August 1998 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - August 1998 Issue

The Telescope Nut
by Jeff Baldwin

Polishing your Mirror

It's now time to polish that mirror! The pitch lap is made, channeled, and texturized. Let's get started.

There is mirror-on-top (MOT) polishing, and there is tool-on-top (TOT) polishing. Both will polish the mirror. There are times when the lap is the same size as the mirror, and there are times when the lap is smaller than the mirror. I am going to start with the lap equal in size to the mirror.

The mirror is currently frosted, as it was last ground with 5 micron grit. Our goal right now is to make it shiny without deviating too far from a spherical shape. We will accomplish this by rubbing the pitch lap back and forth and side to side (W strokes) on the mirror with a polishing agent between the two. The heat from this friction melts (literally) the glass' top few molecules and causes them to flow. Here's an analogy: Take a piece of lead, file it, and it looks fuzzy. Torch that piece of lead up to melting temperature and it suddenly becomes shiny. When it freezes to a solid, it is still fairly shiny. If we did this with the glass, we'd ruin it. We have to do this with extreme control and the polishing lap allows this.

I use barnesite as a polishing agent. It contains cerium oxide. Cerium oxide and rouge are other polishing agents, I no longer use them for specific reasons. Any of these three will work.

WARNING: Cerium oxide and barnesite contain cerium, an alpha emitter. Eating or breathing this stuff will make you sick, possibly terminally. Keep it wet, never eat nor breath it, and always wash your hands when you are finished working with it.

Place your lap in warm water to soften the pitch. Place a polishing agent onto the mirror. I mix my barnesite with water in a bottle and squirt a little on the mirror, which is now face up on the barrel. The entire environment must be fairly sterile or tiny dust or grit particles may scratch your mirror. Take the lap out of the warm water and place it on the mirror over the barnesite. Press until the contact feels good (you'll have to experience this a few times to know "the feel"). Don't press too long or the tiny facets you've put on the lap will blob out. This pressing charges the lap with barnesite, and this trapped barnesite will stay on the lap rather than be shoved off the side of the mirror while working.

OK, for the next 20 hours rub W strokes on the mirror, keeping it wet and occasionally charged.

While working a mirror, most opticians tend to work slightly harder in particular areas. If you work the center more than the edges, your sphere will deepen in the center, and if you work harder on the edge than in the center, that part will change the sphere too. For this reason, expecially for beginners, I recommend that you work MOT for a spell, then TOT for a spell, and keep changing back and forth during the entire polishing process. We will change this when we parabolize the mirror, but for now this is a good idea.

An important concept is the rate at which you stroke while mirror making. This varies, with factors being size of mirror, size of tool, temperature, individual philosophy, hardness of pitch, concentration/daydreaming and other factors. I feel that a rate of 1 hz (across and back 1 time per second) is the appropriate starting rate for beginners. You will increase and decrease from this, but that's a good place to start. Too fast and you will sleek the mirror, break the lap, scratch the glass, and most likely dog-biscuit and develop a turned down edge. When parabolizing, I even work as slow as 0.1 hz (6 strokes per minute). It's painfully slow, but the best mirror smoothness I've ever seen came from excruciatingly slow strokes with hard pressure. This is a feedback training phenomenon that you as an individual will have to discover.

Your mirror will very quickly appear to become polished. It is very easy to quit after 3 hours because you think it is done. If a mirror is underpolished and then aluminized that way, it will disappoint you when it returns from the coaters. It will look fuzzy on the outer areas where the underpolish is greatest.

A way to tell if the mirror is truly polished out is to laser test it. This is done by cleaning off the mirror very well (Windex) and shining a pen laser onto the mirror's surface. If the mirror is underpolished, there will be pits on the glass that you might not see until you shine a laser onto the glass. When the laser hits these pits, light goes in all directions, including back to your eye. This is seen as a red spot on the glass. However, if the mirror is polished out, some of the light will refract through the glass, hitting the other side of the mirror, and some of the light will reflect off the mirror, shining onto the wall. None of the light will scatter off the surface, because there are no pits.

Bottom line: If you laser the glass and see a spot on the surface, it is not finished being polished yet, and if you absolutely cannot see the spot, then it is polished. This must be true for the entire mirror surface, clean and dry.

Summary:

We've ground the backs flat, curve generated, coarse and fine ground, polished, and checked to see if the mirror is polished out.

Missed stuff:

TDE while polishing, astigmatism while polishing, scratches while polishing, edge bevel upkeep, glass thickness needs, choices of glass. I'll catch up on these topics next month.

It is important to astigmatism test your mirror about an hour into polishing. If it fails, then you have to go back to about 320 grit and work your way back. If you wait until the mirror is polished out to discover astigmatism, then you will have done hours of polishing work, only to have to go back to 320 grit. I'll explain this test next month.

Clear Glass...Jeff Baldwin
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