LSST: A New Telescope Concept
"If we knew what the discoveries were likely to be, it would make no sense to build such a telescope."
Engineering concept for the LSST telescope. The folded optical path allows a compact, stiff, and stable optical support structure.
The large mirrors, 8.4 m diameter primary, 3.4 m secondary, and 5.0 m tertiary. The optical design has been optimized and the primary and tertiary mirror surfaces are being fabricated in a single monolithic mirror blank. Light enters the LSST design and reflects off of the 8.4-meter primary mirror back up toward the front of the telescope. There it is reflected by a 3.4-meter secondary mirror down to the third, 5.2-meter mirror. The light reflected by this tertiary mirror then goes to the camera.
The Large Synoptic Survey Telescope (LSST), has been designed to overcome many of these difficulties. It will open up the "time domain" to astronomy by mapping the entire sky deeply, rapidly, and continuously. It will provide all-sky maps of unprecedented depth and detail, and keep doing so frequently and for years to come. By providing immediate public access to all the data it obtains, it will provide everyone, the professional and the "just curious" alike, a deep and frequent window on the entire sky. Cosmic cartography will become cosmic cinematography, forever changing the way we view the heavens.
This change will be much like the paradigm shift of predicting the weather from a single ground-based station to a geosynchronous satellite. LSST will change the way we observe the sky. Rather than using other telescopes to follow up its discoveries, LSST, with its unique capability of frequent, deep imaging of the entire visible sky, will provide its own follow-up.
Whenever we look upon the world in a new way, it reminds us of its inexhaustible richness. Some new discoveries lead immediately to better understanding, while others provide hints of new wonders to explore. In 1928 George Ellery Hale proposed to build a new telescope with the unprecedented aperture of 200 inches. A member of the Rockefeller Foundation's International Education Board asked Hale: "What discoveries will you make?" Hale answered: "If we knew what the discoveries were likely to be, it would make no sense to build such a telescope."
Serendipity is the life blood of science, but we must plan for serendipity, building new ways to encourage it and preparing ourselves to recognize it when it appears. LSST will make the unusual commonplace and the singular observable. The greatest advances to come from LSST are thus almost surely unanticipated.
Such new capabilities are made possible by the confluence of several technological developments. New fabrication techniques for large optics developed for the most recent generation of large telescopes can be extended to novel optical designs which allow large fields of view. New detector technologies allow the construction of cameras which can capture these wide-angle images on focal planes paved with billions of high-sensitivity pixels (picture elements). Recent phenomenal advances in microelectronics and data storage technologies provide greatly enhanced facilities for digital computation, storage, and communication, and new software innovations enable fast and efficient searches of billions of megabytes of data.
It is now possible to fabricate large mirrors of very deep curvature accurately and inexpensively. Large mirrors collect more light, enabling detection of fainter sources. Deep curvature brings light to a focus only a short distance above the mirror's surface. This short focal length significantly decreases the overall length of the telescope. A shorter telescope is lighter, stiffer, and thus more resistant to image-blurring vibration. It is also less expensive to construct and fits in a smaller building, further reducing costs.
With its folded optical design, the 8.4-meter LSST (left) fits easily into the low-cost dome housing the 6.4-meter Magellan Telescope at Las Campanas, Chile (right).
LSST's innovative science relies upon another aspect of short focal lengths well known to photographers; for a given image size, shorter focal lengths provide wider fields of view. Combining a large diameter with a wide field leads to an optimal design for surveying the cosmos. With the effective light-collecting area of a telescope seven meters in diameter, LSST will have a field of view encompassing ten square degrees of sky, roughly 50 times the area of the full moon. Such a field is over a thousand times that of existing large telescopes, yet the light-gathering capability will be among the largest in the world. This wide field will be achieved through a three-mirror design; light gathered by the 8.4-meter primary mirror will be reflected back up to a 3.4-meter convex secondary mirror, and down to a 5.2-meter tertiary mirror before being directed upward again to a camera at the center of the secondary. This triple-folded design is even more compact than traditional telescopes; the 8.4-meter LSST would fit comfortably in an enclosure like that of the 6.5-meter Magellan telescopes in Chile.
