Sardinia Radio Telescope

Sardinia Radio Telescope:General Description,Technical

Commissioning and First Light

P.Bolli *,A.Orlati ?,L.Stringhetti ?,?,A.Orfei ?,S.Righini ?,R.Ambrosini ?,M.Bartolini ?,C.Bortolotti ?,F.Bu?a §,M.Buttu §,A.Cattani ?,N.D'Amico §,G.Deiana §,A.Fara §,F.Fiocchi ?,F.Gaudiomonte §,A.Maccaferri ?,S.Mariotti ?,P.Marongiu §,A.Melis §,C.Migoni §,M.Morsiani ?,M.Nanni ?,F.Nasyr §,A.Pellizzoni §,T.Pisanu §,M.Poloni ?,S.Poppi §,I.Porceddu §,I.Prandoni ?,J.Roda ?,M.Roma ?,A.Scalambra ?,G.Serra §,A.Trois §,G.Valente §,G.P.Vargiu §and G.Zacchiroli ?

*INAF –Osservatorio Astroˉsico di Arcetri,Florence,Italy

?INAF –Istituto di Radio Astronomia di Bologna,Bologna,Italy

?INAF –Istituto di Astroˉsica Spaziale e Fisica Cosmica di Milano,Milan,Italy

§INAF –Osservatorio Astronomico di Cagliari,Cagliari,Italy

?luca@https://www.wendangku.net/doc/9f9243365.html,ano.inaf.it

Received 2015May 5;Revised 2015September 15;Accepted 2015November 30;Published 2016January 18In the period 2012June –2013October,the Sardinia Radio Telescope (SRT)went through the technical commissioning phase.The characterization involved three ˉrst-light receivers,ranging in frequency be-tween 300MHz and 26GHz,connected to a Total Power back-end.It also tested and employed the telescope active surface installed in the main re°ector of the antenna.The instrument status and perfor-mance proved to be in good agreement with the expectations in terms of surface panels alignment (at present 300 m rms to be improved with microwave holography),gain ($0.6K/Jy in the given frequency range),pointing accuracy (5arcsec at 22GHz)and overall single-dish operational capabilities.Unresolved issues include the commissioning of the receiver centered at 350MHz,which was compromised by several radio frequency interferences,and a lower-than-expected aperture e±ciency for the 22-GHz receiver when pointing at low elevations.Nevertheless,the SRT,at present completing its Astronomical Validation phase,is positively approaching its opening to the scientiˉc community.Keywords :Radio telescope,technical commissioning,radio astronomy.

1.Introduction

The Sardinia Radio Telescope (SRT)(Lat.39 2903400N –Long.9 1404200E;600m above the sea level)is a new Italian facility for radio astronomy whose commissioning was completed at the end of 2013.Its formal inauguration took place on 2013September 30.The antenna is a fully steerable,wheel-and-track dish,64m in diameter,located 35km north of Cagliari,on the island of Sardinia.It completes a set of three antennas devoted to radio

astronomical science in Italy (Fig.1),all managed by the Italian Institute for Astrophysics (INAF).

The SRT is a general-purpose radio telescope aimed at operating with high aperture e±ciency.Once all the planned devices are installed,it will observe in the frequency range from 300MHz to 100GHz and beyond (from 1m to 3mm in wavelength).The an-tenna gain is expected to vary from 0.50to 0.70K/Jy for the frequency range 0.3–50GHz and to be around 0.34K/Jy in the 3mm band (70–115GHz).

A key feature of the SRT is its active surface,in total composed by 1116electromechanical actua-tors,able to correct the deformations induced by gravity on the primary surface (or \mirror").E?ort is underway to employ this facility to also correct

This is an Open Access article published by World Scientiˉc Publishing Company.It is distributed under the terms of the Creative Commons Attribution 4.0(CC-BY)License.Further distribution of this work is permitted,provided the original work is properly cited.

Journal of Astronomical Instrumentation,Vol.4,Nos.3&4(2015)1550008(20

pages)#c The Author(s)DOI:10.1142/S2251171715500087

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for non-systematic errors,such as temperature/wind-related e?ects.

The SRT is capable of hosting many microwave receivers,located in four di?erent antenna focal positions —primary,secondary and two beam-waveguide foci —able to cover almost continually its frequency range.The SRT will operate in single-dish (continuum,full Stokes and spectroscopy),Very Long Baseline Interferometry (VLBI)and Space Science (Ambrosini ,2011a )modes.

Thanks to its large aperture and versatility (multi-frequency agility and wide frequency cover-age),the SRT is expected to have a major impact in a wide range of scientiˉc areas for many years to come.A full description of the potential SRT sci-ence applications is beyond the scopes of the present paper,as it is provided in a separate paper dedi-cated to the Astronomical Validation activities (Prandoni et al.,in preparation).Here we illustrate some of the main areas where we think the SRT can play a major role in the next future.

Operations in the framework of international VLBI and Pulsar Timing networks are of top pri-ority for the SRT.SRT is going to be one of the most sensitive European VLBI Network (EVN)stations,together with E?elsberg and Jodrell Bank.Its large aperture is also of extreme importance for Space VLBI observations with RadioAstron.Thanks to its active surface,the SRT will also represent a sensitive element of the mm-VLBI network operating at 7-and 3-mm bands,where a substantial improvement in collecting area and in the coverage of the sky Fourier transform plane is of vital importance for increasing the number of targets accessible to the array and the quality of the images.Once the ˉber optic connection to the site is completed,the SRT will also participate in real-time VLBI observations (e-VLBI).The availability of three antennas (Fig.1)will moreover allow the constitution of a small Italian VLBI network,exploiting a software correlator (DiFX,already operating).The SRT will be also in-cluded in the geodetic VLBI network.

The SRT is one of the ˉve telescopes of the Eu-ropean Pulsar Timing Array (EPTA),which,together with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav),and the Parkes Pulsar Timing Array (PPTA)share the goal to detect gravitational waves.Being the southernmost telescope of the EPTA collaboration,the SRT will allow a better coverage of pulsars with declinations below à20 ,hence a better overlap with the PPTA.Thanks to its dual-band L/P receiver,SRT will be of great impor-tance in measuring accurate dispersion measure var-iations,crucial to obtain ultra-precision pulsar timing,and search for signatures of space-time perturbations in the pulsar timing residuals.The SRT is also part of the Large European Array for Pulsars (LEAP),a project which consists in using the EPTA telescopes in tied-array mode,obtaining the equivalent of a fully steerable 200-m dish.

The SRT is expected to have a major impact also for single-dish observations.In particular,we aim at exploiting its capability to operate with high e±ciency at high radio frequency.Equipped with multi-feed receivers the SRT can play a major role in conducting wide-area surveys of the sky in a fre-quency range (20–90GHz),which is poorly explored,yet very interesting.

For instance the ˉrst-light K-band 7-beams re-ceiver will be exploited to obtain extensive

mapping

Fig.1.

The SRT and the location of the three antenna sites.

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of the NH3in Galactic star-forming regions,in close synergy with existing IR/sub-mm continuum sur-veys of the Galactic Plane.The ammonia molecule,through its (1,1)and (2,2)transitions,is a good tracer of dense cores of molecular gas and is con-sidered an excellent thermometer.Another inter-esting application of the K-band multi-feed receiver is a H2O line survey of nearby galaxies.This will increase the number of detected water masers and derive distances,dynamical models and total masses of (luminous tdark )matter of the galaxies in the Local Group.

Wideband multi-feed receivers operating at higher frequency (40–90GHz)—currently under development —will allow us to get access to unique molecular line transitions in our own Galaxy,like for example those associated with deuterated molecules (e.g.DCO te1à0T,N2D te1à0T),cru-cial to constrain the kinematic and chemical prop-erties of pre-stellar cores,as well as to uncover the cool molecular content of the Universe in a crucial cosmic interval (redshift $0.3–2),through the mapping of redshifted CO low-J transitions.Wide-band radio-continuum (and spectro-polarimetry)mapping of extended (low-surface brightness)Ga-lactic and extra-galactic sources (e.g.SNRs,radio galaxies,nearby spirals),on the other hand,will permit resolved studies aimed at a better under-standing of the physics of accretion and star for-mation processes.In addition high-frequency receivers will allow us to conduct (high spatial res-olution)follow-up observations and monitoring experiments of AGNs,GRBs and other transient events in connection with high-energy experiments (FERMI,MAGIC,CTA),a research ˉeld where the Italian astronomical community is very active.

Due to an agreement between INAF and ASI on the use of the instrument for space applications,SRT will also be involved in planetary radar as-tronomy and space missions (Tofani et al.,2008;Grue?et al.,2004).

This paper presents an overview of the SRT system in the light of the results obtained during the commissioning phase (Ambrosini et al.,2013).Sec-tion 2describes the overall SRT system,including technical speciˉcations of the main antenna modules.Section 3shows the results of the tests performed in the commissioning phase and the timeline for the main milestones.In Sec.4some conclusions are drawn on the overall results of the commissioning.

For the convenience of readers,Table 1provides a glossary of many acronyms used throughout the paper.

2.The Sardinia Telescope System 2.1.Antenna technical characteristics The antenna design,whose schematic view is shown in Fig.2,is based on a wheel-and-track conˉgura-tion.The main re°ector (M1)consists of a back-structure that supports —through actuators —the mirror surface,itself composed of rings of re°ecting panels.A quadrupod,connected to the back-structure,supports the sub-re°ector (M2)and the primary focus positioner and instrumentation.The main and secondary mirrors and the quadrupod lie on the alidade,which is a welded steel structure

Table 1.

Glossary of the terms used.

Acronym Description

ACS Alma Common Software ACU Antenna Control Unit ASI Agenzia Spaziale Italiana AV Astronomical Validation BWG Beamwaveguide

EPTA European Pulsar Timing Array DBBC Digital Baseband Converter DFB Digital Filter Bank

DiFX Distributed Fourier spectrum Cross-multiplied DISCOS Development of the Italian Single-dish Control System

DSP Digital Signal Processing EER Elevation Equipment Room e-VLBI Real-time VLBI

EVN European VLBI Network FEM Finite Element Method FOV Field of View

HPBW Half-Power Beam Width

IF Intermediate Frequency band INAF Istituto Nazionale di Astroˉsica IRA Istituto di Radioastronomia

IRIG-B Inter-Range Instrumentation Group,time code format B

LCP Left Circular Polarization

LEAP Large European Array for Pulsar OAA Osservatorio Astroˉsico di Arcetri OAC Osservatorio Astronomico di Cagliari OTF On-The-Fly

PFP Primary Focus Positioner PSD Position Sensing Device RCP Right Circular Polarization RFI Radiofrequency Interference

ROACH Reconˉgurable Open Architecture and Computing Hardware SDI Single-Dish Imager

SRT Sardinia Radio Telescope T&F Time and Frequency

VLBI Very Long Baseline Interferometry WVR Water Vapor Radiometer

XARCOS

Arcetri Cross-Correlator Spectrometer

SRT:General Description,Technical Commissioning and First Light

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standing on a large concrete tower that forms the bulk of the antenna foundation.Three large rooms were built behind the primary mirror to contain the secondary focus receivers,the beam-waveguide mirrors and several electronic instrumentation and cable distributions,respectively.A fourth room is located in the lower part of the alidade,where the power drivers for the motors,the antenna control unit and the cryogenic compressors are installed.The alidade also supports the elevation wheel,which is a conical truss anchored to the re°ector back-structure through a massive pyramidal structure.

The radio telescope is supported by a rein-forced-concrete foundation excavated into rock;an outer ring beam sustains the azimuth track,while a central building accommodates the azimuth pintle bearing support,the azimuth cable wrap and the encoder system.The wheel-and-track structure con-sists of 16wheels lying on the rail,which is a contin-uous welded ring 40m in diameter.This is connected to the foundation by 260pairs of anchor bolts and by a ˉber-reinforced grout.The overall structure of the antenna weighs approximately 3300tons.

The antenna is steerable around the azimuth and elevation axes.The rotation around the two axes is managed by a servo control system,con-sisting in 29-bit absolute encoders (peak-to-peak position error 0.8arcsec),12brushless asynchronous motors (eight in azimuth and four in elevation)and an ACU available on a BECKHOFF hardware platform employing an IRIG-B generator.A proper torque bias is applied to the motors to overcome the gearbox backlash and to improve the antenna pointing accuracy.

The primary re°ector surface consists of 1008individual aluminum panels divided into 14rows of identical panel types.Each panel has an area ranging from 2.4to 5.3m 2.It is built using alumi-num sheets glued,by means of a layer of epoxy resin,to both longitudinal and transversal Z-shaped aluminum sti?eners.The basic back-structure is composed of 96radial trusses and 14circumferential trussed hoops supported by a large center hub ring.The sub-re°ector surface consists of 49individual aluminum panels with an average area of about 1m 2,whereas its back-structure is formed by 12radial trusses and three circumferential trussed hoops supported by a center hub ring.Three of these trusses are directly connected to a triangular steel frame,which has the function of a transitional structure to the six sub-re°ector actuators.These actuators deˉne the sub-re°ector position,and provide for sub-re°ector motion with ˉve degrees of freedom.

Further mirrors beneath the Gregorian focus,arranged in a Beam WaveGuide conˉguration,

allow

Fig.2.SRT:mechanical structure.The radio-frequency re°ecting surfaces are highlighted in red.

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the addition of four more focal points with magniˉed and de-magniˉed F =D ratio in the intermediate frequency bands (Fig.3).

Two of the four designed BWG layouts have already been constructed.They use three mirrors:M3as a shared mirror,M4for the ˉrst layout and M5for the second one.The re-imaging optics for BWG layout I were designed for maximum focal ratio reduction (right side of Fig.3);whereas BWG layout II was designed so that the output focal point F4lies beneath the elevation axis of the antenna (left side of Fig.3).By an opportune rotation of M3,which takes 1.5min,the desired BWG layout is selected.All the mirrors are portions of ellipsoids and present quite large apertures,as shown in Table 2.The two missing BWG layouts will be added in a later stage to the present conˉguration using two more mirrors and an appropriate rotation of M3;they will be dedicated to Space Science applications.

A rotating turret (Gregorian positioner assem-bly)is mounted eccentrically on the focal plane of the antenna dish,and can house eight separate cryogenic receiving systems and the associated feed horns for operating up to 115GHz.A drive system can rotate the turret (within 2min for a complete rotation of 340 )so that any of the feed horns can be positioned on the focal plane.The servo control system consists of two brushless servo motors with drivers and a position-control computer.

A mechanism selecting among di?erent recei-vers exists also in the primary focus.Several receiver assemblies,the exact number depending on their dimensions,can be allocated at the secondary mir-ror back-structure side.An arm controlled by two servo motors can,in less than 4min,place the re-quired receiver box in the primary focus position (Fig.4),which can accommodate low-frequency receivers whose dimensions are not mechanically compatible with the Gregorian positioner assembly.

The remote and automatic control of all these movements makes the antenna available in fre-quency agility,switching among all the observing bands in a fast and unmanned way.

Suitable atmospheric conditions (see Table 2for the environmental speciˉcations)together with the active surface will extend the use of the instrument up to 100GHz.This capability also requires an advanced metrology system for accurate antenna pointing (see Sec.2.3).The availability of a Water Vapor Radiometer (WVR)and a Weather Station,already in operation,will also allow the dynamic scheduling of the observations,further improving the telescope productivity.

A complex helium 5.5(99.9995%pure gas)plant consisting of seven pairs of supply-and-return lines assures that the microwave receivers,distrib-uted in the various positions,can operate at cryo-genic temperatures.The system is dimensioned so that each line can serve up to three microwave receivers.The total length of the lines reaches 1.5km,and they are composed of both rigid and °exible

tubes.

Fig. 3.Technical view of the Gregorian and BWG rooms containing the Gregorian turret and the BWG mirrors,respec-

tively.

Fig.4.Technical view of the primary focus area.The primary focus positioner,containing the primary focus receivers,and the re°ecting surface of the sub-re°ector are highlighted in light blue and red,respectively.

SRT:General Description,Technical Commissioning and First Light

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For the radio frequency links (Fig.5),more than 3km of coaxial cables connect the focal positions to a central point located in the elevation Equipment Room.The highest frequency useable with the co-axial cables is 4GHz,with a maximum attenuation of 15dB (at 2GHz)for the longest path (100m)and with a matching coe±cient better than à20dB over the whole frequency band.An optical ˉber cabling,based on both single-mode and multi-mode ˉbers,was deployed within the radio telescope for digital and analog data transmission.The optical ˉbers are responsible for the connection of the radio telescope to the external infrastructures.Some customized optical links allow the transport of the astronomical signal,in the IF frequency band (0.1–2.1GHz),from the EER to the remote control and data processing room.This solution,thanks to a link gain of 0dB associated with a dynamic range better than the dynamic range of the receiver itself,ensures that the overall speciˉcations of any receiver are maintained.

Table 2.

Speciˉcations of the SRT.

Characteristics Details

Value

Range of motion Elevation 5–90

Azimuth 180 ?270 Slew rate Elevation 0.5 /s Azimuth 0.85 /s

Mirrors size

M1–Primary mirror 64m (axially symmetrical)M2–Sub-re°ector

7.9m (axially symmetrical)M3–BWG shared between layouts I and II 3:921?3:702m M4–BWG layout I 3:103?2:929m M5

–

BWG layout II

2:994?2:823m Number of individual panels M1–Primary mirror 1008(14circular rows)M2–Sub-re°ector

49(3circular rows)Expected surface accuracy M1panel

65micron rms (El ?45 )in precision M1alignment (photogrammetry;holography)290;150micron rms environment conditions

M2panel

50micron rms Back-up structure (with active surface)0

Actuator accuracy and linearity

15micron rms Other (meas.errors;thermal and wind e?ect)49micron rms Total without holography 305micron rms Total with holography

178micron rms Pointing accuracy Normal conditions without metrology systems 13arcsec Precision conditions with metrology systems 2arcsec F/D

F1–Primary focus 0.33F2–Gregorian focus 2.34F3–BWG layout I focus 1.37F4–BWG layout II focus 2.81Frequency range

Primary focus 0.3–20GHz Gregorian focus 7.5–115GHz BWG foci 1.0–35GHz Precision environment conditions

Wind <15km/h Solar

Absent Precipitation Absent

Temperature

à10 C to 30 C Temperature drift <3 C/h Humidity <85%Normal environment conditions

Wind <40km/h Solar

Clear sky Precipitation Absent

Temperature

à10 C to 40 C Temperature drift <10 C/h Humidity

<90%

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In order to connect the radio telescope to the Internet,mainly to transfer data during real-time VLBI observations,SRT will make use of a 10-Gbps link infrastructure,currently under construction.2.2.Microwave receivers and back-ends Notwithstanding the telescope's enormous collect-ing area and its advanced systems,able to ensure high e±ciency at all the operating frequencies and over the whole elevation range,state-of-the-art mi-crowave receivers and digital and analog back-ends are necessary in order to perform breakthrough astronomical discoveries.The microwave receiver guides,ampliˉes,ˉlters and down-converts the in-coming astronomical signal re°ected by the mirrors.It consists of a cascade of di?erent microwave pas-sive and active components,some of them cooled to cryogenic temperatures.The receiver mainly deˉnes the frequency bandwidth,the antenna illumination,the receiver noise temperature and the cross-polar purity.The radio receiver group of INAF is responsible for the design of the whole receiver chain,both from an electronic and mechanical point of view,for the assembly and testing and ˉnally for the integration in the radio telescope.

The ˉrst-light of SRT was obtained using the following three receivers:the L-and P-band

dual-frequency coaxial receiver for primary-focus operations (Valente et al.,2010),the C-band mono-feed receiver designed for the BWG layout I (Nesti et al.,2010;Orfei et al.,2011;Poloni ,2010;Peverini et al.,2011)and the K-band multi-feed receiver (seven corrugated feeds)for the secondary focus (Orfei et al.,2010).

For the very ˉrst astronomical tests,the C-band receiver was installed in the Gregorian focus with a cone section added in front of the feed-horn to match the di?erent F/D numbers.This choice was made to avoid possible BWG mirror misalign-ments in the ˉrst pointing tests of the antenna.

Table 3describes the main technical char-acteristics of the receivers currently installed,together with those foreseen to be completed in the next three years.For each receiver the table lists the observable sky frequency bandwidth,the focal position,the number of simultaneous beams in the sky (specifying the number of polarizations),an estimation of the expected antenna gain and of the system noise temperature when the antenna points to the zenith.Finally,the last column indicates the status of the receiver.

All the multi-feed receivers (S-,K-and Q-band)are equipped with a built-in mechanical rotator,whose aim is to prevent sky ˉeld rotation while the telescope is

tracking.

Fig.5.Schematic view of the signal transfer lines from the front-ends to the back-ends.

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The W-band receiver,which was decommis-sioned by the Plateau de Bure radio telescopes op-erated by IRAM,is mainly aimed at testing the SRT active surface,and at acquiring know-how in the 3-mm band.However,even though it is a single-pixel,one-linear-polarization and narrow-instanta-neous-bandwidth (600MHz)receiver,its installation on the SRT could play a relevant role in the mm-VLBI network,signiˉcantly extending its baselines.

The front-end outputs are connected to the back-ends either by coaxial cables (in case they are placed within the radio-telescope,in particular the Total Power and XARCOS back-ends)or via opti-cal ˉber in order to reach the distant control room.

Back-ends represent key elements to the success of the observations.They can be either general-purpose or tailored to speciˉc scientiˉc activities.All the back-ends listed in Table 4were installed and are now either commissioned or under test at the telescope (Melis et al.,2014).

2.3.Optical system and active surface The SRT optical system is based on a quasi-Gre-gorian proˉle with shaping applied to both the primary and the secondary surfaces.The present geometry results from a trade-o?between two goals:minimizing the overall system noise temperature,

Table 3.

Microwave receivers installed and under construction for the SRT.

Receiver

Freq range [GHz]

Focal position Pixels ?polarizations Expected antenna

gain [K/Jy]Expected system temperature at zenith [K]Status

L-and P-band coaxial feed

0.305–0.410F11?20.47–0.5950–80Commissioned 1.3–1.8

1?20.50–0.6017–23C-band mono-feed 5.7–7.7F3

1?20.64–0.7024–28Commissioned K-band multi-feed 18–26F27?20.60–0.6640–70Commissioned

S-band multi-feed 2.3–4.3F15?20.7654Under construction C-band (low)mono-feed 4.2–5.6F41?20.62–0.7030–35Under construction X-and Ka-band coaxial feed 8.2–8.6

F11?10.64120Under testing 31.8–32.3

1?10.57190Q-band multi-feed 33–50F219?20.45–0.5645–120Under construction W-band mono-feed 84–116

F2

1?1

0.34a

115

Under refurbishment

a With

a surface accuracy of 178micron and the metrological systems in operation.

Table 4.Back-ends available at SRT.In brackets are given the applications currently under testing or reˉnement and planned to be fully operative within 2016.Back-end Main features and usage Inputs Max input IF band (GHz)

Integrat.time Max spectral channels

Min spectral resolution (Hz)

Total Power

Bandwidth selectable 14

0.1–2.1

1–1000ms

—

—

Attenuation selectable IF distributor Continuum

DFB ADCs,10bit resolution 40–1.0240.1ms –4s 8192$1000

10bit DSP Pulsar

(Spectro-polarimetry)XARCOS Spectro-polarimetry 140.12510s 2048$250DBBC 8bit DSP 40.5120.1–1s 4096125,000

VLBI

(RFI monitoring)(Spectroscopy)

ROACH ADCs,8bit resolution 20.5121s 819262,500

LEAP -Pulsar,array (Pulsar,single dish)(Spectroscopy)

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mainly due to spill over,and reducing the standing wave pattern (due to internal re°ections between the primary and the secondary mirrors and thus detrimental to wideband spectroscopic observa-tions)without excessively sacriˉcing the Field of View available from the Gregorian focus (Cort ès-Medellin ,2002).The shaped parabola –ellipse pair,in fact,provides a wider focal plane than fully shaped re°ectors;the FOV's radius is 20 at 10%of aperture e±ciency loss.The design placed more emphasis on reducing the standing wave than on maximizing the aperture e±ciency.

One of the most innovative features of the SRT is the active surface that consists of 1116electro-mechanical actuators mounted in the backup structure,beneath the primary mirror panel cor-ners,and distributed along radial lines.Its ˉrst aim is to reshape the mirror to compensate for the re-peatable deformations due to gravity.Exploiting advanced real-time measurements,it will also cor-rect for wind and thermal e?ects (Orfei et al.,2004).Each actuator moves either upward or downward at the corners of four adjacent panels,in the direction normal to the local surface,with a maximum stroke of ?15mm.Such a large stroke was required in order to achieve the second aim,i.e.to modify the shaped proˉle back to a surface that is parabolic enough to increase the maximum operating frequency observable from the primary focus (Bolli et al.,2014).A further application of the active surface is also to recover the manufacturing deformations of the sec-ondary mirror (Bolli et al .,2003).

In order to perform high frequency observations the surface accuracy must be around 150 m.The contractual requirement for the alignment of the primary re°ector was 500 m with a \goal"of 300 m.Successive accurate photogrammetry mea-surement campaigns have shown a lower limit in the total RMS of the primary mirror equal to 290 m at 45 of elevation (S üss et al.,2012).These measure-ments,carried out in six di?erent elevation posi-tions,allowed the production of a look-up table,used to correct the gravity deformation with the actuators in an \open loop"conˉguration.In order to further improve the measurements of the primary mirror proˉle,microwave holography measure-ments will be implemented by using the hardware setup successfully tested at the 32-m Medicina radio telescope (Serra et al.,2012).

Moreover,several solutions are being studied and implemented so as to perform real-time

measurements of the re°ector deformations and the misalignments errors in the subre°ector position,with the ˉnal aim of correcting them in a \closed-loop"strategy (Pisanu et al.,2012;Prestage et al.,2004):(i)improved FEM analysis —to understand the e?ects due to gravity,temperature and wind on the behavior of the mechanical structure of the antenna,and to guide the optimized design of the sensor systems;

(ii)inclinometers —to monitor the status of the

track,the inclination of the azimuth axis and the variations of the alidade deformation as a function of the temperature;

(iii)position-sensing devices —to optically monitor

the lateral shift of the secondary mirror;

(iv)optoelectronic linear sensors —to measure the

deformations of the primary mirror;

(v)optical ˉber rangeˉnders —to measure the

deformations of the legs of the quadrupod in order to control the sub-re°ector position;(vi)thermal sensors —to map the distribution of

temperature in the most sensitive areas of the radio telescope and predict the deformations induced by thermal gradients.2.4.Antenna control software

The SRT consists of a large number of modules and devices to be managed and controlled in a timely and precise fashion.The many observing modes available must be commanded and harmonized with the data acquisition and the recording of time-stamps and housekeeping data.The overall control software performing all of these actions is called Nuraghe ,a software package exceeding half a mil-lion lines of code.It runs in the framework of the ALMA Common Software (ACS,a distributed-object framework in turn based on CORBA),which provides a general and common interface for high-level software,hardware and other software parts above the operating system.

For single-dish operations,various modes are supported such as raster scan,On-The-Fly (OTF)cross-scan and mapping,sky dipping,position switching and focusing.The system is also able to operate all the three Italian antennas under a common interface (user interface and scheduling system),as well as to accommodate external appli-cations such as VLBI or pulsar observations.

All data coming from the integrated back-ends are stored in a FITS ˉle.The current release of

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Nuraghe (Orlati et al.,2012),which is written in C ttand Python,enables the operator to control most of the subsystems —e.g.the antenna mount,the receiver chains,the back-ends,the active surface and the minor servo (Buttu et al.,2012).2.5.Auxiliary instrumentation

We now discuss several associated ancillary activi-ties such as radio spectrum monitoring,weather parameters measurement and time-frequency stan-dard distribution,as they are fundamental aspects for the optimal operation of the SRT.

A mobile laboratory is available at the site to carry out dedicated measuring campaigns to moni-tor and identify Radio Frequency Interference (RFI)in the frequency range between 300MHz and 40GHz (Bolli et al.,2013).Special attention is paid to the frequency bands observed by the ˉrst-light receivers and to those allocated by the Italian Fre-quency Allocation Plan to the Radio Astronomical Service (see Sec.3.3).

A rather sophisticated ground-based set of weather measuring instruments are deployed at the SRT site.Besides conventional sensors —ambient thermometer,hygrometer,wind speed/orientation measurer and barometer —a multi-frequency radi-ometer is employed.It generates real-time estimates of di?erent microwave brightness temperatures,opacity,zenith water vapor and liquid content.The measurements conducted so far show a 40–45%probability of ˉnding an integrated water vapor column density of less than 10mm in winter time.The absence of cloud cover can be found 50%of the time in winter and 80%during summer;typical liq-uid water values range between 0.2and 0.7mm.

In winter,the opacity at 22GHz is lower than 0.15Np for 90%of the time (40%during summer).At higher frequencies,in the 3-mm band,the winter opacity is lower than 0.15Np for 35%of the time.

The Time and Frequency laboratory consists of an active hydrogen maser producing the reference signals for the receiver local oscillators,the 1PPS (Pulse per second)timing for data acquisition and all the related equipments.A dedicated GPS re-ceiver derives the local clock o?set (as a di?erence from UTC GPS)for VLBI and provides the time to the antenna for its pointing via an IRIG-B genera-tor (Ambrosini et al.,2011b ).

3.Performance Test 3.1.The ˉrst radio source

SRT detected its ˉrst radio source in Summer 2012.The observation was performed with the C-band receiver,installed in the Gregorian focus at the time.The main uncertainties at that time were the encoder alignments of the two main movement https://www.wendangku.net/doc/9f9243365.html,ing the Moon,a wide and bright target,as a ˉrst reference radio source and moving the antenna in steps of a half beam-size in a cross-scan,we mea-sured the following encoder o?sets:t1.3 in azi-muth and à0.5 in elevation.The successive OTF scans were performed across 3C218,chosen because,being not too far from the Moon on that day,it allowed us to use the just-measured \local"o?sets,in the absence of a pointing model.The antenna response is illustrated in Fig.6;the noticeable pat-tern asymmetry is due to the employment of ˉxed optics,observing far from the elevation of their mechanical alignment (45

).

Fig.6.The ˉrst observed radio source,2012August 8.

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3.2.Pointing accuracy

The SRT software was designed to manage a com-plete and independent model for each receiver and each focus.Three models were thus processed,ini-tially considering 11.30,2.70and 0.80as reference Half-Power Beam-Widths (HPBWs)for the L-band,C-band and K-band receivers,respectively.

To achieve a preliminary pointing model,we applied the encoder o?sets obtained during the ˉrst radio source observation and produced many raw maps over bright and compact sources in order to measure the displacement of the optical axis with respect to the ideal one.This procedure permitted the creation of a basic set of parameters,which was used as a basis for the pointing models for all the three foci.

As a next step to improve the pointing model accuracy,we performed several series of spot obser-vations (OTF cross-scans along the Horizontal frame)of selected pointing calibrators,each time measuring the pointing o?sets.In order to minimize thermal e?ects on the structure we usually performed the observation during the night,in any case avoiding the sunrise.Once the whole azimuth/elevation plane in the sky was satisfactorily sampled,the pointing model polynomial was ˉt to the dataset (Maneri &Gawronski ,2002;Guiar et al.,1986).This process required several sessions to converge to an acceptable solution,for which the model residuals are required to be lower than one tenth of the beam size.

The RMS residuals after the ˉt are presented in Table 5and show considerably less than one tenth of the beam size up to 23GHz.The pointing perfor-mance at higher frequencies (the beam dimension will be around 12arcsec at 3mm)will take advantage of metrological systems (as discussed in Sec.2.3).3.3.RFI

Dedicated campaigns were carried out,both with the mobile laboratory and through the telescope,in

order to detect and identify the interfering signals.Generally speaking,the RFI environment was found to be fairly quiet in the C-band (especially above 6400MHz)and the K-band (almost everywhere in the band),while the P-and L-bands appeared more polluted.Figure 7shows the spectral acquisitions performed with the various SRT receivers.These scans were carried out spanning the whole azimuth range,repeating such 360 circles for di?erent ele-vations (every 3 above the horizon).These charts were obtained in \max hold"mode:all the instanta-neous acquisitions are overplot and,for each fre-quency bin,the maximum recorded value is displayed,thus the diagrams represent a sort of \worst case scenario"of the signals detected in the above speciˉed az –el ranges.

The L-and P-band environment is the most \polluted"at the site.Under these conditions,in order to ˉnd a suitable frequency conˉguration for the Total Power back-end,whose narrowest built-in band is 300MHz,we installed external tuneable (5%of the central frequency)band-pass ˉlters.For L-band acquisitions,an e?ective compromise was found and we successfully observed between 1696.5and 1715.0MHz.No solution,allowing reliable and repeatable measurements in the P-band,was found.Almost all the interfering signals were identiˉed to be self-generated by the apparatus installed in the telescope.Among these devices we can mention the VOIP phones in Alidade and elevation Equipment Rooms,the XARCOS back-end,the encoders of the PFP,and some devices of the control electronics of the K-band receiver.

As concerns the C-band receiver,the general panorama was quite satisfactory;we discovered the presence of only a few interfering signals,mainly concentrated in the lower part of the receiver bandwidth.The signals at 5900and 6016MHz have been identiˉed to be self-generated.The former is produced by one of the local oscillators of the multi-feed system,the latter by the device that guarantees Internet connection to the station through a satel-lite link.The RFI signals coming from external sources were all due to digital links,and they appeared to be much attenuated when observing southward.

The K-band turned out to be almost free from RFI.All the identiˉed polluting signals came from external sources were related to ˉxed links for mo-bile operators.These signals can be practically neglected by avoiding known azimuth and elevation

Table 5.Pointing model residuals and beam size for the dif-ferent receivers.

Observed center freq.

Pointing model residuals [arcsec]Rec.[GHz] az el Beam size [arcmin]

L 1.7057711.12C 7.3510.87.2 2.58K

23

3.9

3.2

0.805

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positions.The bands allocated to radio astronomical service (22–22.5and 23.6–24.0GHz)were conˉrmed to be fully usable.

We emphasize that several interfering signals turned out to actually be \self-RFI",i.e.they were produced by devices installed at the SRT site.Since most of those devices are going to be moved into a properly shielded room in a very near future,we expect that the RFI environment will greatly improve and we thus do not consider the acquisi-tions performed during the commissioning to be representative of the ˉnal site conditions.It must also be stressed,for the sake of the future users of the SRT,that spectral back-ends —and speciˉc mitigation techniques —are being developed in order to cope with RFI and are expected to be operative within 2016(see Table 4).3.4.BWG focus commissioning 3.4.1.System temperature

The C-band receiver measured system temperature versus the elevation position is shown for both polarizations (LCP and RCP)in Fig.8.Values were obtained for every 10 of elevation in the 6.7–7.7GHz sub-band.The atmosphere opacity was estimated by performing a skydip scan ( 0?0:014).

The single expected contributions to the system noise temperature are listed in Table 6.The sum of these elements (23.7K,22.0K)is comparable to the obtained measures,27and 25K,respectively for LCP and RCP.The 2-K di?erence between the ex-perimental curves is consistent with the di?erent receiver temperatures (T rx ),whereas the slight dif-ferences between theoretical and experimental values can be explained with a

higher-than-predicted

Fig.7.Spectral plots illustrating the RFI a?ecting the various receiver bands.These \max hold"plots show all the signals received while observing in the whole azimuth range,for several elevations above the

horizon.

Fig.8.Tsys measured in the band 6.7–7.7GHz.Circles:LCP.Diamonds:RCP.

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spill-over temperature (mainly coming from the BWG mirrors).

Table 6includes an extra-noise term caused by the cover of the Gregorian room,a 1-mm Te°on ˉlm 1.5m in diameter,which protects the apparatus from the external environment.The ˉlm trade name is Virgin PTFE,and the refraction index (not pro-vided as a function of frequency)declared by the manufacturer is 1.35,yielding a re°ection amount-ing to 4%.We measured the Tsys increase by tem-porarily removing the cover;results are summarized in Table 7,where the average of the recorded increments is shown for di?erent frequency bands.As shown in Table 7,such a contribution appears in the C-band measurements,but it is deˉnitively more signiˉcant in K-band.3.4.2.Gain curve

The e±ciency measurement and the gain curves were produced exploiting the continuum back-end,performing cross-scans in the Horizontal frame on bright,point-like calibrators (Table 8).Atmo-spheric opacity was extrapolated from skydip scans.

This campaign was repeated in two phases,re°ecting two di?erent telescope conˉgurations.During the ˉrst phase,the main mirrors were ˉxed to the reference mechanical alignment.The second session was carried out when the active surface and the sub-re°ector were aligned according to photo-grammetry,in order to compensate for the gravi-tational deformations that vary with elevation.

The antenna gain in the 7.0–7.7GHz band is shown in Fig.9for the ˉxed optics.The plot

compares the theoretical values (triangles)with the experimental measurements for the LCP channel (circles).The expected curve was obtained taking into account both the receiver and the antenna ef-ˉciency parameters,including the surface e±ciency computed from the RMS of the mirrors,in the hy-pothesis that the optics were aligned at all the ele-vations (not achievable in real measurements,as the subre°ector was not tracking).The trend of the measured curve re°ects the expected one,however a gain o?set (about 10%–15%)is evident at all ele-vations —including 45 ,where the antenna optics were supposed to be e?ectively aligned.

The plots of Fig.10show the BWG receiver gain curves for both the circular polarizations,obtained with active surface and subre°ector tracking.The °atness of the curves along all the elevation range demonstrates a great improvement due to the compensation for structural deforma-tions;on the other hand,the peak is slightly below expectation (0.61instead of 0.66K/Jy).We might

Table 6.

Contributions to the Tsys in the 6.7–7.7GHz band.

Contribution

Notes T rx

LCP 8.5K RCP 6.8K Lab measurements Gregorian cover 1.4K Observations

Atmosphere tCMB 4.1K ?0:014at zenith Ground spill-over 2.7K Simulation BWG spill-over

7.0K

Simulation

Table 7.

Tsys contribution by the Gregorian room cover.

Cover,Tsys contribution (K)

6.7–

7.7GHz 18–20GHz 20–22GHz 22–24GHz 24–26GHz 1.4

11.2

9.2

8.3

9.8

Table 8.List of the °ux density calibrators used for C-band measurements (Ott ,1994).

Source RA J2000Dec J2000Flux density at 7.35GHz [Jy]

3c4801:37:41.2971t33:09:35:118 3.79753c14705:42:36.1379t49:51:07:234 5.29113c28613:31:08.2881t30:30:32:960 5.70713c309.114:59:07.578t71:40:19:850 2.34333c29514:11:20.6477t52:12:09:141 4.05883c161

06:27:10.096

à05:53:04.72

4.3614

Fig.9.Gain with mechanically aligned mirrors:measurements (red circles)and expected values (triangles).

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then conclude that the optical alignment of the main and secondary mirrors is quite satisfactory for this wavelength,and that the employment of the active surface is able to deliver an almost constant gain.

3.4.3.Beam size

Figure 11shows the beam deformation (")with respect to the estimated HPBW (beam size, e ),for both telescope conˉgurations (ˉxed and active sur-face).The beam size is a reliable indication of how

the optics of the telescope re°ect the theoretical design;accordingly,the deformation of the beam along the elevation span could be a symptom of a °awed telescope structure.The beam deformation is computed through Eq.(1),where az and el are the beam sizes measured along the azimuth and eleva-tion axis,respectively.The comparison of the two curves led us to the same conclusion inferred through the gain curves:the e±ciency reduction observed with the ˉxed alignment is almost completely recovered using the active surface and the tracking subre°ector.

"?????????????????????????????????????????????????

e az à e T2te el à e T2p :e1T3.5.Gregorian focus commissioning

The characterization of the K-band receiver,located in the Gregorian focus,was obviously complicated by the impact of weather conditions on radiation at these wavelengths.It was thus decided,so as to achieve very accurate and e?ective measurements,to limit the commissioning activities to the time intervals showing low and stable opacity.During these clear-sky periods the opacity at the site turned out to be particularly favorable.3.5.1.System temperature

The K-band receiver Tsys measurements were per-formed following the same procedures employed for the BWG receiver,for both the polarizations,repeating the procedure for each 2-GHz sub-band.Figure 12shows the LCP values for the central

feed.

Fig.10.C-band measured gain curves with the active surface

and the sub-re°ector movement operating:LCP (top),RCP (bottom).Dashed lines indicate the expected values in an ideal

scenario.

Fig.11.C-band beam deformation.Squares:with ˉxed optics.Circles:with active surface and tracking subre°ector ( ?1:45

arcsec).

Fig.12.Tsys versus elevation measured in 2-GHz sub-bands,LCP of central feed.The zenith opacity values are also reported.

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The atmospheric contribution was evaluated by means of skydip scans,whose results were compared to the radiometer measurements.Table 9lists the main Tsys contributions.

Even taking into account the extra system temperature contribution due to the cover ˉlm,we ˉnd that the measured values do not match expec-tations.The sum of these contributions reaches 49K,quite distant from the observed value of 73K if we refer to the 18–20GHz band.The cause of such a di?erence between measured and theoretical was not completely understood and therefore is still under investigation.3.5.2.Gain curve

The list of calibrators observed during the e±ciency measurements is given in Table 10.Each calibrator was sampled after performing preliminary cross-scans to focus the telescope and achieve an opti-mized pointing.The atmospheric opacity was again estimated by means of skydip acquisitions.

A preliminary gain curve,achieved in the 22–24GHz band,was acquired with the ˉxed optics conˉguration.The experimental and the theoretical results,in Fig.13,are comparable.Measurements show lower values with respect to simulations,as the latter were computed taking into account con-stantly aligned —yet non-active —optics,while measurements were carried out keeping the mirrors in their reference positions,i.e.the positions

obtained with their mechanical alignment at 45 of elevation.In this reference position,where the optics alignment should be ideal,a slight loss in the peak gain is still noticeable (0.52versus 0.55K/Jy).This can be explained as a residual inaccuracy in the mechanical alignment.

Similarly,the gain curves for both LCP and RCP were measured enabling the active surface and the subre°ector tracking.We expected the curve to be °at at 0.66K/Jy,taking into account the RMS of the surfaces of mirrors,the alignment of M1—estimated with photogrammetry —and other involved para-meters such as:accuracy of the alignment of the subre°ector panels,errors in the measurements on panels,gravitational,thermal and wind e?ects on panels,positioning accuracy of the actuators.The plots in Fig.14clearly show that,in terms of peak gain,the data match the predictions.On the other hand,even if the overall telescope e±ciency beneˉts from the active surface and the tracking of the

Table 9.

Tsys contributions in all the receivers sub-bands,measured with the central feed.

18–20GHz

20–22GHz 22–24GHz 24–26GHz Notes T rx LCP 23.7K 24.4K 29.7K 33.9K Lab meas.T rx RCP

21.6K 18.0K 22.7K 27.1K Gregorian cover

11.2K 9.2K 8.3K 9.8K Observation Atmosph :tCMB ( )11.4K 12.7K 16.8K 14.7K El ?90 (0.040)(0.045)(0.060)(0.052)Ground spill-over

2.3K

0.6K

0.6K

0K

Simulation

Table 10.Flux density calibrators,used for the K-band gain curve (Ott ,1994).Source RA J2000Dec J2000Flux density at 22.35GHz (Jy)

3c4801:37:41.2971t33:09:35:118 1.20573c14705:42:36.1379t49:51:07:234 1.76553c28613:31:08.2881t30:30:32:960 2.4330NGC7027

21:07:01.593

t42:14:10:18

5.3890

Fig.13.Opacity-corrected gain curves for the K-band re-ceiver,with ˉxed optics.Theoretical values (triangles)were computed considering aligned mirrors at each elevation.Mea-surements (circles)were acquired with both the mirrors ˉxed in the mechanically aligned positions.

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subre°ector,there is still a substantial decrease in the e±ciency below 45 of elevation.This excess of gain loss at lower elevations can easily be translated in a surface accuracy of about 500micron (El ?20 ).Our investigations suggest that gravity deformation measurements were not su±ciently accurate at low elevations.In view of the installation of higher fre-quency receivers,this problem shall be solved;how-ever,this will be done in the reˉnement phase by using the microwave holography technique.3.5.3.Beam size and bidimensional pattern Measurements of the beam size and pattern were performed alternatively enabling and disabling the active surface,in order to verify its e?ectiveness and to roughly evaluate the quality of the optical alignment.The outcome of these experiments was also meant to be exploited to draw further conclusions about the observed gain curves,and to prove whether there was a relation between potential alignment inaccuracies and the gain drop observed at low elevations.

Figure 15compares the beam deformation "(Sec.3.4.3,Eq.(1)),measuring the deviation from the nominal HPBW,observed in the two telescope con-ˉgurations (ˉxed and active surface)by means of cross-scans on non-resolved sources.Figure 16,instead,shows two raw maps in arbitrary counts,acquired on one of these sources.They illustrate the antenna beam patterns,respectively obtained

with

Fig.14.Gain curve in the 22.0–22.7GHz band.LCP (top)and RCP (bottom)for the central feed,acquired enabling the active surface and the subre°ector tracking.Dashed lines rep-resent the expected values in ideal

conditions.

Fig.15.Beam deformation,with respect to the ideal beam size,versus elevation.Squares:with ˉxed optics.Circles:with active surface and subre°ector in tracking mode ( ?2:25

arcsec).

Fig.16.Two az –el raw maps of the same point-like source.Fixed optics (top)and active surface plus tracking subre°ector (bottom).Both for El >60 .

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the ˉxed and active mirrors,in the same elevation range.

The telescope in ˉxed optics conˉguration showed a beam size rapidly deforming as the pointed elevation deviated from the mechanical alignment elevation (45 ).Astigmatism and coma lobes,revealed in Fig.17(top),explain such a dis-tortion,at least for the upper range of elevation (El >60 ),that is compatible with the gain drop shown in Fig.13.The pattern in Fig.17(top)shows also a distortion tilting at about 45 with respect to the az –el axes.This deformation is not aligned with the gravity vector,so it must be a contribution of several di?erent ones.The compensation of gravity deformations obtained employing the active surface of the primary mirror,on the other hand,allowed us to obtain a particularly e?ective alignment for ele-vations above 45 .Figure 17(bottom)demon-strates that most of the distortions of the antenna beam pattern had disappeared in this range.An asymmetry in the sidelobes is still visible,likely due to some residual squint.

Finally,the map in Fig.18,taken at the average elevation of 22 ,highlights a signiˉcant asymmetry and deformation,visible both in the main beam and in the ˉrst sidelobe.This deformation can be con-sidered in agreement with the characteristics of the gain curve presented in Fig.14,as it can be explained by the current status of the optics.3.6.Primary focus commissioning 3.6.1.System temperature

After the preliminary tests,the primary focus re-ceiver turned out to have an issue in the microwave

chain responsible for the signal phase shifting pro-ducing circular polarizations from the native linear ones.The problem a?ected the RCP only:the noise calibration signal was not detectable for this chan-nel.The cause,although immediately identiˉed,could not be ˉxed before the completion of the commissioning,thus no further measurements could be performed.For this reason,this paper discusses only the results for the LCP.

Table 11presents the theoretical contributors leading to an estimated temperature of 27.0K when the telescope is parked at zenith and up to 44.0K when the elevation angle is 5 .Even considering all the caveats, e.g.the intrinsic inaccuracy in the prediction of the noise calibration signal level —the system temperature is higher than expected possibly due to an underestimation of the simulated spill-over and/or other noise sources.Figure 18for example provides hints about the in°uence of RFI on the measured Tsys values:it shows clustered measurements,in particular in the elevation range from 15 to 50 ,creating \jumps"in the Tsys trend.This likely derives from the presence of variable RFI a?ecting the signal level.3.6.2.Gain curve

The L-band gain curve is given in Fig.19.It shows slightly scattered measurements but,in this case,the gain value equal to 0.52K/Jy is entirely in agreement with the expectations (0.50–0.55K/Jy for the band under

test).

Fig.17.Az –el raw map over a point-like source with active surface enabled and tracking subre°ector (average elevation 22

).

Fig.18.System noise temperature versus elevation for the L-band primary-focus receiver (1.6965–1.7150GHz).

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These measurements were performed using the active surface,yet only to re-shape the primary mirror in order to obtain a parabolic proˉle.Ele-vation-dependent corrections were not applied,as it is not necessary to compensate for gravity defor-mations at these low frequencies.The parabolic proˉle allowed us to increase the antenna gain with respect to the original quasi-Gregorian https://www.wendangku.net/doc/9f9243365.html,ing the latter,the antenna gain would rapidly decrease for frequencies above 1.3GHz,reaching 0.4K/Jy at 1.7GHz.The re-shaping of the surface might allow us to proˉtably employ receivers up to a frequency of 22GHz in the primary focus.

3.7.Summary of the commissioning

measurements and milestones The entire process of the telescope commissioning took almost 18months,from mid 2012to the end of 2013.Table 12shows the schematic timeline of the major steps carried out,including examples of the technical advancement that has been taking place in parallel to the AV activities.

Table 13ˉnally summarizes the main telescope measured parameters.

Table 11.Tsys contributions (1.6965–1.7150GHz).

Contribution

Notes T rx

LCP 13.0K Lab measurements

Atmosphere tCMB El ?90 5.0K Observations and El ?5 22.0K

simulations Ground spill-over 9.0K

Simulations

Fig.19.Gain curve for the L-band receiver (1.6965–1.7150GHz).The dashed line indicates the expected peak gain.

Table 12.Main milestones of the commissioning and post-commissioning technical activities.Task description

Execution period Antenna Software Interface tests

2012June Gregorian Feed Rotator and M3Rotator commissioning 2012July

Minor Servo Software integration and veriˉcation 2012July –2013September Gregorian focus receiver integration

2012July First radio source with the C-band receiver 2012August

Gregorian Focus commissioning

2012August –2012October Primary Focus Positioner and Subre°ector commissioning 2012August –2013September BWG Focus commissioning 2012October –2012December Primary Focus commissioning

2013May –2013July

Active Surface Software integration and veriˉcation 2013January –2013September Inauguration

2013September 30

Installation of back-ends (successive to the Total Power back-end)2013October –2014December Upgrade of the Nuraghe control system to comply with the AV needs

2015

June

Table 13.Summary of the telescope performance measurements for the ˉrst-light receivers.Tsys and peak gain are measurable with an accuracy of 5%due to the intrinsic uncertainty in the temperature of the noise calibration signal.

Observed center freq.[GHz]

Beam size [arcmin]Tsys,El ?90 [K](?5%)Tsys,El ?30 [K](?5%)Peak gain [K/Jy](?5%)Rec.LCP RCP LCP RCP LCP RCP L 1.70511.12?0.1134n.a.35n.a.0.52(44.6%)n.a.C 7.35 2.58?0.022********.61(52.4%)0.60(51.5%)K

23

0.805?0.04

74

7789

92

0.66(56.6%)

0.65(55.8%)

P.Bolli et al.

J . A s t r o n . I n s t r u m . 2015.04. D o w n l o a d e d f r o m w w w .w o r l d s c i e n t i f i c .c o m b y 202.127.29.244 o n 04/19/16. F o r p e r s o n a l u s e o n l y .

4.Conclusions

During the technical commissioning phase,all the devices foreseen for the telescope early activities were successfully tested.The SRT overall perfor-mance was proven to be close enough to expecta-tions.Three receivers (L-,C-and K-band)and a Total Power back-end were fully characterized.The new antenna control system was continuously im-proved and updated during the activities,allowing for the execution of test observations in the most common single-dish modes.The capabilities of the primary re°ector active surface and of the tracking subre°ector were widely demonstrated,as the measurements performed enabling these devices turned out to be almost elevation-independent.Reˉnements regarding the optics are still needed only for K-band observations;further improve-ments,also in anticipation of the installation of even higher frequency receivers (up to 100GHz),will be achieved utilizing microwave holography and me-trology techniques,at present under investigation and testing.

As concerns the environmental conditions of the telescope site,our experiments conˉrmed that the location meets the requirements for high-frequency observations.The occurrence of RFI —aggravated by the local presence of temporarily unshielded ap-paratuses —was found to be an impediment only for P-band observations;the C band was mostly employable and the K band was conˉrmed to be particularly unpolluted —especially above 20GHz.

The telescope was hence made available to the Astronomical Validation team,in charge of asses-sing its scientiˉc potentials,while proceeding with the installation and testing of additional devices —such as digital spectrometers —in view of the shortcoming opening of the SRT to the worldwide community.Acknowledgments

The SRT project was funded mainly by the Italian Ministry of Education,University and Research,joined by ASI and the Sardinian Regional Govern-ment,the latter supporting most of the local infra-structural activities.In the initial phases,the project was managed by CNR,after which it was passed to INAF.Three institutions belonging to INAF made the technical and managerial e?orts:IRA,OAC and OAA.The Nuraghe control system is one of the creations of the INAF project named

DISCOS,aimed at providing the Italian radio tele-scopes with uniˉed managing software.

The contract for the construction of the tele-scope structure was awarded to MT Mechatronics GmbH,Mainz,Germany.

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