by Ken Patton
April 2012: I’m headed to the airport early one Wednesday afternoon. Double check to make sure I have my bags, backpack, and really the one most important thing I need: my passport. This is going to be my first trip to Chile where the telescope for the Dark Energy Survey (DES) is located. I’m a graduate student at Ohio State and I’ve been working on software for DES over the previous half year; this trip is intended to be a ‘mock’ observing run to test the software and hardware on our telescope. And luckily the project is also giving me the first ever opportunity to travel to a destination requiring my passport. I’m flying into La Serena, a town located about an hour and a half drive from the Cerro Tololo Inter-American Observatory (CTIO) where the telescope is located.
The Dark Energy Survey is meant to study the nature of dark energy, a form of energy with negative pressure that is causing the universe to expand at an increasingly faster rate over time. The survey will take images in several different colors over one eighth of the sky, observing over 300 million faint distant galaxies. From statistical properties of the galaxies we can infer the expansion history of the universe. For the ‘mock’ observing run, however, our camera is not yet mounted on the actual telescope; this means we are primarily testing the system without the actual ability to image the sky.
As we are still performing a good deal of engineering work we expected many of the components to have minor issues here or there. However, within the first few days we got derailed by one thing we were not expecting: the cooling system of the telescope dome. It is somewhat equivalent to air conditioning units but it circulates a mixture of water and antifreeze to cool various components inside the dome. It’s an established system that is in use many places, so we did not expect to run into issues with it. This caused a slight panic at first since without the building cooling system we would not be able to cool down the camera for the telescope, which operates at -100 degrees Celsius.
As most experimentalists know, it’s often the little things that have the potential to derail your project. We began to develop a fallback plan for our tests, but fortunately within a couple of days they were able to getting the cooling system back up and running satisfactorily. This allowed us to fully test the software by pulling data off the camera, processing it into images, and sending the data back to Fermilab (the national lab near Chicago hosting our data). Overall the ‘mock’ observing run ended as a success, and it gave us useful information on which systems, such as the building cooling, needed improvements over the next few months before we actually intended to observe the sky.
Returning to Ohio it was back to the daily grind. Unfortunately, most days in the life of a scientist are not always glamorous. Much of your time is spent building, troubleshooting, and debugging rather than analyzing or collecting data. But we live for the interesting events. Final results from an experiment. Publishing a paper. Travelling to conferences, meetings, or in some cases, to remote observing locations.
Most of my time prior to and after the ‘mock’ observing run was dedicated to working on software for the telescope. Before the telescope was being used to observe the sky, a large fraction of the effort in the collaboration was focused on getting all the systems ready. Nonetheless, we still made time for collaboration meetings to discuss the goals of our dark energy science, mostly through constraining the evolution history of the universe. To do this we have four different probes of cosmology: supernova, baryon acoustic oscillations, galaxy clusters, and gravitational lensing (you can read more about these probes here: http://www.darkenergysurvey.org/science/).
Then in early September we had first light. Wooooooooo! It was exciting seeing the first few images of the sky come off the camera because of how much work we had put into the project at that point. In the very first images all of the stars and galaxies looked like large donuts- a sign the system was out of focus. This is pretty much what we expected since we had not gone through the process of calibration yet; in particular we needed to determine the optimal distance from the correcting lenses in the telescope to the focal plane. So that night the telescope operators stepped through offsets based on the first images to focus it and voila, stars and galaxies started to appear. We had real data with which we could begin to do science.
Since then we’ve progressed into a phase of ‘science verification’ before the official start of the survey. In this phase we staff the telescope with regular observing shifts and collect data much like we would for the full survey, but over a much smaller area of the sky with the intention of analyzing the data on a quicker time scale. A typical observing shift consists of four scientists for DES: two observers (in case one falls asleep!), a software expert (often done remotely), and a run manager. The run manager is the one who plans the observations for the night. The observers then actually run the telescope, telling it where to point and verifying the correct images are being taken. And the software expert is just on call in case system issues arise.
Eight months after my ‘mock’ observing run I got to return to the telescope once more for a real observing shift. It was a bit more intense this time around because the telescope had to be run at night (surprise!) unlike the first trip. For this trip I mostly operated as an observer while also providing local software support based on my experience with the code.
During the course of the survey many scientists will travel down to CTIO in order to rotate through week-long observing shifts. We have not officially started the full survey, but are currently finishing up the phase of science verification. When this phase is complete we should be able to publish preliminary results confirming our ability to perform the dark energy science we set out over the next five years. You can follow our progress and see more images here: http://www.darkenergysurvey.org
About Ken Patton
I am originally a Columbus native, but I did my undergraduate degree at Swarthmore College near Philadelphia and then worked in Washington D.C. for three years before returning to The Ohio State University to pursue my PhD in Physics. In my free time I play a lot of soccer, both on recreational teams and in various pick-up games with friends. I got into astrophysics and cosmology after a professor once told me that if I had an interest in general relativity, condensed matter may not be the right area for me.