An upgrade is underway for an instrument on the Magellan Baade 6.5-meter telescope at the Carnegie Institution of Washington's Las Campanas Observatory in Chile. The instrument, called IMACS (Inamori-Magellan Arial Camera & Spectrograph) will have the original CCD detectors in its camera replaced with newer technology devices. The new CCDs will provide better throughput, higher signal-to-noise, and increased resolution than the previous chips and will result in significant efficiencies in the use of telescope time. During the process of ordering the new CCDs the Observatory was made aware of a new enhancement to the detectors that would considerably increase the CCDs responsiveness at both the blue and red ends of the observable spectrum. A relatively new CCD technology called "deep-depletion" significantly improves the response at the red end and new anti-reflection coatings can be added to these devices to improve the response in the blue. This combination will add increased science and discovery capability in particular by enabling intermediate resolution (~10,000 <= R <= ~20,000) spectroscopy of the Calcium triplet and Magnesium I lines in the far-red. This capability will be used to study the chemical composition and kinematics of stars in nearby galaxies, the chemical composition of globular clusters in external galaxies, and the chemical composition of dwarf stars in the bulge of our own Milky Way Galaxy. Funds for these enhanced CCDs are being provided through a RAPID grant by NSF's Division of Astronomical Sciences.
Telescopes focus light from distant stars and galaxies; astronomers use this information to understand the nature of these basic components of the universe we live in. The vast majority of these objects are very faint, for example, stars on the outskirts of the Milky Way Galaxy we live in, and entire other galaxies like our Milky Way seen billions of light years away. For more than a hundred years the challenge of gathering enough light from these faint objects to understand their physical properties, "births" and "deaths," and their connection to our very existence, has led astronomers to build ever larger telescopes: the size of the telescope mirror determines how faint are the stars and galaxies that it can study. Building larger telescopes has become increasingly expensive – the latest generation of the cost approximately $100,000,000 apiece. For this reason, every effort is made to optimize their performance, making sure they to collect as much light as possible. Towards this end, scientists and engineers have vastly improved the method of recording the light captured by the telescope. Observations "by-eye" that dominated 19th century astronomy were replaced in the 20th century by photographic emulsions on glass plates, but even these were very inefficient -- less than 1% of the light was actually captured! A revolution occurred in the 1980’s when an electronic device made of silicon, much like the invention of the transistors that replaced vacuum tubes -– began to replace photography. These "charge-coupled devices" –- CCD detectors --- are superior to photographic plates in important ways, especially that their efficiencies reach 90%, about 100 times better than photography. In effect, replacing a photographic plate with a CCD made a telescope 100 times bigger. However, CCDs could not immediately replace photography for much of astronomy, they were initially expensive and available only in very small sizes, which meant that programs that required covering a large field of the sky could not be carried out with these superior detectors. By the 1990’s astronomers began to assemble mosaics of CCDs to make a larger detectors – this was the end of photography on large telescopes. Beginning in 1996, we at the Carnegie Observatories began construction of a giant instrument called IMACS --- Inamori-Magellan Areal Camera and Spectograph --- for the Magellan-Baade 6.5-m telescope at Carnegie’s Las Campanas Observatory in Chile. The heart of this instrument was a 2 x 4 mosaic of CCD detectors that covered approximately a 5-inch square area. The CCDs were purchased from a subsidiary of the Tektronix Corporation called SITe, at the cost of about $500,000. This "CCD camera" was the principal tool for astronomical research with IMACS when the instrument was commissioned in 2004, and it served very well to carry on unprecedented studies of, in particular, the changes in galaxies over the last 6 billion years of cosmic time, and how galaxies like the Milky Way came to their present form and rate of creating new stars. For all its success, the first IMACS camera using SITe CCDs were becoming obsolete, in particular, the efficiency of these early detectors was only about 65%. Mostly with funds from the National Science Foundation, a second camera was constructed for IMACS, this time with CCDs from E2V corporation with an efficiency of over 90%. This gain of about 50% in the power made possible even more difficult projects, including a search for the youngest typical galaxies (so-called Lyman Alpha Emitters) at a time only 1 billion years after the Big Bang, about 7% of the present age of the universe. Even with the new camera, this program still took years to complete; it found more than 100 of the faintest galaxies ever seen at the dawn of cosmic time. The NSF grant that is the subject of this report enabled an upgrade of the original camera with yet newer E2V that are even better than those of the second camera. When completed, the two different channels of IMACS will each have a state-of-the-art detector that will give astronomers a wide range of options for taking pictures of stars and galaxies, and dispersing their light for spectral analysis that teaches us about how old these objects are, how far away, how rich in the heavy chemical elements that make up planets and, of course, life itself. IMACS has become the most used instrument at the Magellan Observatory, and a renowned contributor to the worldwide effort to push the frontiers of knowledge back to the birth of our universe.
