The ALMA Project: Technical Aspects Alwyn Wootten, National Radio Astronomy Observatory, USA ALMA Technologies • Antenna -- Mechanical Engineering, Materials • Correlator -- Special purpose IC for high speed signal processing • Computing -- Non-linear imaging algorithms • Detectors -- Improving the best in the world • Remote Access -- Bringing the telescope from the 16500 Chajnantor site into the observer’s control • Photonics -- Light waves to radio waves The ALMA Antenna Mechanical Engineering at the Heart of the Array • Must maintain accuracy at 16,500 foot Llano de Chajnantor – Surface accuracy better than 20 microns – Pointing accuracy better than 0.6 arcseconds • Despite – high winds (50 percentile 6.5 m/s) – no vegetation - windblown grit and dust – annual median temperature -2.5 C (range -20 to +20 C) – pressure 55% of sea level--UV radiation 170% of sea level) • Three designs – ALMA/NA Vertex, of Santa Clara, CA much carbon fiber of a novel sort – ALMA/EU EIE, of Venice, Italy considerable carbon fiber – ALMA/JP Mitsubishi refinement of ASTE pre-prototype • Final design after 1.5yrs of tests in New Mexico The ALMA Correlator High Speed Signal Processing • Analog input at 64 x 8 x 2GHz per second digitized and transmitted at 96 Gigabits per second from each antenna • Fiber optic transmission to digital filters, then to correlator • Correlator: Achieving 1.7 x 1016 multiply and add operations per second! cross-correlates signals from 32*63=2016 pairs of antennas on 16 msec timescales; autocorrelates signals from 64 antennas on 1 msec timescales, 32 Gbyte/s output • Design offers flexibility of selection of – Bandwidth – Spectral window placement • Power requirement 150 kW. • Under construction NRAO-CDL Charlottesville for delivery to Chajnantor The ALMA Next Generation Correlator Higher Speed Signal Processing • Twice as many channels in high dispersion mode as the ALMA Correlator; more than an order of magnitude more in low dispersion mode. • Support for the Atacama Compact Array, a set of a dozen 7m antennas designed to enhance the submillimeter performance of ALMA. • Improved sensitivity through three-bit digitization in all modes rather than in the high dispersion modes only as provided by the ALMA Correlator. • Twice as many spectral windows (16) as the ALMA Correlator. • Capability for being a spectrometer for up to 5 subarrays (the ALMA Correlator handles 16). • Minimum continuum time resolution of 1 millisecond. • A research project of institutes in Europe, Japan and North America. Detectors Many Laboratories Worldwide • Radio receivers amplify weak signals, usually after mixing with a locally generated signal (LO) • Receivers will cover the entire observable submillimeter spectrum observable from Earth’s best site • Superconducting tunnel junction receivers (4K) mix and HEMT amplifiers at e.g. 4-12 GHz amplify for frequencies above ~90 GHz • 8 on each of 64 antennas--the most extensive superconducting electronic receiving system in astronomy Front End Specifications • Frequencies from 31 to 950 GHz covered in 10 bands – requires RF bandwidth up to 30% • All bands dual polarization • 8 bands use SIS mixers at 4K • Mixers separate sidebands where possible, and balanced • Highest possible sensitivity and stability – receiver noise close to quantum limit – wide detection bandwith (IF 4-12 GHz) • Highest reliability (1280 systems) • Modular design Complete Frequency Access (Picture of Chajnantor transmissioin with frequency bands superposed) Front End Concept • Ten bands, one 1 m diameter dewar with 70K, 15K and 4K stages • Each band a modular ‘cartridge’ held by flexible thermal links • All bands share focal plane, cartridges plug in from bottom, optics atop Preliminary Cartridge Design • Optics • Two mixers • IF amplifiers • Local oscillator • Cables • Mount Remote Access • Astronomers anywhere can interact with the system, and receive interim images in real time • Requires high speed communication--Chilean communications network development Photonics • LO - IR lasers beat together produce reference frequency for mixing, distributed to all antennas over fiber optics • Key technology is high frequency (>100 GHz) photodiodes--developed by NTT Japan to 300 GHz • After mixing and amplification, signal is digitized and transmitted over fiber optics to correlator