Mirror-making breakthrough!

When last I regaled you with tales of my continuing telescope-making adventures, I had spent four hours working back from an oblate sphere to a paraboloid. Little did I know how much I accomplished in that session. This past Friday, I only had to do about ten minutes worth of polishing before another Foucault test revealed that I might be done! Yes, that’s right, after more than four months of working on this cursed piece of glass, it could be good to go! I say “could” because the Foucault test revealed a pretty marginal 1/3.7 wavefront error, but most of that error came from an anomalous reading on the last zone on the very edge. Taking that reading out, my mirror was a respectable 1/5 – 1/6 wavefront error. Next Friday we’ll definitely do some more testing to determine more accurately what the situation is.

But the ultimate test is how well the mirror performs under actual use. If you pop it in a tube and it resolves clear images of what you’re looking at, all of the Foucault readings in the world are meaningless — the mirror is done! An un-aluminized mirror (aka glass) reflects about 4% of the light hitting it, making it more than good enough for viewing brighter stars and the Moon. So, I’ll try out the mirror as it is now, and if I like what I see, I’ll get it aluminized. If not, I’ll go back to figuring. But to test out how good it is, I actually need a tube to put it in.

Thus, I’ve started construction of the rest of my telescope. I went to the hardware store and bought most of the simple items I need: 3/4″ plywood, paint, screws, springs, silicone, and other stuff. I also put in online orders for all of the specialty parts that can’t be obtained in a hardware store: the focuser, diagonal secondary mirror, curved-vane spider, and eyepieces. I’m still missing a few things here and there, like angled aluminum to hold the mirror in place and teflon for the Dobsonian mount, but I’ll get it all eventually.

So it’s finally starting to look like I will actually finish this project. I will admit, it’s taken a bit longer than I thought to get where I am now, but I’m happy nonetheless. Figuring the mirror is the only truly precision part of the endeavor. Everything else is just basic handyman-type construction work: cutting wood to shape, screwing and bolting it together, painting, etc. That won’t pose a problem. Thanks to my dad and shop class in high school, I’m pretty comfortable around the various power tools I’ll need to use to finish this project.

And, just to tease you, here’s a picture of my tube after a day of painting. The tube is 55″ long and 10″ in internal diameter. I’ve applied one coat of primer on the inside and two coats of primer and one coat of topcoat on the outside. Still to go, a couple coats of flat black on the inside and another coat of topcoat and two coats of glossy acrylic on the outside. But even with just the one coat of topcoat, you can see now neat it looks. I chose a very unique type of paint for the topcoat. I didn’t originally have steampunk in mind when making this scope, but it looks like that’s where I’ll end up!

Telescope tube, 3 coats paint

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