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Tired

Posted by Tom Benedict on 15/08/2014

I realize I haven’t written anything in months. I blame work.

Every large telescope relies on mirrors to collect light. And every mirror relies on something shiny to make them work. That shiny bit can be any number of things: silver, gold (yeah, gold), or in our case aluminum. Sometimes these are over-coated either for protection or to boost reflectivity in a particular wavelength range. Regardless, all coatings are subject to another truth about astronomy: it’s an outdoor sport. Mirror coatings are subject to wind, dust, and sometimes rain and snow. Coatings degrade.

Our mirrors are bare aluminum, which gives us a base reflectivity just below 90%. We put about 850 angstroms worth on our mirrors. It’s a little thin, but it’s what our chamber is capable of. Our primary mirror needs to be re-coated every three years. Our secondary mirror, which faces down and is less subject to rain and dust, needs to be re-coated every five years or so. This year they both came due.

The past two months of my life have been spent coating two mirrors.

As simple as that sounds the truth is a lot more involved. The room was completely stripped and cleaned top to bottom about six times. The chamber needed to be re-prepped, once for the secondary and once for the primary. In between I was ordering new supplies, testing our secondary support system for leaks, helping to take apart and re-assemble the telescope, etc. Except for a brief break for our company’s 35th anniversary party and another for a hurricane, I’ve been at the summit for the better part of two months.

Both coatings turned out well. Last night we finally got the thing put back together and aligned. We still have one last chamber re-prep planned for Monday, but other than that I’m off the hook. This morning when I woke up I looked around the room and saw my KAP bag, my camera bag, my gliders, and my 2m/70cm radio, and realized that for the last two weeks I’ve practically had no hobbies to speak of. Everything has revolved around those two mirrors. And now I’m finally free. At least for a couple of years.

Some time soon I hope to get my cameras back in the air, and to start playing with planes and radios again. But for now my only plan is to sleep.

– Tom

Posted in Astronomy | Leave a Comment »

Quick ‘N Dirty Astrophotography

Posted by Tom Benedict on 10/09/2013

I ran across an astrophotograph on Flickr that really blew my socks off. I don’t like to post other people’s pictures to my blog without asking, so I’m posting a link to it instead:

http://www.flickr.com/photos/71942737@N06/8575933459/

It was made out here in the islands, and features a lighthouse, distant lights on Haleakala, and an amazing rendition of the night sky. I’ve always wanted to do wide-angle astrophotography, but I don’t own a tracking mount. To my utter delight, I saw that this photo was made using nothing more than a tripod and a DSLR. “Hey!” I thought, “I can do that!”

That night I drove up Kohala Mountain Road and pulled off about half a mile from the Kohala Mountain Lookout which is unfortunately tree-bound. My pull-off spot gave me a nice unobstructed view of the sky.

I tinkered around with settings and hit on this:

  • 18-55mm kit lens wide open at 18mm (nothing fancy here)
  • Manual exposure mode
  • ISO 1600
  • f/3.5 (as wide as it’ll go… again, nothing fancy here)
  • 30 second exposure

Lo and behold, it worked! But on a crop sensor camera even 18mm isn’t all that wide. Hey, I have a low-tech fix for that! Panorama!

I made 11 exposures and threw them into Autopano Pro. It stitched without a problem, and produced this:

Stars Upon Thars

There are some artifacts in there, but nothing bad. I could be a little more careful about setting a fixed white balance. And with some thought and effort I could probably get a lot more out of it by processing the original photos using RAW workflow. But hey, it worked!

Now all I need to do is find a foreground subject that’s even on par with hawaiiansupaman ‘s lighthouse! Thanks, hawaiiansupaman for showing me the way.

– Tom

Posted in Astronomy, 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 »

A Minor Thing (But…)

Posted by Tom Benedict on 01/12/2012

More progress on the cameras I’ve been building at work! I spent the past several weeks testing and building the cameras’ getters. (Yes, a “getter” is a noun. “Gettering” is the verb. Even after having done this for years that still feels funky when I say or type it.) Anyway, the getters have finally joined the ranks of the parts that have gone from CAD to reality. It’s show and tell time!

A getter is a type of sorption pump. It typically includes some source of cold (a cryogenic gas, a closed-cycle cryocooler, or magic – your pick), a gettering material (activated charcoal in this case), and the bits and pieces to bring the cold to the gettering material. By cooling the gettering material down to cryogenic temperatures, it becomes a cold trap for residual gases in a vacuum system. Get the thing cold enough, and as molecules of residual gases come in contact with the getter, they tend to stick to it. Voila! Pump!

These cameras were originally designed to take a commercially available getter. But as the project progressed, we realized we needed more performance out of them than the commercial units offered. So we designed and built our own.

The first design criterion was that it pump faster than the commercial unit. The commercial getter we originally intended to use in these cameras is basically a cup with a bunch of activated charcoal glued in it. The easiest way to double the pump speed is to double the amount of surface area that’s doing the pumping. Answer? Use two cups and glue charcoal to the other cup, too! But space is limited, so we stuck the two cups back to back with one facing up and one facing down.

The second design criterion was that we didn’t want carbon dust getting all over the inside of the camera. For starters it’s dirty, and we’re building these things in a clean room for a reason. But carbon dust is also conductive. This is not what you want to have rain down all over a circuit board! Answer? Add covers!

So here’s the design we came up with:

Sitelle Getters - Rendering

Two getters are shown. The one in the foreground is right side up. The one in the background is upside down, showing off the counter-bored screw holes that allow the getter to be bolted to the cold head. Inside are the cavities that take the activated charcoal. On top of each cavity is a cover plate that’s meant to hold filter material in place to keep the carbon dust from leaking out.

Having a design is nice, but it doesn’t do you any good if it doesn’t actually work. The last two weeks have been spent testing different charcoal loads to see what works best. Monday we pumped down our latest setup, and so far it looks good. Our recipe at the moment is 2-3mm of Stycast 2850FT epoxy, poured into the bottom of each cavity with as much activated coconut charcoal dumped in as will fit. Once the epoxy has cured, the excess charcoal is poured out and the cavity is sealed with a sub-micron fiberglass filter to catch any residual carbon dust. This is then followed by a 60 mesh .0065″ diameter wire stainless steel screen to protect the filter (which is quite fragile), followed by the cover plate as shown in the above rendering. So far the lab tests look good. So do the getters:

Sitelle Getters

As exciting as it is to see each part of these cameras in finished form, it’s just as exciting to see the various bits and pieces on my workbench slowly disappearing as they’re incorporated into final form. The cleaner my bench is, the closer I am to done. I can’t wait.

But I also can’t wait to see this instrument in action. The cameras are only a tiny tiny part of what this thing will be. It’s exciting.

– Tom

Posted in Astronomy, Engineering | Leave a Comment »

Reasons to Enjoy the Weekend

Posted by Tom Benedict on 17/11/2012

I thought I was going to spend the week finishing the cameras at work. A nice, quiet, uneventful week. Boy was I ever wrong. A series of projects and emergencies had me and my co-workers at the summit and on the go almost the entire week.

It started with an instrument exchange. These days we have three primary instruments on the telescope: Megacam, our visible light imager, WIRCAM, our infrared imager, and Espadons, our visible spectropolarimeter. Typically two of these will be used each month. When the moon is less than half full – what we call dark time – Megacam is typically on the telescope. WIRCAM and Espadons are less sensitive to moonlight, so they are our bright time instruments. One or the other will be on any time the moon is more than half full.

This was one of the exceptions. Our adaptive optics bonnette (AOB) was at the cassegrain focus of the telescope along with one of our smaller infrared imagers, KIR. AOB/KIR was slated to be removed and Megacam installed. The actual exchange went well. The rest went horribly wrong. Two of us had just finished up a bunch of work on Espadons the week before. The only step left was to put all the covers back on and check the instrument out for the sky. Once the exchange was done we popped all the covers on and ran a quick hardware check. We’ve run hundreds of these over the years without incident. This time it failed.

It turned out one of the actuators on the instrument had died. We spent the next two days coming up with a work-around while our machinist rebuilt the actuator. By the end of the second day we had our temporary fix – a micrometer head bolted to a block of aluminum – all sorted out. We also had the rebuilt actuator re-installed and checked out for the sky.

Meanwhile we were slated to pull WIRCAM out of its upper-end so that another adaptive optics system could be tested on sky. This took the better part of a day, and involved a fair bit of stress because we couldn’t let the camera warm up during the move. As soon as it was out of its upper-end, it was wheeled down to one of our labs where we hooked it back up to its cryogenic system. It never came above 85 degrees Kelvin.

While all this was going on, other members of the team were working on a possible problem with our dome shutter – the big door that has to open every night so we can observe, and that has to close every morning to keep out the sun, the rain, the wind, and the dust. Just as we finished getting WIRCAM bedded down in the lab, I was called on to help with the shutter.

So at 4pm on my third day in a row at 14,000′ of altitude, I found myself fifty feet above the ground staring across space at Megacam while my boss was squeezing himself into a crawlspace in the ceiling barely big enough to fit his body so he could take pictures of the shutter drive units. “Hey!” I thought, “I wonder if I could make a credible HDR photo using a cell phone.” So I pulled out my phone, braced it as best I could against a nearby railing, and did a five frame, five stop HDR stack.

Megacam on CFHT

It turned out pretty well, considering the contrast range, the high pressure sodium lighting, and the fact that I didn’t have a tripod, beanbag, or any other real way to steady the camera. All in all I’m impressed with the cameras that are being shoehorned into cell phones these days. It’s no replacement for a DSLR or even a good compact camera. But it’s honestly pretty darned good. I thought it particularly fitting that I was photographing what was at one time the largest digital camera in the world, using a camera so small it fits inside a cell phone.

And in case you’re wondering how I got to be fifty feet off the ground to make this picture, this is what the view was looking straight down:

Where I Was

Yup, that’s my foot. And the square yellow column just to the left of my foot is all that’s holding me up. For the part of the shutter my boss needed to access, this is the only way to get there.

And maybe this explains a couple of things:

First, I know why I like the perspective KAP offers. It’s a fascinating height to work from. I enjoy going up in the lift just to get that view. Tired as I was, I jumped when my boss said he needed someone to go up with him. It’s the closest I can come to seeing what a KAP rig would see. Things are close enough to look familiar, yet far enough away to give the viewer a fresh take on the world.

Second, it just reinforces how glad I am not to have a fear of heights. It’s amazing how much of my working life here has been spent strapped into a fall harness.

Finally, it’s just one more reason to enjoy the weekend. Even when I think I have a pleasant uneventful week ahead of me, this is typically what happens. Par for the course when working at a telescope.

So what’s in store for next week? I HAVE NO IDEA! And that’s par for the course, too.

– Tom

Posted in Astronomy, Engineering | Leave a Comment »

More Camera Work

Posted by Tom Benedict on 10/11/2012

The new parts for the cameras worked perfectly. In the process I also figured out why the holes on the first set were the wrong size. To make a long story short, I left some stock on the parts for a finish pass of the cutting tool, but didn’t actually include a finish pass in the toolpath. So the holes were precisely 0.0118″x2 too small. (I left 0.0118″ for finishing.) Sometimes the software is too smart for the operator…

But the new parts work great!

CFHT Sitelle Camera Assembly

I got most of the anodized bits assembled earlier in the week. From left to right are the anti-vibration towers for the cryo coolers, the preamplifier enclosures, the cryo cooler hose supports (hiding behind my coffee cup), the vacuum flange clamps for the valve and vacuum gauge, and finally the support rods for the anti-vibration towers for when the camera is off vacuum.

But the real work this week is in the foreground. On the left side are the getter bodies and their caps. On the right are the partially assembled “three ring circuses” that provide the thermal isolation necessary to cool the detector down to -100C while the outside of the camera is still a relatively toasty 0C. Those parts I had to re-make? Those light colored tabs in the three ring circuses are the new parts. They fit perfectly.

But not everything went so smoothly. The three ring circus sitting right in the middle is fitted out with a dummy detector PCB, but it’s not actually attached to the rest of the parts. Why? Because when I placed the nut and bolt order from hell for these cameras, I left out the screws that hold the detector onto the cold part of the camera. I sent out a new nut and bolt order as soon as I realized that, but now it’s a game of hurry up and wait.

In the meantime, though, there’s plenty more to do. So today I built out one of the getters and stuck it into a test cryostat we have sitting in the lab. It’s pumping down now, and should be ready to turn on Tuesday morning. The test for this thing is about as brute-force as you can get: With our PCC cooler loaded with PT-14 gas, our previous getter design would pull the vacuum on the test cryostat down to the high 10x-7 torr range. If this one can’t beat that performance, I have to change the design. It’s that simple. My back of the envelope calculations say it should be able to pull about 1.6x harder than our previous getters, and have a much higher overall capacity. So it shouldn’t be a subtle effect. If it is, it’s back to the drawing board.

There are only a handful of parts left to make for these cameras. Providing the getter testing goes well and the new screws let me finish assembling the camera’s innards, I may be back to serious kite flying in the next couple of weeks. I can’t wait.

– Tom

Posted in Astronomy, Engineering | 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 »

Ready for a Weekend!

Posted by Tom Benedict on 08/09/2012

For a short work week, it’s been a long week at work. But it’s been a good one! Most of the external parts for the cameras are done. If fortune continues to shine on us, it’s looking like we might send out all the external parts for anodizing some time in the next few weeks. We still haven’t settled on a color for these things, but it’s looking like the original orange I used for making the CAD renderings may win out in the end. The only requirement is that the cameras be brightly colored. One of them will be located almost knee-height when it’s on the telescope, in an area where forklifts are in use. If I could make it fluorescent orange with tiger stripes, I would! But I’d settle for fire engine red or hot pink, too.

This is, I hope, the last rendering I post of this camera design. The next picture I post should be the real deal:

Sitelle Camera September, 2012

I’ve still got some internal parts in the queue, though, so the work is far from done. I’m working on the vacuum feed-throughs next week, and hope to make a start on the cold finger/foot/strap assemblies as well. That is a story all its own:

Any time you build a cryogenically cooled camera, you need some way to isolate the cold bits so they don’t let heat from the room-temperature camera body leak in. Along with this, you need some way to attach your cold source to the cold bits so any heat they generate or collect through ambient radiation can be taken away. The first is accomplished by using some thermally insulating material to support the cold bits. In these cameras I’m using G-10 fiberglass tabs to separate the warm parts and the cold parts. The second is accomplished by using some thermally conductive material to draw the heat off. In these cameras that’s done through an unfortunately complicated assembly of copper and aluminum.

Usually these assemblies are comprised of three parts: the cold head (the cold source), the cold strap (the bit that connects the cold head to the rest of the assembly), and the cold foot (the bit that actually connects the cold to the back of the detector chip). In this case our cold head is a Polycold PCC cold head. Our cold strap is a piece of copper 0.20mm thick x 12mm wide x 90mm long. Our cold foot is a block of copper about 35mm square and about 15mm thick.

The trick is that you have to be able to remove the cold head to access the rest of the camera. So somewhere in there you need some sort of a removable joint. In one camera I’ve worked on, this was done using a spring and two pressure plates. One of the plates is connected to the cold head. The other is connected to the cold strap. Push the two together, and the force of the spring keeps things in good solid contact. Voila! The cold head / cold strap / cold foot circuit is complete!

The original design for this camera also used a spring, but concerns about mechanical vibration coupling between the cold head (which does vibrate) to the detector chip through the spring ruled that out. I had to come up with something else. Over the course of the last three months I tried all sorts of ideas, none of which panned out. The best approach I had used a single bolt to bolt the copper cold strap directly to the cold head. Only problem? There was no way to reach the bolt with a wrench! No matter what I did, there was no way to get a tool in there. Time and again, I found myself back at the drawing board, starting over from scratch.

Then one of my co-workers came up with a brilliant idea: Why not use magnets? Stick a magnet on the cold head and a magnet on the cold strap, and let the magnets do the work of holding the two in contact. The idea is similar to how the clasp on a lot of handbags work these days. We tried it on the bench in the lab, and it worked great. So yesterday I mocked up a set of hardware to try this in a real cryostat. Today I installed everything and started pumping it down.

Installing the cryo head was dead nuts simple. As soon as the two magnets got close enough, “Clack!” they came into contact. The magnets are self-centering, so some of the off-center contact problems we had on the spring designs are also solved. Even before getting things cold, I could already tell the idea was working. In order to drive some of the water off the cryostat walls, I stuck a heater on it while it was pumping down. Both the cold head and the cold foot registered the heat being applied. Under vacuum, the only path that would let both parts see the same heat is the cold strap. Only way for that to work? For that joint to be solid. Yaaaay!

I plan to start cooling Saturday morning and see how it cools through the weekend. By Monday we should have good numbers for how it’s likely to behave. If it all works out well, I’ll start cutting parts for the real cameras on Tuesday.

But enough work! It’s time for a weekend!

And speaking of weekends, I finally ordered the tools and parts to replace the main halyard on our boat. My plan now is to finish drawing up the new main halyard winch in CAD, put the old winch back into the mast, and get the boat ready for when the halyard bits all show up. We should be sailing by next weekend. Meanwhile I can start machining the new winch, which is being designed not to let the cable jam ever again. When it’s ready, swapping them out will be about an hour’s work: the perfect project for a weekday evening with the promise of a weekend sail at the end of it.

Time to play.

– Tom

Posted in Astronomy, Engineering, Sailing | 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:

CFHT 1

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.

GAAAAH!

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 »

A Semi-Kinematic Fiber Mount

Posted by Tom Benedict on 29/12/2011

As Designed As Built

I still haven’t wired up my 5.8GHz video downlink for my KAP rig. Life keeps getting in the way. Mostly I’ve been living in the shop at work, making parts. Today I finished the last project I had to get off my plate before building the two cameras for our new instrument. Just in time for the new year!

Before describing what this is, lemme back up a little…

We have an instrument at work that is fed light by a fiber optic bundle. An injection module lives on the telescope whose job is to get light into the fiber. The other end is a spectrograph that’s located in a temperature controlled room deep inside the building. The arrangement works quite well, but the performance of the whole thing depends on very accurate positioning of the fiber. Essentially the light of a star needs to be focused onto a fiber 100 microns in diameter, and positioned to better than a micron. Tight tolerances.

To make it even more fun, the bundle has three fibers in it, not just one. All three need to line up with optics in the injection module to better than a micron. So rotation is just as important as the position.

Ideally a bundle like this would have a precision ground rectangular ferrule with nice registration features on it. As you can see, the ferrule is round. And there aren’t any good registration features to set rotation off of. To be fair, the original design was copied off of another instrument that has given years of good service. But it only had a single fiber, so rotation wasn’t important, and it was installed once and never touched again. This one is removed and installed every time the instrument comes off the telescope. And each time it’s installed, the optics in the injection module have to be re-aligned to match the new position and rotation of the fiber bundle.

Clearly something had to be done.

The idea behind this new mount really goes to our master machinist, who has since retired. He took one look at it and said, “You should make something like the chuck on a ram EDM machine.” And that’s precisely what we did!

The heart of the thing is a stainless steel v-block that’s toward the rear in both pictures. I wound up making it out of some unknown 400-series alloy I grabbed out of the scrap box. I rough machined it, then surface ground it to size. This was the most critical part in the whole thing. I’m new to precision grinding, so it took me almost an entire day to make. It’s really not that complicated a part. I’m just slow. It has a bunch of tapped holes in it to boot, which is always an adventure with stainless. I’m happy to say I didn’t break any taps, for once!

The clamp block is made out of 330 stainless, and the collar and pin is a commercial shaft collar with a 3mm dowel pin pressed into it. That constrains the fiber in rotation. Depth is set by a shoulder in the aluminum base block. The v-block is recessed into the aluminum base block by 6mm, and is locked into place with a pair of button head screws. (I love it when I can work button head screws into a project. They’re so nice looking!)

To install the fiber you push it into the hole, snug down the clamp block screws, rotate the fiber until the two pins touch, then torque the clamp block screws down. That’s it. The fiber bundle is 7.5mm in diameter, and is held over 24mm of its length. That’s a 3.2:1 clamp, so the arrangement is quite stiff once things are cinched down.

Which is good for us! Because this mounts to the telescope, there’s no fixed direction for gravity. The fiber is protected by a stainless steel jacket, so there’s some weight hanging off of it. The long clamp on the v-block keeps it from shifting around as the telescope moves. (Remember that 1 micron tolerance on position and orientation? Yeah!)

The instrument this is intended for is already checked out for its upcoming run, so we won’t get to install this for almost a month. I hope to have good things to say about it once we put it into service.

Now on to my video downlink!

– Tom

 

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