A Tiny Forest Of Resistors Makes For Quick And Dirty Adaptive Optics

July 25, 2022 by Dan Maloney 16 Comments

The term “adaptive optics” sounds like something that should be really complicated and really expensive. And in general, the ability to control the properties of optical elements is sufficiently difficult enough that it’s reserved for big-science stuff like billion-dollar space telescopes.

But that doesn’t mean there aren’t quick and dirty adaptive optics that are suitable for the budget-minded experimenter, like this thermally deformable mirror. As [Zachary Tong] explains, this project, which started quite some time ago, is dead simple — a 4 by 4 array of through-hole resistors stand on end, and these are attached to a glass coverslip that has been aluminized on one side. An Arduino and a couple of shift registers make it possible to individually address each of the 16 resistors in the array. Passing a current through a resistor heats it up a bit, leading to thermal expansion and a slight deflection of the mirror sitting on top of the array. Controlling which resistors heat up and by how much should lead to deformation of the mirror surface in a predictable way.

The video below shows some of [Zach]’s experiments with the setup. Unfortunately, he wasn’t able to fully demonstrate its potential — the low-quality mirror didn’t cooperate with his homebrew interferometer. He was, however, able to use a dial indicator to show deflection of the mirror in the 2- to 3-micron range by heating the array. That alone is pretty cool, especially given the dirt cheap nature of the build.

As for practical uses, don’t get too excited. As [Zach] points out, thermal systems like this will probably never be as fast as MEMS or piezoelectric actuators, and many use cases for adaptive optics really don’t react well to added heat. But changing the shape of a mirror with air pressure is another thing.

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Hackaday Links: June 12, 2022

June 12, 2022 by Dan Maloney 4 Comments

“Don’t worry, that’ll buff right out.” Alarming news this week as the James Webb Space Telescope team announced that a meteoroid had hit the space observatory’s massive primary mirror. While far from unexpected, the strike on mirror segment C3 (the sixth mirror from the top going clockwise, roughly in the “south southeast” position) that occurred back in late May was larger than any of the simulations or test strikes performed on Earth prior to launch. It was also not part of any known meteoroid storm in the telescope’s orbit; if it had been, controllers would have been able to maneuver the spacecraft to protect the gold-plated beryllium segments. The rogue space rock apparently did enough damage to be noticeable in the data coming back from the telescope and to require adjustment to the position of the mirror segment. While it certainly won’t be the last time this happens, it would have been nice to see one picture from Webb before it started accumulating hits.

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All About Dichroic Optical Filters

February 20, 2022 by Dave Rowntree 7 Comments

[IMSAI Guy] presents for your viewing pleasure, a nice video on the topic of optical filters and mirrors. (Video, embedded below) The first optical device is a simple absorption filter, where incoming light is absorbed in a wavelength-selective manner. Much more interesting however is the subject of interference or dichroic filters. These devices are constructed from many thin layers of a partially reflective material, and operate on the principle of interference. This means that photons hitting the filter stack will interfere either constructively or destructively giving the filter a pass or stop response for a particular wavelength.

As [IMSAI Guy] demonstrates, this makes the filters direction-specific, as photons hitting the stack at a different angle will travel slightly further. Longer travel means the interference effect will be different, and so will the filtering response. You can see this by playing around with one in your hands and seeing the color change as your rotate it. Dichroic filter films can also make for some stunning optical effects. Very cool stuff.

By creating a filter stack with a wide enough range of inter-layer thicknesses, it’s possible to construct a mirror that covers the full spectrum with excellent reflectivity. Since you can tune the layers, you can make it reflect any range of wavelengths you like. One thing we’ve not seen before is a wedge-like optical filter device, where the layer thicknesses progressively increase lengthways, creating a variable optical frequency response along the length. We guess this would be useful for diagnostics in the field, or perhaps for manually tuning a beam path?

We like the applications for dichroic films – here’s an Infinity Mirror ‘Hypercrystal’. If you don’t want to buy off-the-shelf films, perhaps you could sputter yourself something pretty?

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Put A Little Piece Of The James Webb On Your Wall

February 16, 2022 by Tom Nardi 59 Comments

The James Webb Space Telescope (JWST) has become something of a celebrity here on Earth, and rightfully so. After decades of development, the $10 billion deep space observatory promises to peel back the mysteries of the universe in a way that simply hasn’t been possible until now. Plus, let’s be honest, the thing just looks ridiculously cool.

So is it really such a surprise that folks would want a piece of this marvel hanging up in their wall? No, it’s not the real thing, but this rendition of the JWST’s primary mirror created by [James Kiefer] and [Ryan Kramer] certainly gets the point across.

A CNC router was used to cut the outside shape from a piece of 1/2 inch MDF, as well as put 1 mm deep pockets in the face to accept the hexagonal golden acrylic mirrors. We originally thought the mirrors were also custom made, but somewhat surprisingly, gold-tinted hex mirrors are apparently popular enough in the home decor scene that they’re readily available online for cheap. A quick check with ~~everyone’s favorite~~ a large online bookseller turned global superpower shows them selling for as little as $0.50 a piece.

With a coat of black paint on the MDF, the finished piece really does look the part. We imagine it’s fairly heavy though, and wonder how it would have worked out if the back panel was cut from a piece of thick foam board instead.

Of course this isn’t a terribly difficult design to recreate if you had to, but we still appreciate that the duo has decided to release both the Fusion 360 project file and the exported STL to the public. It seems only right that this symbol for science and discovery should be made available to as many people as possible.

After a dramatic launch on Christmas Day and a perilous flight through deep space, the JWST has performed impeccably. Even though we’re still a several months away from finally seeing what this high-tech telescope is capable of, it’s already managed to ignite the imaginations of people all over the globe.

Working Model Reveals Amazing Engineering Of Webb’s Mirror Actuators

February 8, 2022 by Dan Maloney 30 Comments

We end up covering a lot of space topics here on Hackaday, not because we’re huge space nerds — spoiler alert: we are — but because when you’ve got an effectively unlimited budget and a remit to make something that cannot fail, awe-inspiring engineering is often the result. The mirror actuators on the James Webb Space Telescope are a perfect example of this extreme engineering, and to understand how they work a little better, [Zachary Tong] built a working model of these amazing machines.

The main mirror of the JWST is made of 18 separate hexagonal sections, the position of each which must be finely tuned to make a perfect reflector. Each mirror has seven actuators that move it through seven degrees of freedom — the usual six that a Stewart platform mechanism provides, plus the ability to deform the mirror’s curvature slightly. [Zach]’s model actuator is reverse-engineered from public information (PDF) made available by the mirror contractor, Ball Aerospace. While the OEM part is made from the usual space-rated alloys and materials, the model is 3D printed and powered by a cheap stepper motor.

That simplicity belies the ingenious mechanism revealed by the model. The actuators allow for both coarse and fine adjustments over a wide range of travel. A clever tumbler mechanism means that only one motor is needed for both fine and coarse adjustments, and a flexure mechanism is used to make the fine adjustments even finer — a step size of only 8 nanometers!

Hats off to [Zach] for digging into this for us, and for making all his files available in case you want to print your own. You may not be building a space observatory anytime soon, but there’s plenty about these mechanisms that can inform your designs.

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3D Prints With A Mirror Finish

December 25, 2021 by Matthew Carlson 15 Comments

As anyone who has used a 3D printer before knows, what comes off the bed of your regular FSD printer is by no means a mirror finish. There are layers in the print simply by the nature of the technology itself, and the transitions between layers will never be smooth. In addition, printers can use different technology for depositing layers, making for thinner layers (SLA, for example). With those challenges in mind, [AlphaPhoenix] set out to create an authentic mirror finish on his 3D prints. (Video, embedded below.)

As the intro hints, mirrors need very flat/smooth surfaces to reflect light. To smooth his prints, [AlphaPhoenix] first did a light sanding pass and then applied very thick two-part epoxy, allowing surface tension to do the smoothing work for him. Once dried, silver was deposited onto the pieces via a few different sprays. First, a wetting agent is applied, which prevents subsequent solutions from beading up. Next, he sprays the two precursors, and they react together to deposit elemental silver onto the object’s surface. [AlphaPhoenix] asserts that he isn’t a chemist and then explains some of the many chemical reactions behind the process and theorizes why the solutions break down a while after being mixed.

He had an excellent first batch, and then subsequent batches came out splotchy and decided un-mirror-like. As we mentioned earlier, the first step was a wetting agent, which tended to react with the epoxy that He applied. Then, using a grid search with four variables, [AlphaPhoenix] trudged through the different configurations, landing on critical takeaways. For example, the curing time for the epoxy was essential and the ratio between the two precursor solutions.

Recently we covered a 3D printed mirror array that concealed a hidden message. Perhaps a future version of that could have the mirror integrated into the print itself using the techniques from [AlphaPhoenix]?

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The Ins And Outs Of Casting Lenses From Epoxy

December 9, 2021 by Dan Maloney 10 Comments

If you need a lens for a project, chances are pretty good that you pick up a catalog or look up an optics vendor online and just order something. Practical, no doubt, but pretty unsporting, especially when it’s possible to cast custom lenses at home using silicone molds and epoxy resins.

Possible, but not exactly easy, as [Zachary Tong] relates. His journey into custom DIY optics began while looking for ways to make copies of existing mirrors using carbon fiber and resin, using the technique of replication molding. While playing with that, he realized that an inexpensive glass or plastic lens could stand in for the precision-machined metal mandrel which is usually used in this technique. Pretty soon he was using silicone rubber to make two-piece, high-quality molds of lenses, good enough to try a few casting shots with epoxy resin. [Zach] ran into a few problems along the way, like proper resin selection, temperature control, mold release agent compatibility, and even dealing with shrinkage in both the mold material and the resin. But he’s had some pretty good results, which he shares in the video below.

[Zach] is clear that this isn’t really a tutorial, but rather a summary of the highs and lows he experienced while he was working on these casting methods. It’s not his first time casting lenses, of course, and we doubt it’ll be his last — something tells us he won’t be able to resist trying this all-liquid lens casting method in his lab.

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