by Kevin Reil
The Dark Energy Camera (DECam) is a 570-megapixel camera installed on the 4-meter Victor Blanco telescope atop Cerro Tololo, a mountain in the Chilean Andes. The science mission for the Dark Energy Survey, of which I’m a member, is nothing less than to use this camera to understand Dark Energy. Which is a tall order, since the phrase “Dark Energy” itself is, as some cosmologists say, simply words we use to describe our profound ignorance about the current-day accelerating expansion of the universe. Though this accelerating expansion was a theoretical possibility and predicted by a minority of astrophysicists in the later part of the last century, its actual confirmation through observations of a certain type of supernova (called a Type Ia supernova) in 1998 came as a major surprise to the majority of the community (and larger world). And has continued to puzzle the entire community of astronomers and physicists ever since, with mysteries such as how we square the paradigm of an ever-expanding Universe with a specific start time for it in the past. Thus, we forge ever onward, observing more of the Universe steadily, in the hope that probing further will elucidate the why and wherefore of this most recent profound cosmological mystery.
DECam is one of the tools we use in this grand endeavor, and it first opened its camera shutter in September 2012 and has thus far completed three of its planned five seasons of observation. During the five years of the survey, the instrument will collect information on millions of galaxies and look for and study thousands of supernovae.
We published a paper last year about the details of the camera’s construction and operation which was rather aptly titled, “The Dark Energy Camera.” My favorite part of the paper is really the author list. It summarizes a rule called STP that I know best from the volunteer world. It stands for the “Same Ten People” who show up every time and make sure things happen. For an instrument like DECam that number is actually slightly over 100 people (so we’ll alter the rule to “SHP”) but the idea remains the same: If you want to do great science, one way is to surround yourself with people like those listed as authors on this paper, then do the best work you can as part of an excellent team.
The work done by KIPAC faculty member Aaron Roodman and myself is covered in section 7.3.3 of the paper, “Active Optics System (AOS).” The DECam’s AOS is not to be confused with the adaptive optics system the Gemini Planet Imager, another instrument with a lot of KIPAC involvement, uses. The DECam’s active optics corrects focus and alignment between exposures while the Gemini Planet Imager’s adaptive optics makes correction while exposures are being taken.
The AOS is just one small piece of a very complex system, but a rather critical one. It keeps the camera in focus and optically aligned automatically, without any manual intervention. This allows observers to focus their attention on taking data. The AOS works by using eight wavefront sensors to continuously correct the camera’s focus and alignment and needs no human intervention. With it, instead of the camera drifting out of focus by hundreds of microns through the night, the system quickly and automatically moves to better than 30 microns defocus, and the shapes of the images remain very stable. (See the image below.)
The most immediate benefit of the AOS is survey efficiency: no fiddling with the focus saves time, and time equals money in the calculus of research, as in so many other areas. DES should save several weeks of time over its five-year mission due to this system.
All of DECam’s science benefits from sharp, well-focused images, but of the four pillars of dark energy science being investigated by DES—analyses involving: Type Ia supernovae, the distribution of galaxy clusters, baryon acoustic oscillations, and weak gravitational lensing—weak lensing science likely benefits the most because weak lensing effects are very subtle. In weak lensing, we study tiny alterations in the shapes of distant galaxies caused by the paths of their light rays being deflected by all the mass in the Universe lying between them and us. Correcting focus right between actual exposures minimizes the systematic changes in shapes introduced by the camera, keeping them consistent and allowing us to measure the weak lensing effect.
The system is working well; a few years of effort and several months on the mountain (Cerro Tololo) are captured in the summary sentence of our section: “Closed loop operation [of the automatic focus and alignment] was made the default condition for all observers at the end of DES SV [Science Verification time, in early 2013], and it has remained in stable problem-free operation since that time.” Phew! A huge lot of hard work is summarized in those few words, and for us it meant mission accomplished—at least for that stage of the experiment!
But the AOS is just one piece of DECam; a collection of really smart, dedicated people, each contributing a little piece like this, is necessary for science to work. That’s what I love about it. So you don’t have to read our entire paper, but I encourage you to look at the list of authors and think about all the other contributions that went into building a state-of-the-art scientific camera.