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  • Proposal to join LSST at LUPM

    The Large Synoptic Survey Telescope (LSST) is a wide-field, ground-based telescope, designed to image a substantial fraction of the sky in six optical bands every few nights. It is planned to operate for a decade, starting in 2021, allowing the stacked images to detect galaxies to redshifts well beyond unity. The LSST is designed to achieve a very broad science roadmap, that can be articulated around four major science themes:

    • Probing Dark Energy and Dark Matter;

    • Taking an inventory of the Solar system;

    • Exploring the transient optical sky;

    • Mapping the Milky Way.

    This document outlines the current work plan that the LSST team at LUPM is putting forward to officially join the LSST project and efficiently start their scientific and technical activities. It is intended to serve as a support to the Scientific Council of LUPM, in preparation to the review on October 1-2 2014. In the first part of the document, we summarize the LSST project, the Dark Energy Science Collaboration (DESC), and the LSST-France activities. The second part shortly introduces the LUPM team, and we devote the third section to a presentation of the LSST project and activities at LUPM.

    The LSST project


    The LSST Corporation was formed in 2003, as a non-profit organization aiming to build an 8 meter class telescope, able to serve as a next-generation cosmological instrument. It is a public-private partnership with the National Science Foundation (NSF) and the Department of Energy (DOE) supporting the design, development, and construction of the project. In particular, the DOE, through its contracts with national laboratories like SLAC, BNL, and LLNL, has a strong contribution to the design and construction of the LSST camera. The DOE also supports the LSST scientific program that directly relates to the Dark Energy and Dark Matter enigmas of modern science. In this section, we present the LSST instrument, the DESC international collaboration, and the LSST-France structure and activities.

    Description of the LSST

    The LSST will be constructed on El Peñón Peak, in the Cerro Pachón mountains of the northern Chilean Andes, at an altitude of 2680 meters. This site benefits from the weather data taken over more than 10 years at the Cerro Tololo Inter-American Observatory (CTIO), located 10 km away. These data show that more than 80% of the nights are usable with excellent atmospheric conditions. Furthermore, Cerro Pachón is also home of the Gemini South and SOAR telescopes, which have confirmed the site performance, and provide extensive infrastructure eventually usable by the LSST both at the summit and at the Base Facility, located 57 km away at La Serena. The telescope design is a three-mirror anastigmat, with the primary mirror reaching a diameter size of 8.4 meters (6.5 meter effective radius). All three mirrors will be actively supported to control wavefront distortions introduced by gravity and environmental stresses on the telescope. The dome that contains the telescope has been designed to reduce dome seeing and to maintain a uniform thermal environment over the course of the night.

    The LSST camera contains a 3.2-gigapixel focal plane array comprised of 189 4096x4096 CCD sensors with 10 μm pixels. The focal plane is 0.64 m in diameter, and covers 9.6 deg\(^2\) field-of-view with a plate scale of 0.2 arcsec/pixel. The CCD sensors are deep depletion, back-illuminated devices with a highly segmented architecture, 16 channels each, that enable the entire array to be read out in two seconds. The detectors are grouped into 3x3 arrays, all identical and each containing its own dedicated front-end and back-end electronics board, which fits within the footprint of its sensors, thus serving as a 144-Megapixel camera on its own. The CCDs will be maintained at an operating temperature of −100\(^°\)C. The grid also contains two guide sensors and a wavefront sensor positioned at each of the four corners at the edge of the field. The camera body also contains a mechanical shutter and a filter exchange system holding five large optical filters, any of which can be inserted into the camera field of view for a given exposure. A sixth filter can replace any of these five via an automated procedure accomplished during daylight hours. The LSST filter choice (u, g, r, i, z, y) is modeled on the system used for the SDSS, which covers the available wavelength range with roughly logarithmic spacing while avoiding the strongest telluric emission features and sampling the Balmer break. The observation strategy is entirely based on a “universal cadence” that scans the sky “

    Finally, with its planned wide, deep, and fast survey, the LSST will generate a formidable amount of information, which puts the computing and data archive model to unprecedented challenges. For instance, one night of observation will produce about 1.5 TB of data, about 2 billion objects will be routinely monitored for photometric and astrometric change, and online processing needs to guarantee that transient events will be posted in less than 60 seconds to the worldwide community. In ten years of survey, the LSST database is expected to serve information for about 20 billion objects, and will grow in size to  30 PB eventually.