First M87 Event Horizon Telescope Results. II. Array and Instrumentation
Authors:   The Event Horizon Telescope Collaboration, Akiyama Kazunori , Alberdi Antxon , Alef Walter , Asada Keiichi , Azulay Rebecca , Baczko Anne-Kathrin , Ball David , Baloković Mislav , Barrett John , Bintley Dan , Blackburn Lindy , Boland Wilfred , Bouman Katherine L. , Bower Geoffrey C. , Bremer Michael , Brinkerink Christiaan D. , Brissenden Roger , Britzen Silke , Broderick Avery E. , Broguiere Dominique , Bronzwaer Thomas , Byun Do-Young , Carlstrom John E. , Chael Andrew , Chan Chi-kwan , Chatterjee Shami , Chatterjee Koushik , Chen Ming-Tang , Chen 陈 Yongjun 永军 , Cho Ilje , Christian Pierre , Conway John E. , Cordes James M. , Crew Geoffrey B. , Cui Yuzhu , Davelaar Jordy , De Laurentis Mariafelicia , Deane Roger , Dempsey Jessica , Desvignes Gregory , Dexter Jason , Doeleman Sheperd S. , Eatough Ralph P. , Falcke Heino , Fish Vincent L. , Fomalont Ed , Fraga-Encinas Raquel , Friberg Per , Fromm Christian M. , Gómez José L. , Galison Peter , Gammie Charles F. , García Roberto , Gentaz Olivier , Georgiev Boris , Goddi Ciriaco , Gold Roman , Gu 顾 Minfeng 敏峰 , Gurwell Mark , Hada Kazuhiro , Hecht Michael H. , Hesper Ronald , Ho 何 Luis C. 子山 , Ho Paul , Honma Mareki , Huang Chih-Wei L. , Huang 黄 Lei 磊 , Hughes David H. , Ikeda Shiro , Inoue Makoto , Issaoun Sara , James David J. , Jannuzi Buell T. , Janssen Michael , Jeter Britton , Jiang 江 Wu 悟 , Johnson Michael D. , Jorstad Svetlana , Jung Taehyun , Karami Mansour , Karuppusamy Ramesh , Kawashima Tomohisa , Keating Garrett K. , Kettenis Mark , Kim Jae-Young , Kim Junhan , Kim Jongsoo , Kino Motoki , Koay Jun Yi , Koch Patrick M. , Koyama Shoko , Kramer Michael , Kramer Carsten , Krichbaum Thomas P. , Kuo Cheng-Yu , Lauer Tod R. , Lee Sang-Sung , Li 李 Yan-Rong 彦荣 , Li 李 Zhiyuan 志远 , Lindqvist Michael , Liu Kuo , Liuzzo Elisabetta , Lo Wen-Ping , Lobanov Andrei P. , Loinard Laurent , Lonsdale Colin , Lu 路 Ru-Sen 如森 , MacDonald Nicholas R. , Mao 毛 Jirong 基荣 , Markoff Sera , Marrone Daniel P. , Marscher Alan P. , Martí-Vidal Iván , Matsushita Satoki , Matthews Lynn D. , Medeiros Lia , Menten Karl M. , Mizuno Yosuke , Mizuno Izumi , Moran James M. , Moriyama Kotaro , Moscibrodzka Monika , Müller Cornelia , Nagai Hiroshi , Nagar Neil M. , Nakamura Masanori , Narayan Ramesh , Narayanan Gopal , Natarajan Iniyan , Neri Roberto , Ni Chunchong , Noutsos Aristeidis , Okino Hiroki , Olivares Héctor , Ortiz-León Gisela N. , Oyama Tomoaki , Özel Feryal , Palumbo Daniel C. M. , Patel Nimesh , Pen Ue-Li , Pesce Dominic W. , Piétu Vincent , Plambeck Richard , PopStefanija Aleksandar , Porth Oliver , Prather Ben , Preciado-López Jorge A. , Psaltis Dimitrios , Pu Hung-Yi , Ramakrishnan Venkatessh , Rao Ramprasad , Rawlings Mark G. , Raymond Alexander W. , Rezzolla Luciano , Ripperda Bart , Roelofs Freek , Rogers Alan , Ros Eduardo , Rose Mel , Roshanineshat Arash , Rottmann Helge , Roy Alan L. , Ruszczyk Chet , Ryan Benjamin R. , Rygl Kazi L. J. , Sánchez Salvador , Sánchez-Arguelles David , Sasada Mahito , Savolainen Tuomas , Schloerb F. Peter , Schuster Karl-Friedrich , Shao Lijing , Shen 沈 Zhiqiang 志强 , Small Des , Sohn Bong Won , SooHoo Jason , Tazaki Fumie , Tiede Paul , Tilanus Remo P. J. , Titus Michael , Toma Kenji , Torne Pablo , Trent Tyler , Trippe Sascha , Tsuda Shuichiro , van Bemmel Ilse , van Langevelde Huib Jan , van Rossum Daniel R. , Wagner Jan , Wardle John , Weintroub Jonathan , Wex Norbert , Wharton Robert , Wielgus Maciek , Wong George N. , Wu 吴 Qingwen 庆文 , Young André , Young Ken , Younsi Ziri , Yuan 袁 Feng 峰 , Yuan 袁 Ye-Fei 业飞 , Zensus J. Anton , Zhao Guangyao , Zhao Shan-Shan , Zhu Ziyan , Algaba Juan-Carlos , Allardi Alexander , Amestica Rodrigo , Bach Uwe , Beaudoin Christopher , Benson Bradford A. , Berthold Ryan , Blanchard Jay M. , Blundell Ray , Bustamente Sandra , Cappallo Roger , Castillo-Domínguez Edgar , Chang Chih-Cheng , Chang Shu-Hao , Chang Song-Chu , Chen Chung-Chen , Chilson Ryan , Chuter Tim C. , Rosado Rodrigo Córdova , Coulson Iain M. , Crawford Thomas M. , Crowley Joseph , David John , Derome Mark , Dexter Matthew , Dornbusch Sven , Dudevoir Kevin A. , Dzib Sergio A. , Eckert Chris , Erickson Neal R. , Everett Wendeline B. , Faber Aaron , Farah Joseph R. , Fath Vernon , Folkers Thomas W. , Forbes David C. , Freund Robert , Gómez-Ruiz Arturo I. , Gale David M. , Gao Feng , Geertsema Gertie , Graham David A. , Greer Christopher H. , Grosslein Ronald , Gueth Frédéric , Halverson Nils W. , Han Chih-Chiang , Han Kuo-Chang , Hao Jinchi , Hasegawa Yutaka , Henning Jason W. , Hernández-Gómez Antonio , Herrero-Illana Rubén , Heyminck Stefan , Hirota Akihiko , Hoge James , Huang Yau-De , Impellizzeri C. M. Violette , Jiang Homin , Kamble Atish , Keisler Ryan , Kimura Kimihiro , Kono Yusuke , Kubo Derek , Kuroda John , Lacasse Richard , Laing Robert A. , Leitch Erik M. , Li Chao-Te , Lin Lupin C.-C. , Liu Ching-Tang , Liu Kuan-Yu , Lu Li-Ming , Marson Ralph G. , Martin-Cocher Pierre L. , Massingill Kyle D. , Matulonis Callie , McColl Martin P. , McWhirter Stephen R. , Messias Hugo , Meyer-Zhao Zheng , Michalik Daniel , Montaña Alfredo , Montgomerie William , Mora-Klein Matias , Muders Dirk , Nadolski Andrew , Navarro Santiago , Nguyen Chi H. , Nishioka Hiroaki , Norton Timothy , Nystrom George , Ogawa Hideo , Oshiro Peter , Oyama Tomoaki , Padin Stephen , Parsons Harriet , Paine Scott N. , Peñalver Juan , Phillips Neil M. , Poirier Michael , Pradel Nicolas , Primiani Rurik A. , Raffin Philippe A. , Rahlin Alexandra S. , Reiland George , Risacher Christopher , Ruiz Ignacio , Sáez-Madaín Alejandro F. , Sassella Remi , Schellart Pim , Shaw Paul , Silva Kevin M. , Shiokawa Hotaka , Smith David R. , Snow William , Souccar Kamal , Sousa Don , Sridharan T. K. , Srinivasan Ranjani , Stahm William , Stark Antony A. , Story Kyle , Timmer Sjoerd T. , Vertatschitsch Laura , Walther Craig , Wei Ta-Shun , Whitehorn Nathan , Whitney Alan R. , Woody David P. , Wouterloot Jan G. A. , Wright Melvin , Yamaguchi Paul , Yu Chen-Yu , Zeballos Milagros , Ziurys Lucy 
Published: April 2019

Abstract
The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of ∼1.3 mm, EHT angular resolution (λ/D) is ∼25 μas, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit s−1, exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.