Making Instruments Work

Today, I’m at the TusCon Science Fiction Convention in Tucson, Arizona. You can get all the details about the event at http://tusconscificon.com. One of the topics I’ll be speaking about is the work we’ve been doing for the last year, installing the DESI Spectrograph on the Mayall 4-meter telescope. At this point, installation is nearing completion and we’re beginning the process of commissioning the instrument. In short, we’re actually making it work with the telescope so we can get the data we hope to obtain.

DESI isn’t the only instrument that we’ve recently installed. We’ve also installed the NEID spectrograph on the WIYN telescope. While DESI has the goal of making a 3D map of about one-third of the sky, NEID has the goal of looking for planets around other stars. I’ve shared quite a bit about the DESI installation because that instrument is of a scale that it required a major refit of the telescope. The NEID spectrograph has involved a similar amount of time in development, but much of that development has happened off site at places such as Penn State University and the University of Wisconsin. NEID, which rhymes with fluid, takes its name from the Tohono O’Odham word meaning “to see.”

Two weeks ago, the port adapter, built by the University of Wisconsin, and the spectrograph, built at Penn State University, both arrived at WIYN and have been installed at the telescope. You can see the port adapter on the side of the telescope in the photo above. It’s job is to capture light coming through the telescope from a distant star and feed it into fiber optics that run downstairs to a high precision spectrograph.

The spectrograph itself lives in a clean room on the WIYN Observatory’s ground floor. To get the kind of precision needed to see planets around other stars, the temperature within the spectrograph must be carefully maintained and the spectrograph elements must be kept in the same relative position. Footsteps nearby could disturb this device. Because of that, the spectrograph itself is built inside a coffin-like housing. Once the Penn State team gets everything set up, they’ll seal up the coffin and, unless there’s a serious problem, no one will look inside again. I got to peak inside the spectrograph a few days ago and it may be my only view.

Now that the instrument is installed at the telescope, we have to make sure everything works as it should and programmers are working to make sure we have software to assure we can efficiently get the data we need. We’re starting with the port adapter itself. I point the telescope at stars and a team of scientists and engineers check the function of the various parts within the adapter to make sure they understand the alignments on the sky, which are necessary for tracking the stars. They check the tip-tilt electronics, which make sure we get as much of the star’s light as possible down the fiber, and make sure all the calibration functions work. After this, work will begin commissioning the spectrograph itself. This is a process which takes a few months to complete to assure we’re getting the performance out of this instrument that we want.

Commissioning nights are very different from normal observing nights at a telescope. On a normal observing night, it’s often me and an observer. Often the observer isn’t even at the telescope, but working from their home institution, controlling a camera on the telescope over the internet and talking to me through computer chat. On a commissioning night, I can have anywhere from five to fifteen people in the control room with me, all working on different elements of the instrument. This marks a busy and exciting time as we get these new instruments ready for service at Kitt Peak National Observatory.

Cable Wrangling

In previous posts about the DESI spectrograph being installed at Kitt Peak’s Mayall 4-meter telescope, I’ve focused on the 5000 robotic positioners at the telescope’s focal plane, which is up at the top of the telescope, and the ten spectrographs located in a climate controlled room at the telescope’s base. However, I haven’t talked a lot about how the light from the 5000 positioners gets down to those spectrographs. The light travels along optical fibers that run from the telescope’s focal plane down to the room with the spectrographs. The whole distance is roughly 40-meters (or a little less than the length of half a football field).

In the photo to the left, you can see the cables running along the front of the telescope at this angle. They’re draped over the blue horseshoe structure in the foreground. Several of the cables are draped down in the lower left-hand side of the photo. There are ten cables that run from the top of the telescope to the room with the spectrographs. Each cable contains 500 individual optical fibers. Each of these cable bundles feeds one of the spectrographs at the telescope’s base. Since each cable contains 500 optical fibers, they are heavy cables. They’re also very fragile. It would be challenging enough to run these fibers from one point to another if they could be locked down in one position. However, the telescope actually has to move, so we can look at different parts of the sky. This means these heavy, fragile cable bundles have to move too.

Before construction even began on the DESI spectrograph, engineers spent time figuring out the best way to run the cables that minimized how much they had to move. Also, there are devices called e-chains that help assure cables stay nice and neat as the telescope moves. This past week, one of the engineers snapped a photo of me helping to prepare one of the e-chains for installation. He was in a lift up near the telescope’s top and looked down at me and another one of the telescope engineers hard at work. I’m the one in the yellow hard hat.

As I mentioned earlier, these cables are both heavy and fragile. That means there’s been a lot of heavy lifting that requires a great deal of care about where we step and place the cables. We don’t want to bend them too tightly, or they could break. The upshot is that this has been exhausting work. Everyone feels wiped out at the end of the day.

Still, we see the proverbial light at the end of the tunnel, or perhaps that should be the light at the end of the fiber! Once the cables are run, we only need to install the last three spectrographs, then the system will be complete. How soon we’ll start observing with the DESI spectrograph will depend on the results of preliminary testing which has already commenced and will be finished soon after the installation is complete. That said, I am told there’s a very good chance we’ll be pointing DESI at targets on the sky in less than a month. At that point, we may start to understand more about this mysterious thing that astronomers have dubbed dark energy.

Practice Makes Perfect

I spent last week at Kitt Peak National Observatory assisting with the installation of the Dark Energy Spectrographic Instrument on the Mayall 4-meter Telescope. We spent a couple of months running the refurbished telescope through its paces on the sky with a simple commissioning camera and now it’s time to finish installing the complete instrument. As we get ready to install this complex array of 5000 robot-positioned fibers that feed ten spectrographs, I find myself thinking of the old saw “practice makes perfect.” Well, how exactly do you practice building and installing an instrument no one has built and installed before? As it turns out, there are ways to do this.

One of the major tasks this week has been “dummy” petal installation. The photo above shows a view of the 4-meter telescope from the top. We’re facing the primary mirror (which is covered with white covers that say “Danger: No Step”). In front of that, and right in front of the camera is the prime focus assembly. The 4-meter mirror focuses light into the prime focus assembly. In the old days, a camera sat there. Now there will be 5000-optical fibers aligned with objects on the sky by robot positioners. Those robot positioners are quite delicate and take up a lot of room, so a test petal has been created. The petals fit in the pie-shaped wedges you see in the photo. The dummy petal is the one with Swiss cheese, like holes. It’s carefully guided into position by the red mechanical assembly. Lasers are used to make sure the petal is positioned very carefully and put in at just the right place. Here’s what one of the real petals looks like.

The entire fiber petal sits in the silver box. The black structure on the right is the same size and shape as the Swiss cheese dummy petal. Behind that is a tightly packed array of delicate fibers. The real petal above will have to be placed precisely without breaking anything. So, in this case, we practice by creating a mockup to try out all the procedures and check that we know what we’re doing before we start installing all the really delicate, expensive instrumentation. There will be ten petals like the one in the photo above and light from their fibers will go down to ten spectrographs two floors below the telescope. We currently have six of those spectrographs installed in a clean room.

Currently, three of the spectrographs are in the lower layer of racks. Three are in the upper layer of racks. The spectrographs are where the real science happens. Light that comes down the fibers is spread apart into a literal rainbow and we can see the characteristic fingerprint of the chemical elements of the objects that each fiber in the spectrograph is pointed to.

The spectrographs and the petals remind us that practice makes perfect when you do things repeated times. We’re practicing with the dummy petal, but then we’ll install ten real petals. We’ve installed six spectrographs and we have four more to go. Each time we take another step forward, the easier the process becomes.

Of course, practice made perfect on our way to building these spectrographs in the first place. We built other, smaller fiber spectrographs and learned lessons from their construction. We’ve learned about robotics and we’ve learned lessons from other people who also work in the field by following their work.

Writing is much like this. You practice by doing. You might start with some short stories to get the hang of writing. Then you might try your hand at a novel chapter, then you’ll write another. All the while, you should keep reading to see what others are doing and have done. You’ll learn techniques as you try them out. You will likely encounter difficulties, but as you keep reading, you’ll be sensitive to those difficulties and you’ll see how others have solved them. This is just one of the ways that science has taught me to be a better writer and being a writer has taught me to be better at the science work I do.

You can learn more about my writing at http://www.davidleesummers.com

The Backbeat of the Universe

This past week, I’ve been helping to re-commission the Mayall 4-meter telescope at Kitt Peak National Observatory and commission the first components of the new DESI spectrograph that we’ve been installing. In nautical terms, you can think of this as being like a shakedown cruise. We’re making sure the telescope is primed for taking scientific data and we want to assure we’ve worked out all the kinks from the telescope sitting idle for a year while it was rebuilt. We’re also making sure the components of the new instrument work as expected.

I have mentioned in previous posts that DESI is a spectrograph fed by 5000 optical fibers, each of which can be positioned to sit on a specific target in the sky. Those 5000 fibers have not yet been installed. What we have now is more of an optical camera installed at the top of the telescope in the black “can” at the top of the picture in this post. That allows us to evaluate the image quality through the telescope and make sure the light from objects on the sky will actually fall on those 5000 fibers when they’re installed.

We also have the guider that will be used with DESI. A telescope like the 4-meter is designed to track the sky with great precision, but because it’s such a large real-world machine, imperfections always creep in, so we have a camera that watches the sky and makes fine corrections to the telescope’s pointing as it tracks the sky. The commissioning instrument we have on now, will let us put the guider through its paces.

The goal of the DESI’s five-year mission is to make a three-dimensional map of about one-third of the entire sky, by giving us not only precise positions of every object we can see in that area, but by giving us distance as well. So, how can DESI do this? It takes advantage of something cool that happened in the early universe.

Everywhere you look in the sky, as far away as we can see, which also means as far back in time as we can look, is something called the cosmic microwave background. This is the universe as it looked about 400,000 years after the Big Bang. Given that the universe is 14 billion years old, that’s a long time ago! Before the epoch of the cosmic microwave background, light was bound up and couldn’t escape. At 400,000 years, the universe had expanded enough that that light and heat could escape, but there was enough gravity to try to keep that from happening. These competing forces set up acoustic waves throughout the universe. These acoustic waves were everywhere and they collided, setting up beat frequencies. These beat frequencies helped to set up localized points of gravity which drew material inwards. In the fullness of times, those localized points would become galaxies. Here’s what the universe looked like at that time.

Image courtesy WMAP Science Team

Now here’s the cool part, because we understand acoustic theory, we can predict how far apart these localized points will be and we can look to see if galaxies tend to be distributed as you would predict from looking at these acoustic waves. In fact, they are. Galaxies today tend to be separated by factors of about 500 million light years. Statistically, they’re much more likely to be at some factor of that than say, 400 or 600 million light years.

If you know how far apart galaxies are today and you know how far apart the acoustic beats were in the primordial universe, you can use geometry to look at more distant galaxies. We used to use how far a galaxy’s chemical fingerprint was shifted toward the red end of the spectrum as a way to measure distance to those galaxies. However, that assumes you understand the rate the universe is expanding. The separation between galaxies at the same redshift, will tell you how far away they actually are without making assumptions about the way the universe expands.

I will be speaking more about this and the DESI project at two astronomy club meetings in the next month. The first presentation will be for the Astronomical Society of Las Cruces on Friday, April 26 at 7pm. The meetings are held at the Good Samaritan Village in Las Cruces, New Mexico. More information about the location is available at: https://aslc-nm.org/MonthlyMeeting.html.

My other presentation will be given to the Phoenix Astronomical Society in Phoenix, Arizona on May 9 at 7:30pm. You can find more details about the location at: http://www.pasaz.org/index.php?pageid=meetings.

Ramping up the Refit

This past week, I’ve continued my work supporting the refit of the Mayall 4-meter telescope for the upcoming DESI spectrograph. DESI is the Dark Energy Spectroscopic Instrument and it will be capable of measuring of the spectra of 5000 objects at a time. Its mission objective is to collect data to help us understand the nature of Dark Energy in the universe. We don’t yet know what Dark Energy is, all we really know is that appears to make the expansion of the universe accelerate with time. To be able to collect these 5000 spectra, the telescope needs a new top end. Indeed, the first thing I saw when I came to work on Monday morning was the old top end sitting on a flatbed trailer outside the telescope being ready to go into storage.

The Mayall 4-meter is a reflecting telescope and the primary optical component is a big 4-meter diameter mirror at the bottom. The light from that mirror is then focused at that top end and either collected by a camera sitting there at “prime focus” or a sent down to an instrument underneath the telescope using a secondary mirror. The top end held both the prime focus and the secondary mirror and could be flipped end-for-end to allow either to happen. DESI will have its 5000 fibers in a new top end and indeed, part of the reason for selecting the Mayall was to have a telescope sturdy enough to handle that large an instrument. At the moment, the telescope is missing its top end, but the new one will be installed soon. There are work platforms, which enabled people to loosen the old top end so it could be lifted out with a crane. The work platforms also keep the telescope structurally stable while there’s no top end in place.

The top end only holds part of the instrument. It will have 5000 optical fibers which may be precisely positioned onto target objects. The light from those fibers is sent along the fibers to spectrographs in an environmentally controlled room where the light will be spread out and photographed so it can be analyzed. In the dark energy survey itself, most people will be looking at the so-called redshift—how far the characteristic spectral “fingerprint” of certain chemicals shifts to the red as a result of its velocity away from us. However, those same chemical fingerprints may be used to understand properties of the objects being looked at and this data will be available to anyone who wants to use it.

Because dark energy is an exciting topic in its own right, but also because this project will be generating so much raw data that’s useful to so many astronomers, it’s a major worldwide undertaking. To break the light from the fibers into spectra will require ten spectrographs which will reside in a carefully climate-controlled room. An exciting milestone I got to watch this week, was unpacking the first of those spectrographs when it arrived from France. Below, you can see the engineers inspecting the optical elements. Note the rainbow visible on the corrector plate of the right-most optical element. That’s exactly what this device is built to do! Break the light into rainbows.

Today finds me in Phoenix, Arizona for Leprecon 44. If you’re in town, I hope you’ll drop by and check out some of the panels and workshops.