Second week of amateur telescope making

Telescope pic 1On Friday I continued working on my primary mirror at the amateur telescope making workshop in Washington, D.C. I finished grinding the 8″ blank with 60 grit, then 80 grit, then finally 120 grit silicon carbide. Guy, who leads the workshop, must have detected my eagerness to complete my first scope, because he asked me if I wanted to take it home and work on it over the week. Of course I said yes.

So he packaged up some 220 grit, 320 grit, and 400 grit aluminum oxide for me and I brought home the glass blanks and worked on them yesterday out in the backyard. I finished up with the 220 grit but I had to stop at that point. My arms and back were absolutely killing me (and still are). After four hours of rigorous grinding on Friday and another hour on Saturday, I’m totally shot. My entire upper body is sore beyond belief. Making a telescope yourself may save money, but it certainly requires a lot of effort. It’s good exercise too.

The workshop wasn’t as well attended as it was on Friday. Oddly enough, I didn’t see a single person who had been there the previous week (besides Guy, of course). It must be a sporadic activity, with most people attending as often as they have Friday nights free. A telescope project can easily drag out over the course of many months in this manner. But I hope to be finished much sooner. And I remembered to bring a camera this time.

The first photograph (used as the lead-in to this post) shows the two 8″ pieces of glass I’m working with. The one on top is what will become the mirror, and thus it has a concave (curved inwards) surface. The final step with the glass, which I’m a long ways away from, is aluminizing its surface, thus turning it into a mirror. The piece of glass on bottom is the “tool”. It has a convex (curved outwards) surface that matches up with the concave surface of the mirror. The two are ground together with water and 80 grit silicon carbide in between in a specific back-and-forth motion that creates the desired shapes (slowly). As you can see, it’s a messy task, hence the newspapers.

Telescope pic 5

Here I am working up quite a sweat grinding away on those glass blanks. Notice the two water bottles, one for lubricating the grinding and the other for quenching my thirst. Also note the water that has splashed on my shirt from the vigorous grinding. It’s a pretty messy activity. Although there’s no hiding the enthusiasm evident on my face.

Telescope pic 8

This is me inspecting the surface of the glass using a microscope. I really wish I could somehow take a picture of what it looks like through the microscope, because it’s pretty neat. Virgin glass looks pretty uninteresting, but after it’s been ground away with grit, the grit gets stuck into its surface, and it takes on a mottled appearance of white and black. As progressively finer and finer grits are used, the size of the spots seen in the microscope decreases, indicating that the surface is becoming smoother. You know it’s time to move onto the next smallest size of grit once everything becomes uniform and there are no larger sized granules with the current size.

Telescope pic 13

This is a shot of the spherometer in action. It measures deviation (either concave or convex) from flatness in the center of the device by means of a pin that travels through the rexolite plastic. The read-out is directly above the pin. Note that the radius of the spherometer is 2.728 inches and the reading on it shows an indentation of 0.039 inches (meaning it is concave). This is just about right for a sphere with a radius of 6 times the diameter of the glass, thus yielding a mirror with an f/6 focal length. Yes, there’s math involved too!

Telescope pic 15

My mirror blank, covered in water and 120 grit. This was just after I had finished up with 80 grit and was moving on. Note how I also replaced all of the newspapers beneath: you have to clean everything up very thoroughly when moving to a finer grit, because any contamination with coarser grit will cause uneven grinding.

Telescope pic 17

This is Guy showing me how to grind down the edges on the tool. The blank was very sharp when we first got it, which made it somewhat dangerous to work with. The reason we’re doing this underwater is so that we don’t inhale any glass dust, which can cause all sorts of lung problems. Similarly, the reason we use water with the grit while grinding is so that the resulting fine powder doesn’t become airborne and end up getting inhaled.

That’s all I have for this week. I’ll be sure to report back with any progress next week, and ideally, I hope to document the whole process from start to finish with photographs. I want to give anyone reading this a good idea of what it’s like to go through, so you can make an informed decision about whether you want to do it yourself. Of course, my opinion is that it’s totally worth it.

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