May 24, 2015

Large Survey Telescope will image the entire sky every few nights for a thousand fold increase in survey power

The Large Synoptic Survey Telescope (LSST) is a planned wide-field "survey" reflecting telescope that will photograph the entire available sky every few nights. The LSST is currently in its design and mirror-development phases. Site construction is scheduled to begin in October 2014, with engineering first light in 2019, science first light in 2021, and full operations for a ten-year survey commencing in January 2022.

The LSST will image the entire visible sky every few nights, thus capturing changes and opening up the time-domain window over an unprecedented range of timescales for billions of faint objects. Each sky patch will be visited 1000 times during the survey with a pair of exposures per visit. The LSST data will enable qualitatively new science. Billions of objects in our universe will be seen for the first time and monitored over time. Motivated by the evident scientific progress enabled by large sky surveys, multiple national reports have concluded that a dedicated ground-based wide-field imaging telescope with an effective aperture of 6-8 meters is a high priority for astronomy, physics, and planetary science over the next decade. With a thousand-fold increase in survey power in time-volume space over current facilities, LSST is likely to make unexpected discoveries.

The LSST design is unique among large telescopes (8 m-class primary mirrors) in having a very wide field of view: 3.5 degrees in diameter, or 9.6 square degrees. For comparison, both the Sun and the Moon, as seen from Earth, are 0.5 degrees across, or 0.2 square degrees. Combined with its large aperture (and thus light-collecting ability), this will give it a spectacularly large etendue of 319 m2∙degree2.

To achieve this very wide, undistorted field of view requires three mirrors, rather than the two used by most existing large telescopes: the primary mirror (M1) will be 8.4 metres (28 ft) in diameter, the secondary mirror (M2) will be 3.4 metres (11.2 ft) in diameter, and the tertiary mirror (M3), located in a large hole in the primary, will be 5.0 metres (16 ft) in diameter.

The 30 terabytes of data obtained each night will open a new window on the deep optical universe - the time domain - enabling the study of variability both in position and time. This enables control of systematics required for precision probes of dark energy. Rarely observed events will become commonplace, new and unanticipated events will be discovered, and the combination of LSST with contemporary space-based missions will provide powerful synergies

A 3.2-gigapixel prime focus digital camera will take a 15-second exposure every 20 seconds.

Allowing for maintenance, bad weather, etc., the camera is expected to take over 200,000 pictures (1.28 petabytes uncompressed) per year, far more than can be reviewed by humans. Managing and effectively data mining the enormous output of the telescope is expected to be the most technically difficult part of the project. Initial computer requirements are estimated at 100 teraflops of computing power and 15 petabytes of storage, rising as the project collects data.

Particular scientific goals of the LSST include:

* Measuring weak gravitational lensing in the deep sky to detect signatures of dark energy and dark matter.
* Mapping small objects in the Solar System, particularly near-Earth asteroids and Kuiper belt objects.
* Detecting transient optical events such as novae and supernovae.
* Mapping the Milky Way.

It is also hoped that the vast volume of data produced will lead to additional serendipitous discoveries.

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