CIRCE is the Canarias InfraRed Camera Experiment, led by Professor Stephen Eikenberry at the University of Florida. I am one of the graduate students who has been involved in this project over the course of about 15 years and I’d like to write about a bit of what I’ve done here on my blog.
CIRCE is a 750 kg (1650 lb) instrument built mostly out of aluminum. It’s about 1.75 meters (5 feet) long and over 1 meter (3 feet) in diameter. I think of it as a her mostly because the name is derived from the sorceress Circe from the Odyssey. On the outside she’s powder-coated black.
I started working on CIRCE’s cryogenic mechanisms in 2012, testing its linear slide (which holds a focal plane mask and half wave plate rotator) and filter box (with 5 wheels). Both mechanisms are designed to work at both room temperature and pressure and in a vacuum at 77 Kelvin (-321 F), which is the operating temperature of the instrument.
This is a tricky thing for a few reasons:
- Lubricant (for shafts and bearings) turns from nearly friction-free liquid to solid rock. We have to degrease EVERYTHING that is inside the vacuum jacket (the black cylinder) because of this. Additionally, we don’t want the lubricant to off-gas while we pump out all the air; any volatiles that boil off when we remove the air from the inside will settle on the optics and detector, which is extremely difficult to remove and therefore bad.
- Gear teeth must mesh at both room and operating temperatures. That’s tricky to design, especially if you have to use mixed metal gears (such as stainless steel and brass). CIRCE got around that by using custom-cut aluminum gears for the filter box, but it’s still not easy.
- Different materials shrink at different rates. Our shafts are precision-ground stainless steel, but the linear slider bearings are most definitely not stainless steel. As a result, some bearings have to be cut with a slot along their long axis to allow them to grip the shaft without binding.
What this means is we purposely degrease the bearings and shafts as much we can using acetone and isopropyl alcohol. There’s a fair amount of clever design work involved as well — the gearing required by our projects must be carefully chosen so that the gears can be cut and deliver the required step resolution. For example, CIRCE’s filter wheels take 1200 full steps per full revolution and have 4 filter positions and a wedge-shaped ‘Open’ position. We end up 1/8 stepping the motors for better control, which means that the filter positions are about 250 steps from each other. The gearing on the outer rim of the filter wheels is helical; the drive gears are split in half radially and connected by a spring to give minimal backlash so that no matter the direction they are turned they provide nearly instantaneous response to commanded motions.
We don’t use absolute encoders for our mechanisms because 1) they’re expensive to design for cryo conditions, 2) a source of radio frequency interference, and 3) we don’t need something that complex. We find that counting the number of pulses, or steps, the motor is commanded to move sufficient nearly all of the time.
As a result, however, we need to be able to ‘home’, or zero the step counts for all of the mechanisms so that we know where the mechanism is in its motion, especially if the moving part can run into and damage other components.
Testing the filter box and linear slide for repeatability and reliability meant putting it in our small test Dewar, wiring it up, verifying that it worked warm, then pumping out the air and cooling it with LN2 until it reached equilibrium some days after starting the test process. This allows time for the filter wheels to cool via conduction through the bearings, shaft and shaft bearings to the aluminum body. It’s not very efficient, so some of the cooling happens radiatively as well.
We then tested the filter box by commanding something like 250 motions per wheel for 4 gravity vectors (over 5000 motions). We also tested how reliable homing was by triggering the homing switch and counting steps. It was not the most exciting two weeks of my life.
After that the CIRCE team began constructing the active and passive thermal shields. They are made out of aluminum sheets and are designed to minimize the heat load put onto the optical bench (as well as create a nice dark environment inside the instrument. We iterated a lot, working on welds, seams, joints, and aluminized Mylar jacketing to further improve their thermal characteristics. I spent about a month doing little more than trimming the Mylar and aluminized tape to shape the oddly-shaped components. I also helped out in the machine shop, cutting the sheet metal to shape the shields. It was a fantastic learning experience.
CIRCE’s detector electronics arrived in 2013 and we began tuning the detector and readout electronics to optimize the performance. I was only involved tangentially, but I did run the detector on occasion. I also did some design work for some clamps and wire guides both in and out of the Dewar.
The neat thing with the readout electronics is the flex cable system. These cables, about 15 inches long, carry the a data from the detector to the outside world (there are lots of details there, but it’s ancillary). These flex cables have Hirose connectors which are finicky beasts and tend to disconnect if the flex cables are under tension, which (spoiler alert) they always are. Our solution was to print a clamp in three parts, with the joined cables held precisely in place by the clamps (much like making a mold). This brings the chance that the cables will spring apart to 0. However, connecting the flex cables is a bit of a bear; it requires one person to hold the interface plate tight against the vacuum jacket while a second person reaches through the interface hole and connects the flex cables and screws the last clamp piece into place. It’s not my favorite job because any static discharge near the flex cable (which protrudes from the back of the interface plate by about 4 inches) will fry the detector or delicate readout electronics, and if the first person drops or lets the interface plate slip, the flex cables will be damaged beyond repair.
Next was lab acceptance testing, where we ran CIRCE through her paces and demonstrated to the GTC that CIRCE would in fact work at GTC. That went off with only small hitches, such as GTC’s shock at the (admittedly) poor quality of the engineering-grade detector. In CIRCE’s defence, IR detectors are more flawed than optical detectors, and despite 2 of 32 amplifier channels being dead, the CIRCE detector’s flaws are averaged over when dithering. Once we proved to them via simulation that the detector’s flaws were acceptable, they accepted CIRCE as a GTC instrument. That meant we had to build a crate.
Over the course of about a month we built a shipping crate about 10 feet long by 5 feet high and 6 feet wide. It was quite the undertaking, but eventually we had CIRCE safely ensconced and shipped. As our mechanical engineer said, it was a good sight, seeing the truck’s taillights as it pulled out of the loading dock.
While that was going on, we began fabricating the cabling for use at GTC. The cables run from the electronics rack (which is hard-mounted to the telescope elevation ring) to the Folded Cassegrain rotator cable wrap (designed to allow cables to go from a fixed panel to a panel that rotates with the instrument) to the interface plate on the backside of CIRCE. We had constructed lab-testing cables that were about 2 meters (6 feet) long, but we needed about 10 meters (32 feet) of cable with two connectors in between (at the fixed and rotating panels). Building them was an exercise in patience, but I did improve my soldering skills considerably. I also helped build the electronics rack crate, which we had heat-treated in Jacksonville; we then packed and shipped it to the Canaries.
Once we got there….more to come!
One of the joys of astronomy is the travel we get to do. Big observatories are expensive and so are located in the mountainous regions around the world. The best locations are isolated island mountains, such as Hawaii, or ocean-side mountains, such as the Andes in Chile.
UF is a partner in the Gran Telescopio CANARIAS, or GTC (pictured above). GTC is on the top of the volcanic island La Palma, at an altitude of about 23oo meters (7400 ft). La Palma is nearly ideally located; it’s several hundred km from the coast of Morocco and is not heavily populated. GTC is one of several large observatories that make up the Northern European Observatory (a sort-of counterpart to the European Southern Observatory in Chile). Like Hawaii, La Palma is at about 30 degrees north latitude and has an excellent view of the northern sky and an ok view of some of the southern sky.
So far while in grad school (7 years and counting) I’ve spent about 6 weeks at the summit. My first visit was in 2012 and was a day trip that a group of students took; the International School of Astronomical Instrumentation (IScAI) was hosted on the nearby island of Tenerife at the Instituto de Astrofísica de Canarias (IAC), which is also the host institution for GTC. We got to tour several of the telescopes that neighbor GTC. The coolest thing (aside from seeing what €200 million and decades of determination can get you) was that GTC is so finely balanced you can move the whole telescope by hand. It weighs in at around 400 metric tonnes and yet you can move the telescope up and down with ease from the catwalk, and even spin the entire telescope by pulling yourself around on the catwalk. It does has an huge amount of rotational inertia, so you do have to strain a bit to get moving.
My second trip was to unpack CIRCE from its container (above, left), which I also had a hand in building. We arrived in June 2014 and spent two weeks uncrating everything and verifying in the lab that the instrument had survived. While stressful, it was a fantastic experience, made all the better by the fact that when we cooled down CIRCE and tested the detector, everything worked well (I got to travel around the island of La Palma while we waited for the temperatures to stabilize, which takes about 2 days). This is not always a guarantee with infrared detectors — cooling down from room temperature to liquid nitrogen temps has been known to kill them. On top of not killing the detector array we also nailed the clocking of the detector to a few milli-radians, meaning that we didn’t have to futz with the detector position and clocking. That’s important because small changes in tilt of the detector are every difficult to make with CIRCE (imagine trying to hold your camera in place on the lower left corner while moving the upper right corner a fraction of a millimeter, while in a vacuum and at cryogenic temperatures). As it was, we found that we were within a few milli-radian of perfect and so we left it at there.
CIRCE then spent a few months (literally) chilling in the instrument lab while we waited for GTC to install and commission the Folded Cassegrain Rotator. The FC Rotator (as we call it sometimes) moves in such a way as to cancel out the apparent rotation of the sky as seen by GTC (this is a necessary and vital component of modern large observatories, all of which use alt-az telescope mounts). The CIRCE team went over in December 2014 and installed the instrument on the rotator, but the control software wasn’t quite ready and science commissioning was postponed. However, the CIRCE team was able to do many of the engineering-type tasks required, such as verifying that CIRCE could take images and automatically deposit them in the GTC repository, and that communications sent from our control software could make the telescope slew.
During engineering commissioning we discovered that the environmental cover (the shutter attached to the front of the instrument designed to protect the entrance window from dust and frost) was experiencing enough friction to bind up; it was unable to open and close when commanded, so we had to design and build a replacement.
In March 2015 I took part in science commissioning. We had two goals before we started observing in earnest: 1) re-wire the motor control chassis, and 2) replace the environmental cover.
CIRCE’s motors are simple stepper motors that we refurbished for use in vacuum. They’d had given us no troubles in the lab at UF or while testing in the instrument lab at GTC using short (~6 feet), but when we used the ‘flight configuration’ cabling for use on the telescope (which are much longer, ~30 feet), we encountered a swathe of problems.
It turned out that the 30 feet of cables Alan, Nick (our instrument scientist), Scott (our electrical engineer), and I had made in early summer made excellent radio receivers, and the interference that the cabling picked up made it impossible to drive the motors; the signals to move the motors, simple voltage pulses, were swamped out by the radio frequency interference. Poor Alan had to rewire all of the connectors and cables, stripping out and redoing all of the work we’d done in early summer so that the wiring carrying the motor signals were separated from the ground wires and redoing the cable shielding. Once that was done, things seemed to work in the lab and we were able to move each of the mechanisms.
After installation on the telescope (which is terrifyingly exciting to watch, by the way), we noticed that some mechanisms had stuttering problems, meaning that they would not complete movements from position to position, or sometimes not move at all. Scott suggested rewiring the motor controller box (which was a snarl of wiring, designed by the previous electrical engineer) as a way of cutting down on the gremlins, as we referred to the problems. However, time at the summit is precious, and Steve (CIRCE’s Principle Investigator) decided to save that for a later trip. So, in early March, Scott and I went up to the telescope, pulled the motor chassis out of the electronics rack, and rewired it. We also tested the new environmental cover, which worked like a charm.
We replaced the environmental cover, which meant pulling CIRCE off of the telescope, setting it on precision wooden blocks (pictured above), and removing about 2000 tiny screws (actual number of screws: ~30). We also installed a ‘blower’ (pictured at left),
which was designed and 3D printed out of plastic. Its job is to blow compressed air across the entrance window to prevent any fogging or frost from building up on the entrance window . We iterated quite a bit on the design of the blower, which is one reason 3D printing is so great — it costs nearly nothing to produce highly complex shapes with interior hollows that are quite literally un-machinable using traditional subtractive machining techniques.
The next week was full of observing. In the course of 7 nights we observed for 3 first-halves and 3 second-halves of the night, with great weather for most of the run. We sadly didn’t kill all of the gremlins in our motor chassis, as we still had a few difficulties, but the frequency of failure was brought down to acceptable levels. I did a bit of real-time data analysis using superFATBOY (the Florida Analysis Tool Born Of Yearning for high quality astronomical data), which is written in Python by Dr Craig Warner, one of our software engineers.
We had a very successful science commissioning run, marred only by the fact that our half-wave plate rotator mechanism, which was a steel belt driven rotating mechanism, failed to move at all, most likely due to stiction (sticky friction encountered when cooling parts to cryogenic conditions – keep in mind most lubricants make excellent rocks at liquid nitrogen temps!). Even with that failure, we had a spectacular science run and observed many nifty targets, including an extremely red Gamma Ray Burst counterpart search (paper in progress! but not by me), an amazingly deep look at the lensing galaxy cluster SDSS 1004+41, and a few others. Our first target of the run was the star Betelgeuse, which we had hoped would be useful for figuring out the rotational axis location on the detector. When we moved CIRCE off of the Folded Cass rotator and reinstalled it, we used 3 brass locator pins to define the instrument position kinematically. CIRCE is over 3 feet in diameter (<1 meter), and we wanted to test how well we could repeat its position on the rotator. So, by using a bright-ish star (Betelgeuse was WAY too bright!), slightly defocused, we could calculate its center of rotation by rotating CIRCE in 15 deg increments and find the ‘barycenter’ or average position of the star. The repeatability of CIRCE turned out to be extremely good, to within a few millimeters of where it was before — not bad for a 1 Tonne, 1 meter in diameter instrument! As I mentioned earlier, we didn’t get rid of all of the gremlins with the motor chassis rewire, but many of our previous troubles were lessened.
My next journey to GTC was in March 2016 for its first Servicing Mission. We brought a replacement mechanism for the half-wave plate that Yigit (a fellow graduate student) designed. I wasn’t involved with removing CIRCE from the telescope (I followed Steve and Alan a day later), but once I arrived we got to work opening up CIRCE. We replaced the half-wave plate mechanism, added new filters to the filter wheels, and installed the grisms we had liberated from the FLAMINGOS-1 instrument. This necessitated milling out some of the material from the base of the filter box, which we did at the William Herschel Telescope just down the road (they have a full machine shop for parts fabrication).
Alan and I balanced the wheel holding the grisms using steel washers (pictured above to the right) – calculating the weight and position required of the counterweights is not a simple thing because the center of mass of the grisms wasn’t known a priori, nor was their exact clocking.
We then reassembled the instrument, taking care to clean and regrease all of the o-rings. It was an action-packed week, capped when I connected the flex cables together. We use two that are each about 15 inches long; carry the data signals from the detector through the vacuum jacket. We spent a lot of time at UF working on how to hold these two cables together; the flex cables themselves are tricky to work with, as they are enclosed in the Dewar and are totally inaccessible once the air is removed and the instrument cooled down. The manufacturer recommends using tape to hold together the two cables at the connection point; they are springy and have a tendency to spring apart if they’re under tension. The CIRCE team solution was to (of course) 3D print a 3-part clamp
that holds the lower flex cable in place while installing the upper flex cable (which is firmly and permanently attached to an aluminum feed-through plate). It’s a tricky job, one that requires two people working in each other’s space while working with tiny, tiny screws mostly by touch. We did it without a hitch, and I managed not to electrocute the detector, which is directly wired to the flex cables. It’s a bit nerve-wracking to do.
After we closed up and cooled CIRCE down, we ran it thru some lab tests to verify that we hadn’t done anything terribly wrong. We discovered that one of our filter box motors doesn’t work while cold (it worked beautifully while warm). It’s a bummer, but not crippling. We spent quite a bit of time taking spectroscopic standards with the grisms and determining where the spectra were on the detector and what filter combinations worked for us. We also replaced the plastic blower with a stainless steel one (pictured at right) that we ordered from Xometry.com (they are like a broker for 3D printed or CNC machined parts).
After we checked CIRCE out in the lab, we installed it on the telescope. I was given a bright yellow and orange safety harness. We worked alongside GTC engineers to ensure that CIRCE was safely lifted to the Folded Cass station (about 50 feet above the dome floor), and managed to have it installed on the rotator after we broke for lunch. We then ran CIRCE thru her paces, checking that the detector was still functional. However, when we tried to move motors, we were disheartened that none of them moved.
We spent about 3 hours troubleshooting the problem, measuring voltages and amperages and anything else we could think of both inside the electronics rack and at the cable connections. Eventually, after much skull and anxiety sweat, we tried running an extension cable from one of the Nasmyth platforms to the motor chassis directly (bypassing the electronic rack AC switch and Baytech control unit). It turns out that somehow the power delivered to CIRCE via the telescope elevation ring is ‘dirty’, meaning that the pulses that drive the motors are contaminated with noisy, unwanted pulses. By using another power source, we were able to turn all the motors (sans the bad filter wheel motor) and fully check out CIRCE. It took us into the evening and we missed twilight (important for CIRCE because we generate flat fields using the fading IR twilight). However, we were able to observe for about 4 hours and verify that we had both put CIRCE back together properly and that the new mechanism worked as expected.
Here are a few pics that I decided to add because, why not.
Hello potential readers/researchers looking for a post-doc!
Welcome to my neglected blog — I’m writing a few posts that hopefully will illustrate my skills to you. Be sure to check out the #AstronForNoms tag for those posts.
As for the rest of you forlorn visitors, I welcome you too. I’m not much of a blogger — I’ve been focussing on getting my applications to various post-doc positions out, as well as writing up my science papers and, oh yeah, that thesis thing.
I have spent much of my time at UF learning the ins and outs of astronomical instrumentation. I’ve spent months and years working on CIRCE (a near infrared instrument) in the lab at UF and at the Gran Telescopio Canarias in Spain, racking up over 6 weeks at the summit thus far. Most of my time with CIRCE has been hands-on. In addition, I’ve also learned how to design instruments using ZEMAX and SolidWorks for optics and opto-mechanical design work, respectively. (links go to the respective websites)
The GTC is currently the largest steerable telescope in the world, being precisely 10 centimeters larger than both Keck telescopes. It is 10.4 m in diameter.
Here’s a list of my hands-on experience with CIRCE (work overseen by Dr Nick Raines and Prof Steve Eikenberry, as well as Greg Bennett (mechanical engineer) and Scott Mullen (electrical engineer) and other CIRCE team members):
- Assembly, testing, trouble-shooting of cryogenic mechanisms (eg, filter wheels, mask holders) at both room temp/pressure and cryogenic conditions
- Assisted in final assembly of the instrument and construction of the thermal shielding (including about a month spent wrapping everything in aluminized Mylar)
- Built electrical cables that run from the Dewar through the cable chain to the electronics rack (~2 km of wires stretched end to end)
- Packed (and then unpacked once at GTC) the instrument on its handling cart, the electronics rack, and associated tools
- Scientific commissioning of CIRCE on-sky in March 2015 (6 half-nights of observing), including:
- Troubleshot electrical issues in our motor controller box (rewired the box with oversight from our electrical engineer)
- Worked with UF CIRCE Team + GTC staff to install new environmental cover (necessitated pulling CIRCE off of the telescope and reinstalling it)
- Planned observations of relevant scientific targets
- Near real-time data reduction of commissioning data using superFATBOY data pipeline (first beta test)
- Refurbished CIRCE in March 2016 with Prof Steve Eikenberry and Mr Alan Garner
- Replaced the half-wave plate polarizer mechanism with one designed by Mr Yigit Dallilar
- Replaced the slit and polarimetry mask
- Added grisms borrowed from the FLAMINGOS-1 instrument, giving CIRCE low-resolution spectroscopic capabilities
- Added new narrowband filters from FLAMINGOS-1 (JH and HK filters) and others
- 1/2 night of engineering/scientific commissioning to verify instrument performance and spectroscopy mode
- Passed the machine shop course offered at UF (basic milling, lathe)
Other hands-on tasks have involved:
- Lab work with SPIFFI, a speckle-stabilized imaging experiment (funding for the next-generation version (the Speckle Stabilized Science Demonstrator) although approved, was not forthcoming)
- I worked with Dr Mark Keremedjiev to better characterize the ringing of the Fast Steering Mirror and get hands-on experience with the instrument
- 3D printing and rapid prototyping using our Makerbot Replicator 2 and 2X printers
- Testing of MIRADAS probe arms (open air testing and cryogenic)
- Assisted Ms Amanda Townsend in setting up telescopes for her thesis project, a distributed light-gathering telescope made up of 4 smaller telescopes
- Assisted with printing and commissioning Pepito, a pathfinder spectrograph designed for 3D printing (as opposed to traditional machined components)
I also have a lot of design work experience with a handful of instruments.
MIRADAS (also destined to be put on the GTC) is the main instrument for which I’ve done design work. I’ve designed the macro-slicer. If you’re curious, here’s a link to a page with another link to the paper. It’s a bit of a slog, but only 15 pages with figures (lots of those!) and references. I’ve also worked on FRIDA, an imaging spectrograph for GTC, and IGIS (web page coming, TBD).
Here’s a list of my design experience.
- MIRADAS macro-slicer:
- optical design using ZEMAX: 36 slitlets sharing 18 pupil mirrors, 12 beam combiner mirrors, and 6 slicer mirrors
- opto-mechanical design using SolidWorks
- includes precision location dowel pinholes for repeatable mechanical location (‘automatic’ optical alignment)
- includes tool marks from machining mirrors
- 3D printed full-size components
- IGIS (Integrated Grating Image Slicer):
- optical design in ZEMAX of 10 slices and 10 pupil mirrors (each with their grating etched into the pupil mirror)
- optomechanical design in SolidWorks of mirrors, mirror mounts, optical bench and enclosure.
- Fabrication completed in December 2016 by Durham Precision Optics, to be delivered to UF by February 2016 for further laboratory testing