Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.mrao.cam.ac.uk/~bn204/publications/2015/1504.04877v2.pdf Äàòà èçìåíåíèÿ: Fri Mar 4 11:35:22 2016 Äàòà èíäåêñèðîâàíèÿ: Sun Apr 10 10:18:08 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: jet

S

U B M I T T E D TO

APJL

ON

9M

ARCH

2015;

AC C E P T E D O N

10 A

PRIL

2015

A Preprint typeset using LTEX style emulateapj v. 05/12/14

AN OVERVIEW OF THE 2014 ALMA LONG BASELINE CAMPAIGN
A L M A PA RT N E R S H I P, E . B . F O M A L O N T , C . V L A H A K I S , S . C O R D E R , A . R E M I JA N , D . BA R K AT S , R . L U C A S , T. R . 2 2 5 ,6 7 1 ,3 8 1 ,3 9 U N T E R , C . L . B R O G A N , Y. A S A K I , S . M AT S U S H I TA , W. R . F. D E N T , R . E . H I L L S , N . P H I L L I P S , A . M . S . R I C H A R D S , P. C OX 1,3 , R . A M E S T I C A 2 , D . B RO G U I E R E 10 , W. C OT T O N 2 , A . S . H A L E S 1,2 , R . H I R I A RT 11 , A . H I ROTA 1,5 , J . A . H O D G E 2 , C . M . V. I M P E L L I Z Z E R I 1 ,2 , J . K E R N 1 1 , R . K N E I S S L 1 ,3 , E . L I U Z Z O 1 2 , N . M A R C E L I N O 1 2 , R . M A R S O N 1 1 , A . M I G NA N O 1 2 , K . N A K A N I S H I 1 ,5 , B . N I KO L I C 8 , J . E . P E R E Z 2 , L . M . P è R E Z 11 , I . T O L E D O 1 , R . A L A D RO 3 , B . B U T L E R 2 , J . C O RT E S 1,2 , P. C O RT E S 1,2 , V. D H AWA N 11 , J . D I F R A N C E S C O 13 , D . E S PA DA 1,5 , F. G A L A R Z A 1 , D . G A R C I A - A P PA D O O 1,3 , L . G U Z M A N - R A M I R E Z 3 , E . M . H U M P H R E Y S 14 , T. J U N G 15 , S . K A M E N O 1 , 5 , R . A . L A I N G 1 4 , S . L E O N 1 , 3 , J . M A N G U M 2 , G . M A R C O N I 1 , 3 , H . N A G A I 5 , L . - A . N Y M A N 1 , 3 , M . R A D I S Z C Z 1 , J . A . RO D ñ N 3 , T. S AWA DA 1,5 , S . TA K A H A S H I 1,5 , R . P. J . T I L A N U S 16 , T. VA N K E M P E N 16 , B . V I L A V I L A RO 1,3 , L . C . WAT S O N 3 , T. W I K L I N D 1,3 , F. G U E T H 10 , K . TAT E M AT S U 5 , A . W O OT T E N 2 , A . C A S T RO - C A R R I Z O 10, E . C H A P I L L O N 10,17,18 , G . D U M A S 10 , I . D E G R E G O R I O - M O N S A LVO 1,3 , H . F R A N C K E 1 , J . G A L L A R D O 1 , J . G A R C I A 1 , S . G O N Z A L E Z 1 , J . E . H I B BA R D 2 , T. H I L L 1,3 , T. K A M I N S K I 3 , A . K A R I M 19 , M . K R I P S 10 , Y. K U RO N O 1,5 , C . L O P E Z 1 , S . M A RT I N 10 , L . M AU D 16 , F. M O R A L E S 1 , V. P I E T U 10 , K . P L A R R E 1 , G . S C H I E V E N 13 , L . T E S T I 14 , L . V I D E L A 1 , E . V I L L A R D 1,3 , N . W H Y B O R N 1,3 , M . A . Z WA A N 14 , F. A LV E S 20 , P. A N D R E A N I 14 , A . AV I S O N 9 , M . BA RTA 21 , F. B E D O S T I 12 , G . J . B E N D O 9 , F. B E RT O L D I 19 , M . B E T H E R M I N 14 , A . B I G G S 14 , J . B O I S S I E R 10 , J . B R A N D 12 , S . B U R K U T E A N 19 , V. C A S A S O L A 22 , J . C O N WAY 23 , L . C O RT E S E 24 , B . DA B ROW S K I 25 , T. A . DAV I S 26 , M . D I A Z T R I G O 14 , F. F O N TA N I 22 , R . F R A N C O - H E R NA N D E Z 27, G . F U L L E R 9 , R . G A LVA N M A D R I D 28 , A . G I A N N E T T I 19 , A . G I N S B U R G 14 , S . F. G R AV E S 8 , E . H AT Z I M I NAO G L O U 14 , M . H O G E R H E I J D E 16 , P. JAC H Y M 21 , I . J I M E N E Z S E R R A 14 , M . K A R L I C K Y 21 , P. K L A A S E N 16 , M . K R AU S 21 , D . K U N N E R I AT H 21 , C . L AG O S 14 , S . L O N G M O R E 14 , S . L E U R I N I 29 , M . M A E R C K E R 23 , B . M AG N E L L I 19 , I . M A RT I V I DA L 23 , M . M A S S A R D I 12 , A . M AU RY 31 , S . M U E H L E 19 , S . M U L L E R 29 , T. M U X L OW 9 , E . O ' G O R M A N 29 , R . PA L A D I N O 12 , D . P E T RY 14 , J . P I N E DA 20 , S . R A N DA L L 14 , J . S . R I C H E R 8 , A . RO S S E T T I 12 , A . RU S H T O N 32 , K . RY G L 12 , A . S A N C H E Z M O N G E 33 , R . S C H A A F 19 , P. S C H I L K E 33 , T. S TA N K E 14 , M . S C H M A L Z L 16 , F. S T O E H R 14 , S . U R BA N 21 , E . VA N K A M P E N 14 , W. V L E M M I N G S 23 , K . WA N G 14 , W. W I L D 14 , Y. YA N G 15 , S . I G U C H I 5 , T. H A S E G AWA 5 , M . S A I T O 5 , J . I NATA N I 5 , N . M I Z U N O 1,5 , S . A S AYA M A 5 , G . KO S U G I 5 , K . - I . M O R I TA 1,5 , K . C H I BA 5 , S . K AWA S H I M A 5 , S . K . O K U M U R A 34 , N . O H A S H I 5 , R . O G A S AWA R A 5 , S . S A K A M OT O 5 , T. N O G U C H I 5 , Y. - D . H UA N G 7 , S . - Y. L I U 7 , F. K E M P E R 7 , P. M . KO C H 7 , M . - T. C H E N 7 , Y. C H I K A DA 5 , M . H I R A M AT S U 5 , D . I O N O 5 , M . S H I M O J O 5 , S . KO M U G I 5,35 , J . K I M 15 , A . - R . LYO 15 , E . M U L L E R 5 , C . H E R R E R A 5 , R . E . M I U R A 5 , J . U E DA 5 , J . C H I B U E Z E 5,36 , Y. - N . S U 7 , A . T R E J O - C RU Z 7 , K . - S . WA N G 7 , H . K I U C H I 5 , N . U K I TA 5 , M . S U G I M OT O 1,5 , R . K AWA B E 5 , M . H AYA S H I 5 , S . M I YA M A 37,38 , P. T. P. H O 7 , N . K A I F U 5 , M . I S H I G U RO 5 , A . J . B E A S L E Y 2 , S . B H AT NAG A R 11 , J . A . B R A AT Z I I I 2 , D . G . B R I S B I N 2 , N . B RU N E T T I 2 , C . C A R I L L I 11 , J . H . C RO S S L E Y 2 , L . D ' A D DA R I O 39 , J . L . D O N OVA N M E Y E R 2 , D . T. E M E R S O N 2 , A . S . E VA N S 2,40 , P. F I S H E R 2 , K . G O L A P 11 , D . M . G R I FFI T H 2 , A . E . H A L E 2 , D . H A L S T E A D 2 , E . J . H A R DY 41,27 , M . C . H AT Z 2 , M . H O L DAWAY , R . I N D E B E T O U W 2,40 , P. R . J E W E L L 2 , A . A . K E P L E Y 2 , D . - C . K I M 2 , M . D . L AC Y 2 , A . K . L E ROY 2 , H . S . L I S Z T 2 , C . J . L O N S DA L E 2 , B . M AT T H E W S 13 , M . M C K I N N O N 2 , B . S . M A S O N 2 , G . M O E L L E N B RO C K 11 , A . M O U L L E T 2 , S . T. M Y E R S 11 , J . OT T 11 , A . B . P E C K 2 , J . P I S A N O 2 , S . J . E . R A D F O R D 42 , W. T. R A N D O L P H 2 , U . R AO V E N K ATA 11 , M . G . R AW L I N G S 2 , R . RO S E N 2 , S . L . S C H N E E 2 , K . S . S C OT T 2 , N . K . S H A R P 2 , K . S H E T H 2 , R . S . S I M O N 2 , T. T S U T S U M I 11 , S . J . W O O D 2 H
Submitted to ApJL on 9 March 2015; accepted on 10 April 2015
1 ,2 1 ,3 1 ,2 1 ,2 1 ,3 4

arXiv:1504.04877v2 [astro-ph.IM] 24 Apr 2015

ABSTRACT A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to 15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from September to late November 2014, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at 350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy. Subject headings: instrumentation: interferometers--submillimeter: general--telescopes--techniques: high angular resolution--techniques: interferometric

efomalon@nrao.edu 1 Joint ALMA Observatory, Alonso de CÑrdova 3107, Vitacura, Santiago, Chile 2 National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA, 22903, USA 3 European Southern Observatory, Alonso de CÑrdova 3107, Vitacura, Santiago, Chile 4 Institut de PlanÈtologie et d'Astrophysique de Grenoble (UMR 5274), BP 53, 38041, Grenoble Cedex 9, France 5 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 6 Institute of Space and Astronautical Science (ISAS), Japan Aerospace

Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210 Japan 7 Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 106, Taiwan 8 Astrophysics Group, Cavendish Laboratory, JJ Thomson Avenue, Cambridge, CB3 0HE, UK 9 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford, Road, Manchester M13 9PL, UK 10 IRAM, 300 rue de la piscine 38400 St Martin d'HÕres, France 11 National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801, USA


2

ALMA Partnership et al.
600 0 4000

F I G . 1 . -- Example LBC array configuration (in this case the array that was used for the 3C138 Band 6 observations in Appendix A). The black points show the nominal LBC antennas. The five antennas near the center (red points) are not part of the nominal LBC array, but were useful for measuring more extended emission (the number of these antennas varied; see Section 2 for details). The axis units are in meters.

To test the highest angular resolution capability of ALMA using baseline lengths of up to 15 km at selected frequencies, the three-month period from 2014 September to November was dedicated to carrying out the 2014 ALMA Long Baseline Campaign (LBC)43 . The approximate resolutions that can be achieved with the longest baselines are 60 mas at 100 GHz, 25 mas at 250 GHz and 17 mas at 350 GHz (but these can vary by 20% depending on the imaging parameters). The major goal of the campaign was to develop the technical capabilities and procedures needed in order to offer ALMA long baseline array configurations for future science observations. This paper presents an overview of the ALMA LBC, focusing on the technical issues affecting submm interferometry on baselines longer than a few kilometers. In §2, we describe the LBC array and campaign test strategy. §3 describes the effects of short-term phase variation due to the atmosphere and a method for determining if conditions are sufficiently stable for imaging. In §4, we discuss the systematic phase errors found between the calibrator and science target. In §5, an overview of Science Verification (SV) at long baselines is given. Images and initial science results on the SV targets are presented in three accompanying papers (ALMA Partnership et al. 2015a,b,c). An illustration of the quality of the ALMA calibration and imaging is given by a comparison of preliminary ALMA SV and Very Large Array (VLA) images of 3C138 with the same resolution (Appendix A). In §6, we present conclusions drawn from the LBC and recommendations for science observing using long baselines with ALMA.
2. LONG BASELINE CAMPAIGN OVERVIEW

2.1. The LBC Array Since many of the distant antenna pads had not been previously powered or occupied, a coordinated effort was made from April to August 2014 to prepare a sufficient number of antenna stations beyond 2 km from the array center. The con43 The LBC was led by the Extension and O (EOC) team, which includes members from the (JAO) Department of Science Operations. It was international team including members from the Centers, and the JAO expert visitor program.









INAF, Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy 13 National Research Council Herzberg Astronomy & Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada 14 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D¨ 85748 Garching bei Mnchen, Germany 15 Korea Astronomy and Space Science Institute, Daedeokdae-ro 776, Yuseong-gu, Daejeon 305-349, Korea 16 Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands 17 Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France 18 CNRS, LAB, UMR 5804, 33270 Floirac, France 19 Argelander-Institut fÝr Astronomie, UniversitÄt Bonn, Auf dem HÝgel 71, Bonn, D-53121, Germany 20 Max Planck Institute for Extraterrestial Physics, Giessenbachstr. 1, 85748 Garching, Germany 21 Astronomical Institute of the Academy of Sciences of the Czech Republic, 25165 Ondrejov, Czech Republic 22 INAF-Oss. Astrofisco di Arcetri, Florence, Italy 23 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala, Sweden 24 Centre for Astrophysics & Supercomputing, Swinburne University of Technology, Mail H30, PO Box 218, Hawthorn, VIC 3122, Australia 25 Space Radio-diagnostics Research Center, Geodesy and Land Management,University of Warmia and Mazury, Olsztyn, Poland 26 Centre for Astrophysics Research, Science & Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB, UK 27 Departamento de AstronomÌa, Universidad de Chile, Casilla 36-D, Santiago, Chile 28 Centro de RadiostronomÌa y AstrofÌsica, Universidad Nacional AutÑnoma de MÈxico, 58089 Morelia, MichoacÀn, MÈxico 29 Max-Planck-Institut fÝr Radioastronomie, Auf dem HÝgel 69, 53121 Bonn, Germany 30 Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK 31 Laboratoire AIM, CEA/DSM-CNRS-UniversitÈ Paris Diderot, IRFU/Service dAstrophysique, Saclay, F-91191 Gif-sur-Yvette, France 32 Department of Physics, Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH, UK 33 I. Physikalisches Institut, UniversitÄt zu KÆln, ZÝlpicher Str. 77, 50937, KÆln, Germany 34 Faculty of Science, Japan Women's University, 2-8-1 Mejirodai, Bunkyo-ku, Tokyo 112-8681, Japan 35 Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo 192-0015, Japan 36 Department of Physics & Astronomy, University of Nigeria, Carver Building, Nsukka 410001, Nigeria 37 National Institutes of Natural Sciences (NINS), 2F Hulic Kamiyacho Building, 4-3-13 Toranomon, Minato-ku, Tokyo, Japan 38 Hiroshima Astrophysical Science Center, Hiroshima University, 1-31 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan 39 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 40 Department of Astronomy, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903, USA 41 National Radio Astronomy Observatory, Avenida Nueva Costanera 4091, Vitacura, Santiago, Chile 42 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, 1200 E. California Blvd M/C 249-17, Pasadena, CA 91125, US A

12



1. INTRODUCTION The Atacama Large Millimeter/submillimeter Array (ALMA) is a millimeter/submillimeter (mm/submm) interferometer located in the Atacama desert of northern Chile at an elevation of about 5000 m above sea level. The high-altitude, dry site provides excellent atmospheric transmission over the frequency range 85 GHz to 900 GHz (Matsushita et al. 1999). ALMA is currently in its third year of science operations and was formally inaugurated in 2013 March. Until now, science observations have used configurations with baselines from 100 m to 1.5 km, with some limited testing of a 3-km baseline in 2013 (Asaki et al. 2014; Matsushita et al. 2014).

N/S position in m

2000 0 2000 4000 6000 000 8 6000 4000 2000 0 20 00 4000 6000 800 0



E/W position in m

ptimization of Capabilities Joint ALMA Observatory a collaborative effort by an JAO, the ALMA Regional


yewvr.ms, field 2 (3c138)
8 0 00 6000 4000 20 0 0 v (k) 0 -2000 -4000 -6000 -8000 -8000 -6000 -4000 -2000
(spw='3')

2014 ALMA Long Baseline Campaign

3

0 2 0 00 u (k)

4000

6000

8000

F I G . 2 . -- The uv distribution for the the 1-hr 3C138 Band 6 observations. The u-axis is the east-west spacing and the v-axis is the north-south spacing. The axis units are in kilo-lambda (k, where 1000 k = 1300 meters).

the phase transfer and subsequent image errors; (3) Go/noGo tests: development of an online method to determine the near real-time feasibility of long baseline observations (Section 3.2). (4) Cycle time tests: phase referencing tests with different intervals between calibrator scans; (5) Baseline determination: observations of many quasars distributed over the sky for 30 to 60 min to determine antenna positions and delay model errors; (6) Weak calibrator survey: measuring the flux density of candidate calibrators for suitability as phase reference sources; (7) Calibrator Structures: imaging of calibrators at long baselines to search for significant angular sizes; and (8) Astrometry: phase referencing among many close quasars to measure the long baseline source position accuracy. Most test observations were made at 100 GHz (ALMA Band 3). The observed phase fluctuations are associated with variations in propagation time (delays) in the ALMA system or in the atmosphere, which are also described as pathlength variations. The propagation changes are generally non-dispersive so that the phase fluctuations will scale with frequency44 (although there are significant dispersive effects in the contributions due to water vapor at some frequencies above 350 GHz; these effects can be estimated).
3. SHORT-TERM COHERENCE Imaging using phase referencing techniques requires a reasonably phase-stable array. Hence, an early goal of the LBC was to determine the short-term (5 to 60 sec) phase rms properties of ALMA over a variety of conditions. In addition to phase noise, systematic phase offsets between the science target and calibrator were found; in §4, we describe their origin and how they were minimized. One of the main contributions to phase instability at mm wavelengths is the fluctuation of the amount of water vapor in the atmosphere. The ALMA site was chosen for its low average water vapor content and excellent phase stability. Nevertheless, at baselines longer than 1 km, the short-term phase variations may make imaging impossible. A good rule of thumb is that if the rms phase variations are (rad), then the approximate loss of coherence (the decrease of the peak intensity of a point source caused by these random phase fluctuations) is exp[(- 2/2)] (Richards 2003). For = 30 or 60 the coherence is respectively 87 or 58%. Hence, a general guideline is that the loss of coherence is acceptable and reasonably accurate image quality can be obtained if the rms phase fluctuations are < 30 .

figuration process began with an initial test in late August 2014 when a single antenna was moved out to a 7 km baseline. The nominal LBC configuration consisted of 21­23 antennas on baselines of between 400 m and 15 km and was available from the end of September until mid-November 2014 (with the two longest baseline antennas being added in midOctober). In addition, typically 6­12 antennas were available on baselines less than 300 m that were useful for imaging the more extended sources (though since they were not part of the nominal LBC configuration, the number of these antennas on short spacings varied from day to day and with observing Band). Thus, the total number of antennas used during the campaign typically ranged from 22­36, depending on observing date and observing Band. An example configuration used during the campaign (in this case for the SV observations of 3C138; see Appendix A) is shown in Fig. 1. The resultant u-v coverage for a 1-hr observation of 3C138 with this array is shown in Fig. 2. 2.2. LBC Test Strategy The normal calibration mode for ALMA observing is phase referencing (Beasley & Conway 1995). Over the length of an experiment that can last for several hours, this observing mode alternates short scans of the science target and a nearby quasar that is used to calibrate the target data. Hence, the outcome of the long baseline observations depends strongly on the accuracy with which the phase measured on the calibrator can be transferred to the target. The LBC concentrated on the accuracy of this transfer by: (1) performing test observations of quasars to establish the properties of the phase coherence of the array over long baselines; (2) determining how to optimize observing strategy to achieve good imaging results; and (3) observing, calibrating, and imaging SV targets and other test targets to demonstrate the end-to-end capability of ALMA long baseline observations. Key plans for the LBC testing included: (1) Source stares: 30-min observations of a single bright source to determine the temporal phase variation statistics as a function of baseline length; (2) Short phase reference tests: alternating observations of two close sources to determine the accuracy of

3.1. WVR correction and the Spatial Structure Function To estimate the path variations associated with the water vapour component, each antenna is equipped with a Water Vapor Radiometer (WVR). The WVR is a multi-channel receiver system (Emrich et al. 2009) that makes continuous observations of the emission in the wings of the 183 GHz water line along the line of sight to the astronomical source. A description of this system, and of the way in which the measurements are used to estimate the variations in the amount of Precipitable Water Vapor (PWV)45 in the path to each antenna, is
44 A useful conversion is that a path length change of 1 mm will produce a path delay change (assuming propagation at c) of 3.3 psec. The 1 mm path length change will produce a phase change of 120 at 100 GHz (Band 3), 300 at 230 GHz (Band 6), and 420 at 340 GHz (Band 7). 45 Each mm of PWV along the line of sight will result in a path length increase of 6.5mm; Thompson et al. (2001).


4

ALMA Partnership et al. PWV < 2mm are believed to be less than 20 microns, although they can be much larger when clouds of ice or liquid water are present); see Figure 3. These residuals are thought to be mainly due to dry atmosphere (i.e. density) fluctuations (also see Section 4.2). The properties of the phase rms as a function of baseline length are important for deciding when and how to observe at long baselines. Fig. 3 shows a typical relationship of the phase rms, , as a function of baseline length, b, for a target at three stages of analysis. The - b relationship is called the Spatial Structure Function (SSF). The characteristic shape is similar for both the uncorrected data and for the WVR corrected data, except for the decrease of the variations by about 50%. For short baselines, the rms phase increases as b0.83 , indicative of a 3-D Kolmogorov spectrum (Carilli & Holdaway 1999). The slope then decreases to a 2-D Kolmogorov spectrum with dependence b0.33 at about 3 km, which is roughly the scale height of phase turbulence. This scale height is an average of the wet atmosphere and dry atmosphere scale sizes of 1 and 5 km at the ALMA site. After phase referencing, the shape of the SSF is altered, as shown by the orange points in Fig. 3. In this example, the calibrator is only 1.3 away from the target, the cycle time is 20 sec and the integration time on the calibrator is only 6 sec. Only a small fraction of calibrators are sufficiently strong, even at Band 3, to provide adequate signal-to-noise for accurate phase referencing calibration in this short integration time. Even in the ideal case of a sufficiently strong calibrator, for baselines less than 1 km, there is little decrease in the target rms after phase referencing. However, beyond a baseline of about 1 km, the target rms becomes less dependent on baseline length since the phase fluctuations with scale sizes greater than 1 km are well correlated between the target and calibrator with a 20 sec switching cycle time. 3.2. Go/noGo System At the beginning of the campaign, it was hoped that the properties of the rms phase fluctuations (both before and after WVR correction) could be predicted from measurable weather parameters such as the average PWV, PWV rms, wind speed, and pressure rms. If so, then algorithms associated with these measured conditions could be used to indicate in advance if the phase parameters are adequate for imaging at a specified frequency; namely, that the short-term phase rms would be less than about 30 for the longer baselines. This presumption, however, turned out to be not always true. A direct method to determine the current ALMA phase rms is from a short observation of a strong source. A simple observing procedure called Go/noGo was developed, consisting of a 2-min observation of a strong quasar at Band 3, followed by online data analysis that rapidly determines the SSF with the WVR correction applied. To confirm that the Go/noGo structure function phase rms (averaged over many baselines between 5 to 15 km) is well correlated with phase referencing image quality, many Go/noGo observations that were carried out during the LBC were followed by short reference observations of calibrator-target pairs, with a typical 3.5 separation and cycle time of 60 sec. The plot of the Go/noGo rms phase versus image coherence from the phase referencing experiment is shown in Fig. 4. This demonstrates that the target image coherence is reasonably well correlated with the rms phase at the longer baselines of the calibrator. The reason for the somewhat lower image coherence than expected from the

F I G . 3 . -- The spatial structure function (SSF). The phase rms a (squareroot of the SSF; converted to a path length in microns) versus baseline length is shown for a target at three stages of reduction. The experiment was 15 minutes in duration. The black points show the SSF for the original visibility data. The red points show the SSF points after applying the WVR correction for this source. The orange points show the SSF for this source after phase referencing with a calibrator that is 1.3 away from the target with a cycle time of 20 sec. The PWV during this experiment was 1.44 mm with a wind speed of 7 m/sec. a See footnote 44.

1.00 0.95 0.90

Coherence

0.85

0.80 0.75 0.70 0.650 5

rms phase (deg) for 5-15km baselines

10

15

20

25

30

35

40

45

F I G . 4 . -- Average phase rms for 5 to 15 km baselines versus coherence. The rms phase was determined from a set of Go/noGo (see Section 3.2) observations that were followed by short phase referencing experiments. The coherence is the ratio of the phase referenced image peak density divided by the self-calibrated image peak flux density. The thin red line shows the theoretical relationship between the phase rms (radians), , and coherence, i.e. exp(-2 /2) for a random phase distribution (see Section 3).

given in Nikolic et al. (2013). This WVR correction typically removes about half the short-term phase fluctuations, and increases the proportion of time that phase referencing observations will produce good quality images. Even in good conditions, however, applying a correction to the phases based on these estimates still leaves residual fluctuations that are much larger than the estimated errors (which, with clear skies and


2014 ALMA Long Baseline Campaign

5

Baseline measurements changing the atmospheric dry term
0.02

DA60: baseline: 3.5 km, height above reference: -100m

72 0

0.01

0.00

0

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

sin(Elevation)

F I G . 5 . -- The effect of the height difference delay term. The residual delay/phase for one baseline (after fitting for the best antenna positions) is plotted versus the sin(elevation) for 50 quasar scans that form a typical baseline observation. The baseline length is 3.5 km with an antenna height difference of 100 m. The red points show the residual delay/phases that used the nominal ALMA CALC delay model (Section 4.1) that assigned the measured pressure from the one weather station near the array center to both antennas. The blue points show the residual delay/phase for another baseline observation in which the estimated pressure for each antenna was estimated using the pressure lapse rate from the barometer near the array center.

rms phase variations are discussed in §4.
4. SYSTEMATIC PHASE ERRORS

In addition to the stochastic-like phase variations between the calibrator and target described in Section 3, there were systematic antenna-based phase offsets between the calibrator and science target that persisted on timescales of many minutes to hours. These were found to be caused mostly by errors in the correlator delay model. The offsets were found to scale roughly as the calibrator-target separation, but were nearly unaffected by the cycle time. Such systematic offsets can have serious impact on the target image quality because they are persistant and produce image artifacts (e.g. large side-lobes and spurious faint components), in addition to the blurring of the target image that is associated with short-term phase fluctuations. 4.1. The Delay Model The signals from all antennas must be combined precisely in phase at correlation to obtain accurate visibility phases. A critical part of the ALMA online control software, called the delay server, calculates the expected relative delay of the signals between each antenna from the ALMA array parameters (Marson et al. 2008). If the delay model (DM; which is calculated using the CALC46 third-party software) is accurate, the visibility phase for any point-like quasar with known position should be constant with time and independent of the quasar's position in the sky. An important part of the DM is the estimate of the differential tropospheric delay between each antenna from the source. As described above, the wet delay component is calculated from the 183 GHz emission assuming a model temperature profile, and is included in the DM using the WVR measurement. The zenith dry air delay i above antenna i is accurately given by i 0.228Pi where Pi is the dry pressure in mbars
46

at the antenna (Thompson et al. 2001). For an observation of a target at elevation e, the CALC model delay is (i )/sin(e). Given that only one weather station near the array center had so far been available at the time of the LBC47 at ALMA, the estimate of the dry air delay at each antenna is not as accurate as desired. This inaccuracy results in antenna-based phase offsets that differ between a calibrator and science target and hence produce relatively constant phase offsets between them. 4.2. Measurement of Delay Model Errors The presence of DM errors was suspected from the baseline observations that consisted of about 50 to 100 ten-second quasar observations distributed over the sky48 . Many such observations have been made in order to determine the accurate relative positions of the antennas which are frequently moved from one antenna pad to another as the ALMA configuration changes. The a priori antenna positions are usually more than 1 mm in error, so the baseline observations provide the data needed to update antenna positions, generally to an accuracy of about 50 microns. Over a few years, it was found that the measured position changes of fixed antennas between baseline calibration observations, separated by several hours to a few weeks, were often larger than 100 microns and sometimes well over 1 mm for unmoved antennas that were more than 1 km from the array center. These apparent antenna position changes were traced to the implementation of the dry air delay term in the CALC DM. Fig. 5 illustrates the results of an experiment on 2014 September 16 with two 30 min baseline observations which confirmed the DM error for a 3.5 km baseline with a height difference of 100 m between the two antennas. One experiment used the DM in which the pressure at each antenna was set equal to that measured by the one sensor. After fitting for

http://lacerta.gsfc.nasa.gov/mk5/help/calc_01.txt