Fourth week of amateur telescope making

On Friday I continued working on the 8″ f/6.1 telescope that I’m making by hand. I finished up grinding last week and went onto polishing this week. I took a bunch of photographs to document the process.

Polishing requires the use of a pitch lap to polish against. The pitch laps at the workshop are made from a ceramic plate roughly the same size as the glass covered in squares of pitch resin. The reason it’s in squares is because it needs to adapt to the shape of the glass, and if it’s all one big segment, it won’t be able to change shape nearly as easily.

Telescope 46

We started off by making a new pitch lap. This picture shows the process of removing hardened pitch off an old pitch lap using a hammer and chisel in order to reuse the ceramic.

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Melting some new pitch in a coffee can on a hot plate. The pitch was yellow in its original solid form, but when it melted and became all gooey, it turned brown.

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Pouring the hot liquid pitch onto the ceramic plate. Note the use of masking tape around the edges to prevent the pitch from dripping off the edges.

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Using a ruler to create grooves in the pitch. This is a lot easier than trying to put on squares of pitch individually.

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Shaping the still-warm, but no longer liquid, pitch to the shape of the glass. The white liquid is cerium oxide polishing solution. which I would use for the remainder of the night.

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Here’s what the pitch lap looked like after I used it for about twenty minutes. All of the white is cerium oxide. Note the cavities in the center of the pitch lap — it hasn’t fully adjusted to the shape of the glass, so it needs to be reheated and reshaped. By this point my mirror had already “flash polished”. In just the first few minutes of polishing, there is a much more noticeable change than in the previous hours of grinding or hours of subsequent polishing. The cerium oxide works really well. After the flash polish, the glass looks like it’s done. The reason polishing goes on for hours after this is to polish out the deepest pits. The flash polish was my “Ah-ha!” moment. “I’m really going to get this thing working,” I thought to myself.

Telescope 71

This is what a Ronchi test of my mirror looked like after about twenty minutes of polishing. A Ronche test works by placing the mirror at double the focal length distance from a light source, then looking at the mirror through a diffraction grating from a reflection point at the light source. The orientation of the mirror is figured out by aiming a laser pointer from the observation point and lining everything up such that the laser reflects off the mirror and into the light source. The Ronchi test reveals the shape of the mirror. A paraboloid has straight lines down the center that then bow out as the lines go up and down. It’s pretty much the opposite of what this image shows. I still have a bit more shaping to do.

Telescope 75

This is what my glass looked like after about an hour of polishing. The picture is deceptive. Those words are written in permanent marker on the underside of the glass and only appear to be on the surface of the glass because of internal reflection. Ditto for the apparent rough surface. The mirror side of the glass is so smooth by this point that you can’t see its surface in the photograph.

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Using a heat lamp to reheat the pitch lap in order to get a better shaping to the glass. The heat lamp was taking too long though, so we just ended up immersing it in hot water.

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A woman looking through the Ronchi test apparatus. Notice that it rotates to allow the use of different width diffraction gratings. The green light right beneath the diffraction grating that the woman is looking through is the light source that is reflected off the glass that creates the green lines seen in one of the previous pictures.

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The pitch lap after the reheating and subsequent use. You can see the outlines of where the old deformities used to be, and how there are only four remaining that are of much smaller size. One more reheating should get it perfectly conformed to the glass’s shape. Also note that the pitch squares are flattening out and running together. This will be used using a razor blade to widen the troughs between them.

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A Ronchi test on another man’s 6″ mirror. Notice how the lines are much more parallel up and down; this is indicative of a spherical shape. Now he just has to make it a paraboloid and he’s all set. The lines are much smaller than in the previous Ronchi image because the observation point is too far away from the mirror.

Grit comparisons

Here are microscope images of two mirrors. On the left is my mirror after I finished up grinding with 5 micron abrasive. On the right is the mirror of the woman above after she finished up grinding with 120 grit (125 micron) abrasive. The images were taken under different lighting conditions in different places, so ignore the color differences and just focus on the different sizes of the grains. These pictures were taken at the same zoom. I wanted to get a picture of my mirror after polishing it on Friday’s night, but there wasn’t really anything to take a picture of. The grains are too small to be photographed with a 100X microscope.

On a different note, an African-American single mother from DC stopped by and brought her seven year old son. She was under the impression that the workshop was an activity for kids; apparently the workshop is registered in the community center’s list of “courses”, but the details are wrong. It’s not really something for kids. It requires continual parental attention and lots of patience. It can easily take over a dozen hours of grinding and polishing before a mirror is even ready to be aluminized. I don’t think I would have had the patience for it even when I was a teenager.

But Guy’s main job is teaching public school in DC, so he was more than happy to invite them to stick around for awhile and show them the process and everything that the rest of us were working on. Although the kid won’t be joining the workshop and making his own telescope, I think he gained a bit of appreciation for astronomy and the good old “do it yourself” spirit that night. He seemed really excited with the making of the pitch lap and the Ronchi tests. Here’s hoping he’ll stay interested in science as he gets older, unlike so many of his peers.

One Response to “Fourth week of amateur telescope making”

  1. ? Says:

    Why do you feel the need to say she’s “African-American”? If anything, she’s black.