DESI Naked!

This weekend finds me at Bubonicon 51 in Albuquerque, New Mexico. I’m moderating panels about space cowboys and large scale scientific surveys. If you’re in town, click the link to get the details and drop by. I’d love to see you there. Of course, part of my interest in large scale scientific surveys has to do with the work I’ve been helping with over the last year and a half, installing the DESI Spectrograph at the Mayall 4-meter telescope at Kitt Peak National Observatory. During my my recent shift at the observatory, I got a rare look at the new instrument not just “under the hood” but before the hood even went on.

In the photo above, you see DESI on the left, just over the orange platform. Standing on the ground floor in the foreground are just a few of the telescope engineers and technicians who have been installing this new, innovative instrument which will be used to make a 3D map of about a third of the known universe. DESI itself is an array of 5000 fibers mounted on robot positioners that can be precisely centered on targets each time the telescope moves. The light from those objects then travels down fibers two stories below. The fiber bundles are ready to be run along the telescope. You see them coiled up on the white carts to the lower right of the photo above. Each black cable contains 500 fibers. One of my jobs this week was labeling those cables so people can keep it straight which cable is which as they run them along the telescope.

Here are all the DESI fiber positioners mounted to the telescope. You can see each of the ten cables coming up into ten sets of fiber positioner “petals.” Each of these petals was installed into the telescope with great care about a month ago. Light was placed on all the fibers and it was confirmed that in all the transportation and installation, none of the fibers were broken. All of them transmit light as expected! This week, the control electronics are being wired up and routed through the telescope. Once this chore is complete, more testing will happen to assure that the fibers still transmit light and each of the robot positioners moves as expected using the electronics routed through the telescope.

All of those fibers will eventually come into a clean room downstairs to a series of ten spectrographs. Do you begin to sense a pattern? Ten petals, ten cables, ten spectrographs. As it turns out, another job of mine this week was helping to install the seventh spectrograph, which you see in the lower right of the photo above. Western fan that I am, I feel like you can now cue Elmer Bernstein’s score for The Magnificent Seven. Of course, that won’t last long. soon we’ll have an eighth, ninth, and tenth spectrograph.

Each of those spectrographs will be used to examine the light from 500 fibers. To make the map, we’ll be using these spectrographs to see how far characteristic chemical lines in spectra have shifted from where they normally sit within the rainbow toward the red end, which is one measure of how far away those objects are. We’ll compare that to statistics of how far apart they are, which turns out to be another measure of how far away they are. That said, just because we’re mostly looking for the redshifts, there will be all kinds of other spectral data available that can tell astronomers all kinds of information about properties of galaxies all over the sky. One of the most exciting things about the DESI program is that this data will be available to all. In this post, I may be laying DESI bare for all to see, but the whole project will be laying much of the universe bare, and in the process expanding the body of astronomical knowledge.

  • For a fictional and frightening look behind the scenes at an astronomical observatory, read The Astronomer’s Crypt.
  • To take a tour through the wonders of the solar system, read The Solar Sea.
  • To travel back in time to the Old West, check out Owl Dance!

The Biggest Explosions of All

I’ve spent a lot of time in my astronomy career pointing telescopes at some of the biggest explosions of all—type 1a supernovae. This kind of supernova starts with a white dwarf star and another star orbiting each other. White dwarfs are very dense stars at the end of their lives. The only objects more dense are neutron stars and black holes. The white dwarf’s gravity draws material off the companion star until it reaches critical mass and the whole thing explodes. One such star that I had the chance to observe in detail in was Supernova 2011fe in the galaxy M101. Here’s an image from the Mayall 4-meter at Kitt Peak, one of the telescopes I used to observe this object. The supernova is the bright blue star outshining everything else in the upper right-hand part of the image.

Image by T.A. Rector (University of Alaska Anchorage), H. Schweiker & S. Pakzad NOAO/AURA/NSF

These cosmic explosions are pretty interesting in their own right. Our own star is expected to end its life as a white dwarf and these explosions give us a glimpse at the hearts of these stellar corpses. These explosions are also one of the ways heavy elements formed in the cores of stars get distributed out into the universe. Supernova 2011fe was, in fact, one of the closest Type 1a supernovae we’ve ever observed. We caught the explosion soon after it happened, watched the supernova brighten to maximum, then start to fade away.

Type 1a supernovae also have another useful property. Because white dwarfs have a fairly uniform mass, the brightness of the explosion is also uniform. So, if every Type 1a supernova observed were placed at the same distance away from you, they would all, more or less, be the same brightness. This means that by measuring the apparent brightness of the supernova, you can figure out how far away it is. This is a bit of an oversimplification, but there are ways to calibrate that information based on the how fast the explosion brightens and fades.

Back in the 1990s, an astronomer named Saul Perlmutter was granted target-of-opportunity time on Kitt Peak telescopes. In this case, it meant if a type 1a supernova went off, he could ask the telescope to point to it and take an image and calibration data. He and his colleagues hoped to get distances to as many galaxies as possible. I helped acquire some of that data which was combined with a lot of other data from a lot of telescopes to provide evidence that the expansion of the universe is accelerating. Perlmutter would go on to share a Nobel prize with Adam Riess and Brian P. Schmidt for this work.

This is one of those discoveries that shows some of the true fun of science. We learned that the expansion of the universe is accelerating, but that raises an even bigger question. Why is it accelerating? Typically that’s attributed to something called “Dark Energy.” This attribution isn’t meant to be an answer in itself. It’s meant to be a placeholder. It’s “Dark” energy because we don’t know precisely what kind of energy it really is, or even if it’s energy at all! Later this year, a new instrument called DESI will be installed on the Mayall 4-meter which will endeavor to get answers to some of those questions. But like all good science, I expect a veritable explosion of new questions raised for every answer we’re able to get.