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Finishing the DIY Microphones (v.1.1)

Posted by Tom Benedict on 14/07/2015

The more I thought about the hot glue closure on the back of the microphones, the less I liked it. Don’t get me wrong. It works well. But it’s… permanent. I know the BT-EM172 capsules are only $10, and I know the rest of the microphone is largely scrap-boxed, but I hate to make a thing that can’t be serviced when it needs it.

So I re-designed the enclosure to include an end-cap. It’s drilled out 1/4″ to take a cable grommet, and has three #2-56 screws placed every 120 degrees around the periphery to hold it in place.

BT-EM172 Microphone Enclosure - Exploded View

The end caps took about fifteen minutes apiece to make, and were a comfortable fit in the back of the microphone bodies I made previously. Unfortunately, drilling and counter-sinking the screw holes for the end cap meant I needed to re-coat the microphone bodies along with the end caps. Since I had to re-coat them, I added grooves to each mic body to accommodate a Shure RK183T1 lavalier clip. I’m pretty sure a generic clip for a 9/16″ diameter mic would’ve worked fine, but these turn out to be tough to find. There are several listed on Ebay, but if you look at the metric equivalents, the specs say they fit something around 7-9mm in diameter. 9/16″ is closer to 14mm, so I think something was lost in translation. The clips from Shure will fit. (For shure! Har!)

Countersinking Screw Holes

I wasn’t happy with my previous coating job, so I came up with another way to apply the coating. I shoved each part onto a wooden dowel of the appropriate diameter (3/8″ for the end caps, 1/2″ for the mic body), and chucked it in a drill. I applied the Cerakote with the drill spinning. This gave each part a very uniform coating, and let me hit every outside surface without running into my fixture. I loaded the parts in the oven, dowels and all. On a whim I coated the screw heads, too, so I wouldn’t have shiny stainless screws in a black microphone body. Unfortunately the spray gun malfunctioned, so two of the mic bodies didn’t turn out as nice as I’d like. I slated those for the pseudo-SASS array, where they won’t be seen, and saved the two “good” ones for lavalier mics. Note to self: test the spray gun before loading product into it!

Parts Ready to Cerakote

Once the Cerakote cured it should’ve been a simple matter of assembling each of the mics. But I love to fiddle. I assembled the two for the pseudo-SASS array since I already had that cable made. But I needed more cable for the lavalier mics. Even though I’m already using Mogami W3031 cable for the other mics, I ordered 100′ of Mogami W2697 from Redco Audio to use for the generic lavs (only 20′ of which I plan to use). W2697 is almost identical to W3031, except for the way the shield is constructed. W3031 uses a braided shield. W2697’s shield is served (wrapped). Electrically they’re identical. But a served shield is easier to work with when making cables. I’ll have to wait for the cable and clips to come in before finishing the generic lavs.

Completed Mic Bodies

Rather than waiting like I did with the mono mic I built out, I grabbed my pseudo-SASS array and my recorder, and hiked out to the rocks south of Hapuna Beach. The last time I was there the waves were big, and made big, dramatic crash-bam-booms on the rocks. Of course that was in the winter. The summer wave pattern is a lot more bathtub-like, so the sound was a lot more subtle. Still, I ran several side-by-side comparisons of the pseudo-SASS against the built-in mics on the Tascam DR-05. I put together a set of 30-second clips comparing the two. The recording has eight tracks, alternating between the DR-05 built-in mics and the BT-EM172 array, done at four locations. When listening, keep in mind that the gains are different on the two mics, as are the frequency responses. I did no processing on the tracks aside from cutting and fading, so some tracks are louder than others. That’s a function of my technique in the field (or lack thereof), not the microphones themselves. This test was only so I could tell how well the pseudo-SASS array was separating the two channels.

The pseudo-SASS performed well enough I want to build a real one out of some 1/4″ baltic birch plywood I have in the shop. I still haven’t tested my prototype from the air, but it’s easy enough to include 1/4″-20 sockets top and bottom so I can mount it either way. More photos and sound samples to come!

– Tom

P.S. I’m not keen on the way clips from Soundcloud show up on my web site. I’ve seen other people include Soundcloud clips on their sites that are nice, small, and easily worked with. This thing is ungainly! If you know how to fix this, please let me know.

Posted in Audio, Engineering, Machining | Tagged: , , , , , , , , | Leave a Comment »

Bend a Little and Have Fun

Posted by Tom Benedict on 14/03/2015

Years ago I joined a group called Utata – a group of photographers, writers, and like-minded folks who enjoy lively discussion and creating and promoting art. Utata has several ongoing projects as well as two big annual projects. At the time I joined I was almost exclusively doing aerial photography from a kite, so I found myself unable – or to be truthful, unwilling – to participate in many of the projects. One in particular, the Iron Photographer, routinely kicked my butt.

The Iron Photographer project is modeled along the same lines as Iron Chef: All of the participants are given the same three elements to work with – two compositional and one artistic or calling for a specific technique – and are asked to create new, original works. On the face of it it’s a welcome challenge for any photographer. But if you’re limiting yourself to creating only aerial landscapes it’s less of a challenge and more of an impossibility. Take, for example Iron Photographer 211. The elements were: 1 – a bowl; 2 – something broken; 3 – photographed simply. You can make an aerial photograph that would qualify, but it would be a mighty tall order. I quickly became frustrated and stopped participating.

The lesson I didn’t learn back then was this: bend a little. The whole idea of Iron Photographer is to knock people out of their comfort zone and get them to put their thinking caps on. I staunchly refused and missed out on a lot of opportunities to have fun with a camera.

After a three year dry-ish spell I’m finally starting to get back into photography. This time not all of it is aerial. I figured I’d give Iron Photographer another try. I started with IP 212. The elements are: 1 – the photographer’s hand resting on a flat surface; 2 – an object resting in the palm of the hand; 3 – holga-fied. The only element I needed clarification on was that third one. The idea is to make it look as if the photograph came out of a Holga camera. I don’t own one, so I downloaded Holgarizer – a Photoshop action that would produce a similar result.

The Iron Photographer projects make you think. Yeah, I could’ve done a set of photos of my hand on a table with various objects in it. But where’s the fun in that? Better to ask why my hand is lying on a flat surface. Which flat surface is it lying on? What is sitting in my palm? And who chose to make the photograph? Of course for the requirements of the project it must be the owner of the hand. But from the standpoint of the narrative all of these are open-ended questions.

The first idea that sprang to mind felt cliché even before I made the photograph, but I made it anyway.

As Found

It’s not a happy picture. I wanted it to look like a crime scene: a dingy floor, the weak greenish glow of fluorescent lights, a pallid cast to the skin, and stark shadows outlining someone’s final act. In fact I’d just scrubbed the floor clean so I wouldn’t contaminate my prescription medication. The lighting was all provided by daylight-balanced strobes. And I’m actually pretty tan at the moment. But who’s keeping tabs? The only really stressful moment came when I started to clean up and realized I’d misplaced one of my pills. As tiny as these things are, they’d be lethal to my cats. I spent the time to track down every single one.

Then, of course, I saw that another participant in IP212 had come up with the same idea. Darn!

That’s when I started to wonder: Did the owner of the hand have to be the one who put the object in it? When I figured out the answer was “no” the idea for the next photograph came to mind. I opened the door to my daughter’s room and said, “Wanna be a totalitarian? Grab your boots!”

Of all of the events that mark the passing from childhood to adulthood, one my daughter celebrated with no small amount of gusto was the successful completion of her last high school PE class. She proudly announced that her only reason for wearing tennis shoes to school was null and void, and that she wanted combat boots. She and Rydra picked out a pair that would make any real princess proud.

“Ok,” I told her. “I’m gonna lie down on the ground outside, and you’re going to stand on me.”

Stunned silence. “What?!”

Even I had to admit she had a point. But once I described the photo to her she got into the swing of things.

The Regime

It took a while to work out the balance of the lights. Then it took a while to work out the best angle for my arm. Then it took a while for us to work out how she had to stand so it looked like she was bringing all her weight to bear on me without actually crushing my hand under her heel. In the end she wound up with one boot on and one boot off, standing en pointe on one sock-covered foot while squishing my hand with her booted heel. Early on she was tripping the shutter, but the contortions she was having to go through were more painful than what she was doing to my hand. We switched to a self-timer for the final few frames.

Though the Iron Photographer project lets you tag up to six photos for submission, you’re only really supposed to post one to the discussion forum. I chose this one. This becomes important later.

I had a couple of other ideas I wanted to try, but by this time I realized my first two photos were real downers. Despite the smiles and the laughter and the fun my daughter and I had making The Regime, I knew that no one looking at it would feel anywhere near as upbeat as we did. So I set morbid aside and went after something different.

The challenge called for something to be in the palm of the hand. It didn’t say that it had to be a physical object, just that something had to be there. I thought it would be neat to put something less tangible than a physical object in my hand. “I know!” I thought, “Light!”

I went through a couple of iterations on this one: I could have a beam of light coming out of my hand. (I might still try that one at some point, but not as part of this IP.) I could make the palm of my hand glow. The idea I finally settled on was to have an object in my hand influence light rather than generate it: a prism.

Years ago I worked in a lab that etched diffraction gratings into silicon using MEMS techniques. It was kind of a one man show, so I was responsible for the photolithography, the anisotropic etching setup, maintaining the safety and materials in the lab, characterizing the gratings we were making, etc. I also photographed the bejeebers out of everything we did on color transparency film. To see how much power went into each order of the gratings we were making we aimed lasers at them and measured the power in each of the return beams. It was an important step in characterizing the gratings. But it made for an even better photograph.

Each photograph was done as a single long-exposure frame. I’d turn off all the lights in the room, open the shutter, “paint” out the beams using a business card or some other flat white object (my hand stood in a couple of times), then turn on the lights for the prescribed amount of time and close the shutter. As painstaking as it sounds, once you got into a routine it went pretty quickly.

I used the same technique with the prism.

Can We Get There By Laser Light?

Even having used the technique, it took awhile to work out the details for this photo. Initially I illuminated the prism from the side. But the human palm isn’t all that flat. The prism kept rolling toward my fingers, directing the outgoing beam into the table or some other part of my hand. And painting a beam that’s going toward the camera is tough if you’re using a business card. The camera’s looking at the back side of the card! Eventually I figured out I should place the laser under the camera, and aim it back toward my hand. This gave me a way to see how well aligned the prism was to the beam: put the reflected light back into the laser’s aperture. It also made painting the foreground beam a lot easier since the camera could see the illuminated side of the card.

The difficulty was the outgoing beam. No matter what I did, the prism moved around in time with my heartbeat. You can see it as tiny wiggles in the painted beam. I could’ve Photoshopped that out, but where’s the fun in that?

Since that’s my own hand there on the table, I really didn’t have the option of turning on the room lights at the requisite time. Instead I set up a single strobe and a shoot-through umbrella up and to camera right. I kept the wireless transmitter handy on the table. Once I’d painted the beam I triggered the strobe and closed the shutter. It worked like a charm.

For my last IP212 photo I wanted to make something of a visual pun. The two compositional elements were a hand resting on a flat surface and an object resting in the hand. What if the object in the hand was a flat surface? In keeping with the whole optics theme I considered using a mirror, but honestly that’s kind of a boring photo. Besides, I’d already touched on the optics side of what I do for a living. I wanted to touch on the mechanical side, too. What if the flat object in the hand was being made into a flat object? Milling machine!

Hand Work

Before getting into the hows and whys of this I need to point out that I take shop safety very seriously. At no point did I do anything that put my hand or my tooling at risk. The only way to pull that off was to do this as two separate frames – one with the spindle moving and one with it stopped – and combine them.

I milled five of the six sides of this block using the 1″ cutter shown chucked in the mill. I milled the last side halfway, then stopped. Lighting was pretty straightforward: an umbrella in front and to the right, and a stofen bounced off the white wall behind the mill to provide speculars on the block and vise. I brought the spindle down until it was pressing the block into my hand, and made the first exposure. I wanted some motion blur out of the cutter, so I made a second exposure using ambient light, rotating the spindle by hand from above. Once I’d balanced the light between the two in Lightroom, I brought both frames into Photoshop for layering.

While going through the lighting for this a number of other photographs came to mind that didn’t fit into the IP212 requirements, but that nonetheless would make for pleasing photographs of machine work in progress. And that, to me, is the real benefit of taking on Utata projects like the Iron Photographer: The final result isn’t the photographs made for the project. It’s the ideas that the process of thinking through those photographs leaves you with. That’s what I missed out when I joined Utata years ago. I don’t plan on missing out on it again.

To my utter delight, Greg, the moderator who sets up the Iron Photographer challenges, favorited The Regime and wrote a really thoughtful comment on it. This is the first time one of the Utata moderators commented on one of my photos. Even more delightful, Debra Broughton wrote a short piece about it for the front page of the Utata web site and wrote a comment of her own. I admit I banged my forehead on my desk a little at my obtuseness for taking this long to jump into Utata projects with both feet. But thanks to Greg and Debra I did it with a smile.

– Tom

Posted in Machining, Photography | Tagged: , , , , , , , , , , , , , | 2 Comments »

KAP Rig to Pole Adapter

Posted by Tom Benedict on 01/06/2014

In my previous post I described some bits I’d made so that I could swap a rig from one suspension to another. I did this as the first step in getting into pendulum design. I needed a way to move my rig from a Picavet to a pendulum to whatever design I wind up testing so I can compare the performance of each. But even as I was making them I knew I wanted a mating part that would let me flip my KAP rig upside-down and mount it on a pole. Yesterday I got the chance to make the new pole head:

Pole Head

With a KAP rig most of the forces are axial: pulling up or down. With a pole rig the forces are mostly axial, but if for any reason the pole leans to one side the forces become radial: pushing out sideways, wanting to bend things. I knew the joints on the KAP adapters I made were plenty strong, but I wanted something a little beefier for the pole.

The core of it is one of the KAP adapter bits I’d made previously. The outer shell is the new part. It started life as a length of 1″ diameter 6061 aluminum. I bored out the bottom to match the depth and inside diameter of the original pole head. Once that was done I flipped it around. I turned the remaining outside diameter down to 0.6″ and put a 45 degree taper to transition between the narrow and wider parts. There’s a good bit of metal between the bottom and the taper at the top, so it’s plenty strong. Once the outside was done I drilled the inside 0.375″ to match the KAP adapter bits and drilled a #8 clearance hole between the upper and lower halves of the thing. The only step left was to cross-drill the lower half with a 0.128″ hole for the rivets. The original pole head had nice flats for the rivet heads, so I spotted each hole with an 0.250″ end mill to make a flat.

The 0.375″ hole on top is a close fit for the KAP adapter, but not so close that I had to force the part into place. I put some blue Loctite on the screw I used to secure it to the pole head, so it shouldn’t go anywhere. Once the head was pop riveted to the pole, the work was basically done. But I abuse my pole, and I knew the head would take damage if I didn’t do something to protect it. So I chucked up a piece of PVC pipe, faced off both ends, and used it as a cap. With the lock screw backed out just right, the cap is a snug fit and won’t fall off in transit.

I really wasn’t planning on pursuing this during World Wide KAP Week. Not with so many great places to fly. But weather is weather. Sometimes it cooperates, sometimes it doesn’t. The winds in town were gusting so I ducked into the shop to make this while I waited for conditions to improve. This morning the sun rose to a clear sky and almost zero wind. Here’s hoping there’s something to fly in this afternoon!

– Tom

Posted in Engineering, Kite Aerial Photography, Machining, Photography | Leave a Comment »

Sitelle Getter II

Posted by Tom Benedict on 01/03/2013

Two posts ago I alluded to a design change to the cameras at work. Since I posted pretty fab pictures of the first rev of the getter, it’s only fair that this one has its turn to shine.

The original design was novel in a couple of ways. It had two chambers, one on top and one on bottom. Each was filled with its own charge of activated charcoal, and each had its own filter, protective screen, and polished aluminum cap. This made for a fairly high volume / high throughput getter in a neat little hockey puck sized package.

Sitelle Getters

Unfortunately two problems killed this getter. The first was that it simply didn’t fit. There’s a fair bit of material between where the cold head bolts on and where the cold strap attaches. In the original design of the camera, the flex circuit for the electronics feedthrough passed quite close to the bottom surface of the getter. In practice, though, the flex wound up being longer than expected, which caused the flex to touch the getter. The getter is maintained close to -190C, and the flex is maintained with one end at -100C and the other at room temp (typically 0-5C). Having a thermal short to -190C right in the middle was going to cause problems. To get past this, we machined off the bottom chamber of one of the getters, and moved on.

The second problem that killed this design was that the cold strap attached through the force of a magnet embedded in the getter attracting a second magnet, embedded in the top of the cold strap. Unfortunately the cold strap was too stiff to let this arrangement work well. The magnets were too weak to overcome the springiness of the cold strap, no matter how carefully we arranged things during assembly. And moving to stronger magnets would’ve meant the camera was harder to disassemble. So instead we ditched the magnets and moved to a conventional bolt-together design.

All of which completely invalidated this design for the getter. So I started over. I was planning to make another CNC-heavy design with lots of sexy curves, vented screws, stainless mesh, etc. But right as I got ready to fab the thing, I found out our CNC mill was completely tasked to making new filter frames for one of our other instruments. Whoops!

So I re-designed it (again) to be made entirely using conventional hand-crank tools. And I got rid of the screws. And used thinner material – a good thing in a getter since it means the getter is cooling faster, and is pumping before nasty stuff like water vapor can condense on the face of the CCD. The new design is made using 2mm plate and the same 0.010″ thick copper strap we use for our cold straps. They’re not as nice looking as the original design, but they should work.

Sitelle Getter II

And truth be told, they’re probably better than the original design in a number of ways. The overall cavity size is larger, and I managed to pack almost twice the carbon inside as the original getter. There’s a third copper strip hidden inside the getter that provides a thermal path up inside the bulk of the carbon, so it should cool quite quickly. I kept the filter, so still no chance of carbon dust getting into the camera body. And considering I made four of these in three days, it was a quick solution to a nagging problem that’s been hanging over my head for months.

The only real drawback to this design is that there’s no way to service the thing. If the filter tears or finger oils get on the top, there’s no real way to fix it. Toss it and grab another. Which is why I made four of these for two cameras. Spares!

I probably won’t have a chance to test these until next week. Meanwhile I need to make the bolt-together cold strap for the two science cameras. The prototype has been cooled down twice, and is working perfectly.

So now all I have to figure out is what to do with the two surviving first generation getters. They work! They just don’t fit inside these cameras. And it’d be a shame to turn them into paperweights.

– Tom

P.S. Yes, in case you’re wondering, all those holes were drilled by hand. On four parts. Nothing fancy, just a sacrificial plate bolted to the mill with the part clamped on top. The rest was all trig, and a lot of triple-checking my position so I didn’t screw anything up! All four turned out perfectly. But I swear, if I can skip drilling any holes for a while I’ll be ecstatic!

Posted in Astronomy, Engineering, Machining | Leave a Comment »

Back to the Cameras

Posted by Tom Benedict on 03/11/2012

I haven’t posted anything recently because my life was almost entirely out of my hands for a while. Since my last post I’ve been cutting down four of my trees every spare moment I’ve had from work. And when I’ve been at work I’ve been working on two of our existing instruments in addition to working on the new one we’re building. In the middle of all this my son fell behind in his school work, so I’ve been a tutor as well. Non-stop action!

And not much sleep. But my son is caught up with his work, my trees should be done this weekend, and the servicing on the two instruments at work is nearly complete. That just leaves the cameras for the new instrument. As luck would have it, there has been progress on that as well!

CFHT Sitelle Parts Anodized

The parts came back from the anodizer! And the color? Love it!

There’s still a good bit of work to be done on the cameras, though. For starters, you can’t have an anodized surface inside a vacuum vessel. So our machinist at work is going to take the two camera bodies, machine off all the anodizing on the insides while he’s doing all the finish-machining, add all the side ports, and then either he or I will polish every internal surface to a high shine. This isn’t so much for looks (though it does look cool!) It’s to minimize the surfaces to which gases can stick when we pump the cameras down to hard vacuum.

In the meanwhile our machinist has been making most of the other camera internals. He just finished those today as well, so I’m test-fitting everything to make sure it goes together as it’s supposed to. And therein lies the rub, so to speak. Already I’ve found one set of parts I made that had the wrong size screw holes in them. I needed to re-make one set, anyway, so I’m going to make entirely new parts with the right hole size so they’ll mate with the parts our machinist made. Then there’s the whole set of experiments to characterize the getter. And then the experiments to characterize the cryogenics. And then the pump-down and hold-time tests. And then fitting the actual detectors. And and and… Yeah. Still some work to be done.

But we’re a lot closer now! And maybe, just maybe, I’ll get a chance to fly a kite soon.

– Tom

Posted in Astronomy, Engineering, Machining | Leave a Comment »

A Day In The Life

Posted by Tom Benedict on 28/08/2012

People sometimes ask me, “What kind of work do you actually do at a telescope? Isn’t it just looking up at the stars?”

The answer is “Yes and no.” We do have people here who look at the stars. Or rather, we have observers who operate our facility to take data for astronomers around the world. But most of our staff is tasked with keeping the place running. That’s where my job fits in. When people ask what our job description is, I like to say that it reads: “To do anything and everything necessary to collect science-quality photons every single night of the year.” Mostly, that’s keeping things in good shape and making sure everything runs smoothly at night. But sometimes it means taking on project work.

So what’s a typical work day like? Well… There isn’t one. Every day is different. Some projects we get are big, and span several years. Others are small projects and may only span a few days or a few hours. Here’s a recent example of a small project that is only taking a couple of days of actual work:

Recently, the need came up to measure the width of the slit on our dome. The “slit” is the opening in the dome that the telescope looks out through.

CFHT and the Mauna Kea Atmospheric Monitor

This is a photograph of our facility showing the shutter in the closed position. (Unfortunately I still haven’t made any photos of our telescope with the slit open. Note to self…) The big white panels are the shutter, which covers the slit through which the telescope looks. At night the shutter rolls back like a giant garage door.

To measure the open part of the slit, we stuck two Leica Disto laser measuring units back-to-back and attached them to the shutter. As the shutter opened, we triggered the units to take measurements off to either side. Add the two measurements up, and voila! You have the width of the shutter at that point along the track.

Aaaah! But the shutter has a back side, too, that it slides down as the shutter opens:


Measuring this was a little trickier…

It started about a week ago. One of my co-workers and I climbed to the top of the building to test-fit the Leica Disto unit. Unfortuantely we couldn’t put it smack dab in the middle of the shutter the way we did on the lower edge. But we were able to position it off to one side.

Dual Disto Rail - Installed

Because each Disto will measure over a hundred feet with millimeter accuracy, this still works. We should still be able to add up the two numbers and get the width of the slit. Unfortunately the chunk of metal I mounted the Disto unit on wasn’t entirely flat. One laser beam intersects the arch girder on its thin edge. But the other laser beam was so skewed, it actually hit the enclosure on the far end. I needed to get several degrees of tilt to make everything line up correctly.

So that’s what I did today. I tore the thing down and muddled through a way to get more adjustments out of the mount. Here’s what I came up with:

Dual Disto Rail - Overview

This is an overview of the Disto rail mount. The two Leica Distos are mounted back-to-back on a rail. The rail keeps them pointing in opposite directions, and offers some protection from knocks, dings, and other things that go bump in the night. The rail is in turn mounted at 90 degrees to a second rail along which the first rail can slide. This offers some lateral adjustment in case the Distos need to be offset from the base. The second rail is then mounted in the roll/pitch/yaw platform.

Dual Disto Rail - Roll Pitch Yaw

(I dare you to find an uglier collection of nuts and bolts!) It’s a mish-mash of metric (M6 set screws) and SAE (#10-24, #12-32, and 1/4″-20 for the roll and yaw adjustments). This is one stark reality about working at a place like this: Unless you have time to plan a thing, it is, by default, a scrap box project. I started designing in SAE, intending to use 1/4″-20 for everything. Then I found out our 1/4″-20 screw selection had been ravaged. So I grabbed some #12-32 screws. Lo and behold, I couldn’t get both lengths I was after. Rather than chop screws, I grabbed the next size down and found some the right length. Then I found out we didn’t even have SAE set screws. So I had to use metric.


So it’s a frankenbeast. But it works! I adjusted the two M6 set screws to take out residual roll (I got it to better than 0.002″ across that whole bar!) Now the 1/4″-20 hex head bolt at the back lets you tip the laser beams by several degrees.

I’m slated to go up tomorrow to test-fit this thing again and see if I can bullseye the two laser spots on the edges of the arch girders. If I can, we’re in business for taking these measurements. And if not? I’ve got a machine shop and a scrap box waiting for me on ground floor.

– Tom

P.S. In case you’re wondering how the thing actually attaches to the shutter, that base is essentially a giant fridge magnet. On the bottom are three neodymium magnets I stripped out of some dead hard drives. See? Scrap box project!

P.P.S. So what’s the view like when you’re standing on top of a telescope dome? In a word: Outstanding.

A Day In The Life

Posted in Astronomy, Engineering, Machining | 4 Comments »

Tool Capacity

Posted by Tom Benedict on 14/01/2012

Recently I had an odd lesson in tool capacity. Before launching into what I learned, I need to go back a little:

Around 2003 I made some vacuum feed-throughs for one of our instruments. Because of some space constraints and the decision to use a flex print rather than discrete wires to carry the signals, it wound up having a really oddball shape on the outside so the connector could be tilted at an angle. This let the flex reach the inside surface of the connector without getting crimped.

The design required a 3D contoured surface on the outside. At the time I didn’t have freedom of the shop at work, which includes a large CNC mill, so I made the parts at home. I’ve owned a Taig benchtop 4-axis CNC mill since about 2001. This was before I did all of the rework on the control electronics, so making those parts really stretched the capacity of the tool, but it worked. I made the two parts and swore I’d never do that again. Since then I replaced the control electronics, so doing contouring at home is well within the scope of what my mill is capable of. Nonetheless, I never did make more than those two original parts.

A recent problem with the electronics on one of the feed-throughs put this back on my plate: I need to make four spares. I hadn’t even opened the files since 2003, so this involved some head-scratching. But eventually I got everything back in order. These days I’m the only machinist at our headquarters location, so I do have full run of the shop. I know I can do these at home, but what a great project for the big CNC mill! It has a much bigger spindle than my Taig as well as a larger work envelope. The combination of servo motors and ball screws give it better backlash characteristics than my Taig, which uses open loop stepper motors and lead screws. But the real kicker is that it has flood cooling, whereas I have to stand by my Taig to squirt coolant at the tool while it runs. The mill at work would be perfect for this!

Or so I thought…

My first surprise came when I tried to load the finishing toolpath onto the mill. Because it’s a contouring job and because it’s a finishing path that moves the tool in many many small steps, the file wound up being 3.4MB. No problem for my little Taig at home! I use USB memory sticks to move part files around, so I never even batted an eye when I ran the part in 2003. But the mill at work uses floppies. 1.44 MB floppies. Oh… The file wouldn’t even fit.

The mill at work has, in almost every way, a larger capacity than my mill at home: work envelope, spindle horsepower, maximum slew rate, etc. But it’s limited by how large a part file it can run. In that respect my little Taig, which I can pick up with two hands, wins by a landslide. The only limit to the size part file I can run on it is the size of the memory stick I use. And if those are too small, I can finally get around to running a network cable out to the shop so I can just transfer part files via file sharing.

I found a work-around for the mill at work. I’ll be able to finish the parts in plenty of time. But it made me laugh when I realized how a minor problem like this can bring a project to a halt. And I realized how nice a tool I have at home. Now all I need to do is upgrade the big CNC at work so it can compete with my Taig.

– Tom

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Science Fair Projects for Big People

Posted by Tom Benedict on 18/11/2011

I have three kids in school. All of them do science fair projects. Sometimes they’re enthusiastic about them, but most of the time they treat them as if they were homework assignments rather than fun opportunities to explore the universe around them. On several occasions they’ve told me about things they’re doing outside of school and I’ve mentioned that they would make good science fair projects. “But that’s not our assignment.” ??! Science fair projects shouldn’t BE assigned. They should stem from curiosity! Unfortunately I’ve had a rotten time communicating that to them.

But the truth is over half of what I do at work would qualify as a science fair project. So that’s how I’ve started describing my work to them. “Hey, I worked on my science fair project again today!” Since we’re hoping to present our most recent work at the SPIE Astronomical Instrumentation conference next year, and since my stuff is likely to be done with a paper and poster rather than a paper and presentation, the whole science fair analogy is actually quite close.

So here’s my science fair project this year:

We run a number of instruments at the place where I work. All of these are run under vacuum, and are kept cold to varying degrees. One of our instruments lives around 75K. Others live at a balmy 150K. None of them operate warmer than -120C. Our goal with all our instruments is to get them pumped, get them cold, and leave them that way for as long as possible. Thermally cycling electronics and mechanics is a great way to break stuff and wear them out. The fewer cycles, the better. So the longer things stay pumped and cooled, the happier we are.

Two problems come up:

The first is we need to be able to monitor things like temperature and vacuum. Temperature is easy. Vacuum is more difficult. Vacuum gauges that measure vacuum near atmosphere are pretty cheap, and pretty easy to use. Gauges that measure higher vacuums like the ones we use are more expensive. Worse, all of them emit light. Since we’re talking about instruments that measure very VERY dim light levels, having something inside the instrument that’s blasting light poses something of a problem. So we tried to use low range gauges for reasons of cost, complexity, and light level.

That’s when the second problem came up: We found out that the high range gauges we’d been using were acting as fairly efficient vacuum pumps. This is nothing new. The cold cathode vacuum gauge is basically an ion pump. Dedicated ion pumps are just bigger. So we wanted to find out what the pumping speeds are for various gauges. We tested a number of gauges, and found the cold cathode gauge won hands-down. (Yes, we have numbers to back this up. No, I won’t present them here. Yes, they’ll be in the SPIE paper in 2012.)

Which brought us back to the first problem: How do you make it so a cold cathode gauge doesn’t spill light into your camera?

To test this we outfitted a small vacuum system with a window so we could look down the throats of the gauges we were testing. To give you an idea of how bad this problem is, this is what a hot ion gauge looks like, hooked to our vacuum system:

Ion Gauge Emissions

And this is what it looks like when you look into the window:


Gauge View Hot Cathode (Ion Gauge)

I’m sure some of you are thinking, “But ion gauges are basically light bulbs! Cold cathode gauges don’t emit nearly as much light.” And you’d be right. They emit a lot less. (Again, we do have numbers to back this up. Look for them in the 2012 SPIE paper.) But they still emit light. And if your camera is doing an hour long exposure, every little bit matters. This is what a cold cathode gauge looks like when you poke your head inside:


Gauge View Cold Cathode (Cold Cathode Gauge)

Imagine putting one of these inside your camera, pointed at the chip or the film, and ask yourself if you’d be able to use it for photography. Then consider astronomical instruments sometimes make exposures that are minutes to hours long. All the while the detector is accumulating the photons being emitted by the gauge. You can see it’s a problem.

The problem is, light baffles for vacuum gauges really don’t exist as a commercial product. We looked. We did find a vacuum baffle, but it was really designed to keep crap like coating chamber gunk from getting into gauges, not to keep gauge light from getting into a detector. It dumped the light by a factor of ten, but that’s it. We were looking for factors of millions or more. Since nothing existed, we started designing.

This project is a collaboration between two people. One of us is an optical engineer, the other (me) is a mechanical guy. The optics guy is doing the design analysis, I’m making the prototypes, and we’re both pitching in on the testing and data analysis. The optics guy went through a number of design ideas we came up with while brainstorming one day. All of them panned out, but not all of them could be built. One of my ideas seemed simple at first, but by the time he’d tuned it to get us the attenuation we needed, the design would’ve called for annular slots of 0.2mm or less on a 25mm circle. No freakin’ way. In the end there was a clear winner design. So we built a prototype and tested it.

The attenuation was better than we’d dreamed of. In case you’re wondering, those numbers will also be in the 2012 SPIE paper. (This is a really cheesy way of saying, “We’re still collecting data and don’t have an answer for you yet.”) In any case our problems with light leak are more or less done. We found which gauge we need in order to continue pumping our instruments once cold, and we now have a clear path forward for new instrument development. Yay!

See? Science fair projects are fun, even for big people.

– Tom

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Just Getting it Airborne

Posted by Tom Benedict on 11/08/2011

The editor in charge of the Flickr Collection at Getty Images got back with me and said he’d check with his Creative Research group to let me know if they had identified any areas of their collection of Hawaii images they wanted to expand. So it’s a start! But so far no word on what those areas are.

Meanwhile this led to a discussion with my wife and father, both of whom are good critics. What I mean by that is that both of them understand that the ultimate purpose of criticism is to improve the thing being criticized. So neither holds back punches, and both have provided me with good insight in the past. My wife pointed out that there’s a difference between grand sweeping panoramic landscapes and tightly focused single-frame images. Of my KAP work, she preferred the latter rather than the former. My father gave me some additional information, but agreed that the single frame, carefully composed photograph of a single subject stood on its own better than the panoramic landscapes. Each has a place, but of the two it was the single-frame photographs they preferred.

Up until now most of my design work with the T2i rig revolved around the idea of it being a special purpose rig for making panoramas. Well that needed re-thinking! I’m still working on the design for a highly stabilized KAP rig, but now it looks like making it more general-purpose would be a better idea. In the mean time, it’s pretty clear that my first priority should be to just get the T2i airborne.

While Michael Layefsky was out, I got a chance to see his DSLR rig. He uses a different body from mine, but the weight and the overall build is similar. The T2i rig I built for a photographer here in Hawaii used the Maxi parts from Brooxes, but Michael’s rig was built using the normal BBKK components. Well heck! I have a spare utility frame on my masthead rig! Yesterday I pulled it off and gave it a good looking over. Sure enough the T2i not only fit, it fit with room to spare.

But that’s where convenience ended and reality took over. The center of gravity of the T2i with 18-55mm kit lens is quite a bit forward of the tripod socket. It’s actually almost in line with the lens mount, but off to one side. Some careful work with the utility frame, the camera, a three-point support, and a postage scale nailed the CG and let me plan where to drill the hole in the frame. Unfortunately the answer was that the tripod socket hole needed to be drilled about a quarter inch off the side of the frame, out in space. ARGH!

I made a wider shelf for the utility frame, match-drilled it and bolted it onto the frame, and went from there. Now I have a frame that’ll take the T2i or the A650, and mount either one to my BBKK KAP rig, or my masthead rig. The shelf was made using 0.050″ aluminum, so it hardly added any weight. And since most of the weight is borne by the utility frame, the thinner material isn’t a huge concern. Besides, I always always use a safety tether between my cameras and my rig, so even if the shelf separates in flight or on top of the mast, the camera isn’t coming down.

The only bits remaining are a new metal geared servo for the tilt axis (my current plastic geared on oscillates, even with the A650 and Ho/Ver axis), and the GentLED-Focus cable that’ll let me trip the shutter on the T2i through an R/C remote. I placed the order for those bits from Brooks over the weekend, and should have them in-hand by tomorrow at the latest.

So at long last, months after I bought the T2i, I’m finally going to have it airborne. It sounds goofy to say this since I’m the one who’s been dragging his heels, but I can’t wait!

– Tom

P.S. Yes, I’m still planning to build the ultra stabilized rig. But for now? Time to do some KAP!

Posted in Kite Aerial Photography, Machining, Photography | Leave a Comment »

Rules and Missed Opportunities

Posted by Tom Benedict on 15/06/2011

Very recently, the place where I work has had to comply with a set of rules known as ITAR. It restricts international arms trade. And it turns out that infrared array detectors are considered to be arms-grade devices. All infrared array detectors are arms-grade devices. So all of our infrared cameras fall under ITAR rules. No pictures, no sharing, no talking, no nothing. It’s a bummer.

Where it really gets to be a bummer is that I like to photograph the machining work I do. The parts are small, they’re fun to make, they’re even more fun to photograph, and I get a kick out of sharing the geek, so to speak. But anything having to do with infrared array detectors is strictly off-limits. Rats!

I’m in the middle of a number of projects at work, all of which take top priority, of course. One of them is building out a test cryostat for (you guessed it) an infrared detector. This poor cryostat has been around the block way too many times. It started life as a NICMOS IR camera back in the day. Later, when we needed to test another device, the optics and  mechanics were stripped out and it was retrofitted as a generic cold bench. I fabricated and installed a miniature optics bench with a pattern of 1/4″-20 holes on 1″ spacings, just like an optics table you’d find anywhere in the world. I added a heat shield, heat shield sleeves for the mechanical feed-throughs, all sorts of goodies. As each detector went through this thing, I made a number of electrical feed-throughs. Different numbers of conductors, different connectors, different numbers of connectors, the whole nine yards.

This most recent round required yet another electrical feedthrough, though otherwise we’re not making any changes to the mechanics. It was one of the more challenging designs I’ve had to stick on the thing, and it was the first of these feed-throughs I’ve designed in a 3D CAD package. I was excited! I wanted to share! But no. ITAR rules.

So I can’t show you a picture or even describe the design. I can’t describe the quirky machining challenges involved, or how I got around them. All I can say is that I did it and leave it at that. Which is a bummer, because a joke with no punchline isn’t a joke at all.

– Tom


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