PoS(RTS2012)034 http://pos.sissa.it/ 100–4000. In 2002 the DASI made the first detection of CMB polariza- ≈ 10000) in the 1980s and then in the late 1990s and early 2000s made ground- ∼ † ` ∗ -range ` [email protected] Speaker. CD acknowledges an STFC Advanced Fellowship and an EU Marie Curie IRG grant under the FP7. Interferometry has been acrowave background. very Interferometers provided successful the first tool constraintsangular on scales for CMB ( anisotropies measuring on small anisotropiesbreaking in measurements the of cosmic thecovering mi- CMB the power spectrum at intermediate and small angular scales tion which remains a majoralso goal made for current major and contributions futureeffect to in CMB galaxy the experiments. clusters. detection Interferometers have andIn surveying this of short review the I cover Sunyaev-Zel’dovich the key (SZ) ments aspects and that made summarise interferometry some well-suited of to CMB theand measure- central in observations particular that to have HI been intensity made.of mapping interferometry. I at look high to redshifts the that future could make use of the advantages ∗ † Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

Resolving the Sky - Radio Interferometry:April Past, 17-20, Present 2012 and Future -RTS2012 Manchester, UK Clive Dickinson Jodrell Bank Centre for Astrophysics, AlanUniversity Turing Building, of School Manchester, Oxford of Road, Physics Manchester, & M13E-mail: Astronomy, 9PL, The U.K. CMB interferometry PoS(RTS2012)034 2 2 3 3 4 4 4 6 6 , in 2 (1.1) | ) u ( V | , since it is ` C Clive Dickinson 100). The multipole & ` -plane which corresponds v , given by the van Cittert- , ) y u , convolved with the window ) , u x vy ( π + I 2 ux ( i = π ` 2 ]. e ) at 13 y ` , x C ( I ) y , , are samples of the x ) 2 ( v , u ( V dudvA Z ) = v , u ( V , gives a (noise-biased) estimate of is the primary beam response of the antennae. Essentially, the visibilities are u ) y , resolution can be improved by mosaicing [ x ( ` A . This is accurate in the flat-field approximation (typically SZ interferometers Interferometry basics Advantages of CMB interferometry Disadvantages of CMB interferometry Early measurements at high resolution The era of the CMB interferometers W where The primary cosmological quantity of interest is the CMB power spectrum, An interferometer measures the complex correlation between pairs of antennas. In the flat-sky 1.1 1.2 1.3 2.1 2.2 2.3 . Summary of observations . The future . CMB interferometry known that the CMB anisotropiesthe are FT very (or close more to stricty,power Gaussian. the spectrum spherical The almost transform) power of directly. spectrum theshells is Essentially, sky, of essentially and averaging constant thus the interferometers squarefunction measure of the the visibilities, window function is determined by the FTalthough of the the primary beam (the aperture illumination function), samples of the FT of skythen distribution be multiplied inverted by to the produce primary images beam of function. the The sky visibilities distribution can convolved with the synthesized beam. approximation, the observed “visibilities”, 1. CMB interferometry 1.1 Interferometry basics 2 3 Contents 1 CMB interferometry to the Fourier Transform (FT) of the sky intensity distribution, Zernike equation: PoS(RTS2012)034 ] 29 (1.2) (1.3) Clive Dickinson is the longest baseline. The . For measuring diffuse (ex- 1 d , of the array, as defined by f is then given approximately by S 2 . In fact, fringe-rate filtering can be put to ) 1 Ω d 2 b / λ noise), which is a major problem for total-power f D kT ( 3 2 f N / = = and flux density S f sampling which is known to high accuracy. b is the dish diameter, and T v , u D . Interferometers are inherently more stable than total-power receivers 1 1 and is the optimal that can be achieved. This is one of the principal and the largest angular scales (larger than the primary beam) are essentially ≈ 2 f 1) ≈ f synthesised beam because of resolution effects i.e. loss of flux on large angular scales. is the number of antennas, N >> DASI and CBI used co-mounted dishes which did not suffer from the effects of shadowing and achieved very high Strictly, the DASI/CBI interferometers could not utilise fringe-rate filtering due to their non-tracking antennas, and There are however some drawbacks, as summarised in Table Interferometers have a number of key advantages compared to total-power experiments; these Another key advantage is the attenuation of non-astronomical signals, which have different 2 1 filling factors ( had to use differencing techniques to remove local correlated signals. tended) sources, such as theJy/unit CMB, area) it that is matters, the rather surfacethe than brightness dishes point is sensitivity not source (e.g. the sensitivity in critical (in quantity, units units but Kelvin, rather Jy). or the filling Therefore, factor, the size of impossible to measure. For thescales same reasons, aperture synthesis maps are difficult to quantify on For a single dish, drawbacks for interferometers, anddetector this arrays is are why now total-power preferred.which (single The are dish) other inherently drawback experiments is difficult with inresults with large measuring in an shadowing the interferometer. largest angular scales, They require very short baselines, which where for a demonstration). Accurateparticularly in calibration the is presence of crucial brightCalibration for foreground of sources measuring (radio interferometric the galaxies, data faint diffuse ising foregrounds CMB etc.). images well-studied fluctuations, with and enormous has dynamic beenprecision ranges. since shown it Furthermore, to depends the be only synthesised on capable beam the of is produc- known to high relation between brightness temperature 1.3 Disadvantages of CMB interferometry CMB interferometry 1.2 Advantages of CMB interferometry are summarised in Table since they measure the correlation ofpower signal (baseline) between response pairs of and individual thussuch detectors. are not as Interferometers dependent the do on 2.7 not the K measure total- vantage. CMB the Long temperature DC time-scale and drifts signal, the of receivers bulk (1 of atmospheric emission, which is a major ad- good use for filtering out known sources including Sun, Moon and other local signals (see [ experiments is also mitigated by implementingcorrelation. Automatic Phase-switching Gain naturally Correction provides (AGC) further in suppression thethermal of IF noise systematics. before to This be allows realised the even for very long (months and years)fringe integration rates times. to signals from within the field-of-view PoS(RTS2012)034 Clive Dickinson noise, pointing) spectrum and on f ` / ) of interferometry for CMB ]. 7 bottom scales [ ◦ and 2 coverage known exactly) ◦ v , u 4 ) and disadvantages ( top High dynamic range Rejection of DC signals ). These gave constraints on the very high Maps via synthesis imaging 2 . A summary of interferometric CMB measurements is given in 3 Not as sensitive as total-power (filling factor) Largest angular scales difficult (shortest spacing) Accurate window function (FT of primary beam) ]. A number of similar measurements were made using the OVRO, Ryle, 5 Direct access to power spectrum (square of visibilities) Accurate beam knowledge ( . It was originally designed to have several 1.5 GHz bands but funding was ◦ Easy access to wide range of angular scales (array configuration) –2 0 10 Maps are difficult to quantify (flux loss on large angular scales/resolving out) ∼ ]. In many ways the CAT experiment paved the way for the more sensitive arrays in the A list of some of the advantages ( 24 . 2 The author knows of early attempts by Prof Rodney Davies using the Lovell Mk1 and Mk2 dishes at Jodrell Bank The first published work that the author is aware of was with the newly commissioned Very In the late 1990s, three semi-independent collaborations began designing much more sensitive The VSA was a 30 GHz 14-element interferometer that could be reconfigured to probe angular The first dedicated CMB array was the Cambridge Anisotropy Telescope (CAT). Built by There have been a number of early attempts of detecting CMB anisotropies with interferome- 500 [ 3 ∼ Strong rejection of systematics (baselines, rejection of non-astronomical signals, no 1 but these were never published due to lack of sensitivity. 2.2 The era of the CMB interferometers and flexible arrays dedicatedBank to and measuring IAC groups the worked CMBCaltech together group power to formed construct spectrum. two independent the collaborations Very The to Smalland build the Array Cambridge, the Cosmic Degree (VSA). Background Jodrell Scale Imager In Instrument (CBI). the (DASI) Theseprecision U.S., three measurements instruments the of were the pivotal in CMB making power the spectrum first on sub-degree scales. scales from Table Large Array (VLA) by [ next decade. A dedicated singleat baseline 33 interferometer GHz, was and also managed to employed detect in anisotropies Tenerife, on operating 1 ters going back to the early 1980s 2.1 Early measurements at high resolution the contribution from unresolved point sources. Cambridge University at Lord’s Bridge,and it consisted operated of at 3 15 antennas GHz.` located inside It a was ground screen the first interferometer to detect primary CMB anisotropies at 2. Summary of observations BIMA and ATCA arrays (see Table Table 1: CMB interferometry analyses compared to total-power experiments. PoS(RTS2012)034 ] ] ] 26 ] ] ] ] ] , ] ] ] ] ] 9 12 15 1 8 3 7 4 , , , 20 25 27 24 22 6 5 , 23 14 Clive Dickinson ` 4500 [ 67506000 [ [ 3400 [ 4000 [ 20–100 [ 110–220 [ 339–722125–700 [ [ 630–3500 [ 5000–9000 [ 3000–70000 [ plane, which was improved v ]. This proved to be essential , u 29 1 150–1600 [ 0 0 ◦ . 4 00 2 0 0 0 0 0 -range 140–900 allowing detection ◦ ◦ ◦ ◦ / /28 / 0 0 ` 6 ◦ . 5 measurements. The CBI was the first experiment to ]. ` 30 (GHz) (GHz) Primary beam Frequency Bandwidth Reference ]. DASI was reconfigured to measure CMB polarization, with each 6 dish N ]. 4 ]. 9 Summary of interferometric experiments to measure CMB anisotropies. This is an update of a The DASI instrument was located at the South Pole and used 13 20 cm horns located on a table The CBI was located in the Atacama desert, Chile and operated from 1999–2008 and utilised 150–1600 [ = IRAMRyle France England 3T-WIAC-Int 8 Tenerife 88 U.S.VSA 15 2CBI Tenerife 2 0.4 33 14 0.4 Chile 43 26–36 55 13 0.5 6 1.5 ... 26–36 2 4 10.0 2 45 OVRO U.S. 6 30 2.0 4 Name Location VLA U.S.BIMAATCA 27 U.S. AustraliaCAT 6 10 8DASI England 30 9 South Pole 3SZA 0.2 13 13–17 0.8 0.1 U.S. 26–36 5 0.5 8 10.0 6 8 30 & 90 2 3 8 12 for observing during the daysignals (to that were filter found out to be the strong Sun onsingle-baseline the and interferometer shortest Moon) consisting baselines. and of The VSA two also also 3.7This for employed m allowed an filtering dishes for additional out a located correlated survey in of theiran the own earlier brightest ground 15 sources GHz screens. in survey each of VSA thesame field VSA time. (positions fields The were by VSA provided the produced from Ryle accurate` telescope) measurements at of the the CMB same power frequency spectrum and over at the the range mounted onto an az-el mount. ItIts operated small at 26–36 horns GHz and with 10 table channel provided 10 GHz good digital sensitivity correlator. over the antenna measuring a singledetection polarization of (left CMB or polarization right whichcosmology circular). provided [ remarkable confirmation In of 2002 the DASI standard obtained theory the of first many of the sameantennas components also as located its on “sister” aDASI, instrument, table the DASI. antennas that did The could not CBI be track moved consisted giving a along of constant 3 13 coverage axes in 0.9 (az,el,parallactic the m angle). Like only given to utilise asystematics, single both 1.5 instrumental GHz and channel. astrophysical.ground-screen The while The VSA the entire was tracking telescope carefully elements was on designedastronomical located the signals to tip-tilt inside such minimise table a as all allowed cross-talk large fringe-rate and filtering and of ground non- spillover [ of the first 3 acoustic peaks [ by rotating the telescope in parallacticcorrelator angle. as The CBI was utilised the used2000), same for 10 the channel DASI. 10 CBI GHz After digital group discussions focussed between on the high- CBI/DASI/VSA groups (circa Table 2: previous version of this table produced by [ convincingly show the drop in power due to the Silk damping tail expected from the standard theory CMB interferometry PoS(RTS2012)034 1 mK) . ] as well ]. 2000–3500 23 10 [ ∼ ` Clive Dickinson ` ]. 26 4 GHz) arrays, aiming . 1 . ]. Later, the CBI was upgraded 22 10 arcmin. More recently, the idea ∼ ]. The Arcminute MicroKelvin Imager 6. This involves measuring tiny ( 12 & z 6 ] at moderate redshifts has emerged, which potentially could allow is the number of antennas). The need for sensitivity necessarily leads ]. The CBI also showed evidence for excess power at 17 n 16 ( 2 n 100 GHz. One of the earliest measurements of SZ was made with the Ryle 1000 [ ]. The QUBIC experiment is aiming to deploy a small array at Antarctica to test . ]. The VSA, CBI and DASI experiments all made significant contributions to the 28 ∼ ]. 8 , z 19 18 ], although this has been debated and has not been confirmed to-date. The CBI was also 21 The future for CMB interferometry appears to be limited due to the great need for much In recent years, there has been great interest in low frequency ( The study of clusters via the inverse Compton scattering of CMB photons from hot electrons, The Sunyaev-Zel’dovich Array (SZA) is an 8-element 3.5m dish interferometer located at , 11 fluctuations of the redshiftedof HI HI 21 cm intensity line mapping [ on scales of towards the Square Kilometreduring Array the Epoch (SKA). of A Reionization (EoR) major at science redshifts goal of the SKA is to detect HI higher sensitivity. This is importantpolarization for where both the signal intensity is (e.g. much weaker.the for The SZ large-scale main surveys) goal B-modes for and, which CMB in experiments requirecorrelations particular, is 1000s scales in for measuring of as detectors.to For an very interferometer, large the arrays, numbercorrelators possibly of (although with some multiple correlations feeds, couldAlthough that be this may ignored) would be operating therefore possible with requirechallenge. with very One extremely the possible large advent exception large bandwidths. to of this digital isworks the (software) as bolometer an correlators, interferometer adding concept, it interferometer which is (as essentially opposed abenefits to formidable of multiplying) which an gives interferometer some (but butbolometers not with all) [ the of the higher sensitivity and larger bandwidths available from 3. The future this concept [ 2.3 SZ interferometers the Sunyaev-Zel’dovich (SZ) effect, has become an importantogy. topic Interferometers in are radio well-suited astronomy to and measuring cosmol- thesize SZ of effect a since few clusters arcmin, have which a requires typicalments relatively angular working large single-dish at telescopes for ground-basedinterferometer instru- [ as numerous SZ detections(AMI) including is a a blind 15 survey GHzdishes, [ in experiment combination located with at theas larger Lord’s Ryle making Bridge, telescope targeted dishes Cambridge, observations for of consistingAMIBA point known array of source clusters, is subtraction. eight a it As 90 3.7 is GHz m well originally surveying array configured the located with at sky seven the 0.6 for m Mauna new antennas Loa clusters. and (Haiwaii) then with The upgraded a to 10 1.2 m m platform. antennas [ It was Owen’s Valley Radio Observatory90 in GHz California. it has made With important sensitive contributions receivers to operating both at primary 30 anisotropies and at high- converted to measuring polarization and detected polarization [ number of SZ detections. 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E., Joy, M., & Grego, L., GHz CD acknowledges support from an STFC Advanced Fellowship and an EU Marie-Cure IRG [8] [9] [6] [7] [4] [5] [2] [3] [1] [10] [11] grant under the FP7.providing useful comments: CD Keith wishes Grainge, toLucio Jean-Christophe Piccirillo Hamilton, thank and Erik the Paul Leitch, Scott. following Tim Pearson, people for reading anReferences early draft and Acknowledgments CMB interferometry detection of Baryon Acousticexciting Oscillations areas require and similar thereby sensitivities andbut constrain angular in dark scales as the energy. for presence theInterferometers of Both CMB may primary much these anisotropies be larger new a foregroundsgroups good and e.g. way [ potential of systematic achieving errors this and and calibration. this is being pursued by a number of PoS(RTS2012)034 ] Small-Scale ] Clive Dickinson , 2002, PASP, ] , 2011, MNRAS, , 2006, Presented at ] 2003, Memorie della ] The Australia Telescope ] , 2003, ApJ, 591, 556 ] , 2010, ApJ, 713, 82 arXiv:astro-ph/9712195 ] Interferometric Observation of , 1996, ApJL, 461, L1 , 1997, ApJ, 483, 38 Extended Mosaic Observations with the arXiv:1012.1610 The Anisotropy of the Microwave Background arXiv:astro-ph/0205385 Polarization Observations with the Cosmic A Measurement of Arcminute Anisotropy in the , 1998, MNRAS, 298, 1189 The Cosmic Background Imager QUBIC: The QU bolometric interferometer for arXiv:astro-ph/0402359 arXiv:1010.0645 Cosmological Constraints from a 31 GHz Sky Survey The Cosmic Background Imager 2 A Fast Gridded Method for the Estimation of the 8 arXiv:astro-ph/0409569 First results from the Very Small Array - I. 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