Silvering testing continued

I tried something that should have occurred to me before now: using multiple layers of paint.  Obviously.  I get a much smoother, shinier surface with multiple coats of primer and chrome paint, 2-3 layers of each.  Now it's looking like a mirror.  It's still a very, very poor quality mirror (this brand of chrome paint will not result in a smooth chrome surface regardless of substrate smoothness) but it's reflecting more light and focusing light more sharply.  This is very encouraging.

Silvering test

I've silvered a couple of test objects.  They work as parabolic mirrors, although the main goal here was to improve my silvering technique.  The results are good enough that I think I'm ready to move on to fabricating a hand mirror.

Test objects

I printed up a couple of test objects to see how well a 3D printer could produce the substrate for a first-surface mirror.  Just a couple cm long.  Pretty good results, with some concerns.

- slump.  There's a real danger that the mirror will sag while bing printed.  Very bad given that I need soe degree of precision, especially if I'm building an array of small mirrors.  I'll need to take preventative measures in mirror design (vertical support for the curved surface from the bottom up)
- the thickness of the layers affects surface smoothness and slump

Next task: mirroring.  I've picked up some chrome paint.  Chrome paint has a pretty poor reputation for actually producing an even shiny surface so we'll se how it goes.

First you get the sugar...then you get the power...

Disappointingly, my PLA plastic has yet to arrive.  I'm wondering if it's been misdirected in shipping.  I'll need to look into it.

While waiting I had an idea: sugar.  There's precedent for 3D printing sugar objects, though my idea is a bit different.

Move the focal point of a mirror over a bed of sugar.  A point heat source can caramelize sugar; think of the way a butane torch scorches a creme brule.  Sucrose caramelizes at 160°C.  Having browned a 2D pattern, add a thin layer of sugar on top of the bed and repeat.  The layers should be thin enough that each layer fuses to the one before.  When all necessary layers are complete, the printed object can be lifted out of the bed of sugar, extra sugar being shaken off.  Might work.  One challenge: sugar darkens as it caramelizes, and a darker surface absorbs light better.  The rate the sugar is being heated would go up just when heat is no longer required.

Maybe I'm just feeling a bit bitter.

Handheld mirror: Plan A

So, here's the first proper test mirror I intend to make.  The plan:

- First surface mirror
- 3D printed PLA plastic substrate
- Off-axis mirror
- approximately 20cm x 20cm

Details inside

Every mad scientist needs a PhD

On Tuesday of this week I successfully defended my thesis.  Congrats to me.  I now have a bit more time on my hands to make mirrors.

Safe and stylish

Look what's arrived: 

My new welder's helmet.  I went with a #10 lens.  In direct outdoor sunlight I get a dim, green-tinted view of the world.  It's bright enough that I can see what I'm doing with my hands.  I wouldn't rule out the possibility of going to a darker #12, but I'm reasonably pleased.  This upcoming week is going to be extremely busy for me, but at some point I'm going to need to post my calculations of reflected light intensity to give a sense of why I thought serious protection was a necessity.

As a bonus, I'm now halfway to a fun Halloween costume: Darth Vader, of the planet Vulcan.

Mylar mirrors - recent published research

Pneumatic Mirror Telescope Research.  It turns out that the idea has proven to work passably well as a way of creating lightweight telescopes.  Spray on an epoxy layer and the mirror can hold its pressure for a year.  Perhaps I should have taken the idea more seriously.

Needless to say the pneumatic approach won't work in space, though perhaps some sort of electrostatic tensioning might.  Not something I care to speculate about just now.

Bruce D. Holenstein and Richard J. Mitchell.  "Robert H. Koch’s Work on Lightweight Medium-Aperture Mirrors."  J. Astron. Space Sci. 29(1), 79-84 (2012)

Material test: aluminized mylar

I'm going to try some small scale mock-ups of paraboloid mirrors over the next few weeks.  First up: aluminized mylar, a thin plastic sheet with an aluminum coating.

I knew that there were some pretty serious disadvantages to using this material, and those turned out to be even more serious than I'd thought.  The only real advantage is cost.  I was able to make my handheld concave mirror for less than $5, and this video by Dan Rojas demonstrates that it's possible to build a concave mirror more than a meter in diameter for less than $25. His method involves stretching the mylar across a frame then applying suction to make it concave.

Caution: do not try this at home.  Fire hazard, blinding hazard.  This isn't just boilerplate. The blinding hazard turned out to be quite a bit worse than I'd expected, and I had thought I was already being a bit paranoid.  I would strongly recommend against copying the techniques I've used here; what I was doing was quick and crude, intended only to get a sense of how the material behaves.

Everybody needs a hobby

A few weeks ago I had an idea for an odd little hobby project: solar sintering.  I was inspired by this art project by Markus Kayser.  It struck me that a much smaller scale project would be a novel way of 3D printing plastic objects.  Fun.  Using paraboloid mirrors rather than a Fresnel lens would make it quite inexpensive to build.

Part of the appeal of the idea was that there was the option of taking it in a more ambitious direction, with some difficulty.  As demonstrated by Kayser, it could be scaled up to 3D print ceramic objects by melting sand into glass.

The idea was also appealing for another, somewhat more whimsical reason.  I like to read about space-related technology, but I suspect that any large scale project outside Earth orbit would be economically impractical without space-based manufacturing.  There's a problem of where to begin.  Something like lunarcrete is a good idea (more on that later) but fabricating it would require assembling a whole factory.  In contrast, a small array of mirrors capable of sintering lunar regolith could be made comparable in mass to probes currently being sent to the moon on a semi-regular basis.  An idea worth considering, perhaps, or at least amusing to imagine.  Call it a Lunar Industrial Module (LIM), hence the title of this blog.