitch on it is now referred to as a pitch lap, or just the lap. Channels must be cut into the lap. This will allow the lap to not stick to the mirror due to tight sealing, the vacuum that occurs between the lap and the mirror. It also allows the polishing agent a place to go rather than to be pushed off the side of the mirror to go to waste. I use razors to cut channels, others heat the lap up and press straight things onto the lap. Either way, the idea is to grove the lap with channels. I usually make rows and columns, so that the lap looks like a grid of squares. It is important to not have a square dead center on the lap, nor an intersection of channels. This causes "zoning" on the mirror, which is bad.

The end result is now your tool with pitch on the same face that did the grinding, it has channels in it that leave the tool appearing like streets with city blocks and it is extremely clean.

Before using the lap it must be pressed onto the mirror to make a perfect fit. Heat the mirror up in a tub of hot water to soften the pitch. Lay a screen onto the mirror. I use plastic screen door screen to impress little itty bitty doodads into the squares while I press. Now lay the lap onto the plastic screen and press. The mirror should come up and off with the channels not squished together and with tiny criss crosses on the blocks. If the channels get squished together, then you must recut them and repress. If the criss crosses are not on every block, then the mirror is not completely in contact with the lap and a second press must occur.

To use the pitch lap for polishing we'll want to wait until next month. Right now we have a good pitch lap made and we can store it upright in a safe, cool, sterile location. Don't set anything on it--that includes anything heavier than a feather. Pitch doesn't like to move quickly, but it will move slowly with the least amount of force imaginable. A fly can bury himself in pitch just by standing on it. Wrapping the lap in plastic will more than likely ruin it if the plastic is in contact with the pitch.

Where we've been: We curve generated, course ground, fine ground; we know the curve is the right depth by the sagitta test; the mirror and the tool are spherical and in contact; they have been ground to 5 micron; and the pitch lap has been poured, channeled and microfaceted (little criss crosses).

Where we're going: We will polish the mirror until it is totally polished; we'll make sure it is a sphere without T.D.E. or astigmatism; we'll parabolize the mirror and measure it with a variety of tests; we'll aluminize it and place it into a telescope.

See you next month.

Clear Glass...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.