LSST: A New Telescope Concept
The "etendue" or "throughput" of a telescope is the product of the light-collecting area in square meters and the camera sky coverage in square degrees per exposure. This is shown for all optical telescopes with wide-field cameras. LSST's unique high throughput will create a paradigm shift: by scanning the sky deeply and often, LSST will provide follow-up of its own discoveries.
OBSERVING STRATEGIES
A primary goal of LSST is to detect change. Objects in the sky can vary both in brightness and in position, and searching for these changes presents design challenges. LSST must go faint fast. The ability to detect faint sources is directly proportional to the amount of light that can be captured at the source's image on the telescope focal plane. The total amount of light captured is the product of the intensity of the light and the exposure time. One can increase the intensity by harvesting light over a larger area, by building an optical system with larger aperture. When observing objects which do not change, one can go fainter by exposing for a longer time, adding up the light falling on the detector until enough is accumulated to show the object against the background brightness of the sky.
Transient objects, however, may not linger long enough for extended exposures. To catch these objects before they fade, the only recourse is increased collecting area. Long exposures also limit the amount of information which can be obtained about a transient source. A faint blip on a long exposure might have been caused by a faint, persistent source. It might equally well have come from a bright flash which lasted only a small fraction of the exposure time. To discriminate between these possibilities, exposure times must be as short as possible. Again, to go faint fast, over all of the sky, a large value for the product of telescope aperture and field-of-view is the only recourse.
Objects which move during an exposure spread their light across the image. More exposure time results in a longer, but no brighter, streak. The faster the object moves, the less time is spent at any one point in the image. The less light is accumulated, the fainter the trail. The only way faint, fast-moving objects can be seen is again to increase the intensity through increased collecting area. The objects of interest to astronomers with the fastest motion in the sky are the Near-Earth Objects, or NEOs, asteroids whose orbits carry them close to, and even into, the Earth. For some of these objects, an exposure of 15 seconds leads to trailing in the image; longer exposures will not result in detecting fainter objects. It will take two seconds to transfer the image from the camera to the image-processing computers and five more seconds to re-point the telescope whenever necessary. Taking shorter exposures means that an increasing fraction of the telescope's time is spent reading the camera or re-pointing, and not looking at the sky. 15 seconds is an appropriate compromise between making efficient use of telescope time and detecting as many transient objects as possible. Telescopes with less than LSST's throughput would require longer exposures.
These same data will be a treasure trove for breakthroughs in other areas of astronomy. Because modern detectors simply count the number of photons striking their photosensitive surface, images are represented by the number of photons detected at each point. Two or more images of the same location on the sky can be combined simply by adding these numbers pixel-by-pixel so that the result is virtually identical to that of a single, longer exposure. By "co-adding" images in this way, the ten-second exposure time required for catching transient sources in the act will not limit LSST's ability to detect very faint, persistent sources through long exposures.
