Mem. S.A.It. Suppl. Vol. 10, 19c© SAIt 2006

Memorie della

Supplementi

SRT: design and technical specifications

L. Olmi1 and G. Grueff2

1 INAF–Istituto di Radioastronomia, Sez. di Firenze, Largo E. Fermi 5, I-50125 Firenze2 INAF–Istituto di Radioastronomia, Via P. Gobetti 101, I-40129 Bologna

Abstract. The Sardinia Radio Telescope (SRT) is a general purpose, fully steerable radiotelescope currently being built in Sardinia, that will be operated by the National Institutefor Astrophysics (INAF). The SRT will be a large-aperture antenna (64-m diameter) with aclassical Gregorian configuration and the possibility to use three different focal positions.The primary and secondary reflectors are shaped to minimize spillover and the standingwaves between the feed and the subreflector. The primary active surface will give the SRTthe possibility to operate in the 3-mm band, and will thus contribute significantly to manyscience areas. At present, the foundation of the antenna is almost complete, and the compo-nents of the alidade and of the track are under construction.

1. Introduction

The SRT project has been proposed and man-aged by the Istituto di Radioastronomia, for-merly a scientific structure of the NationalResearch Council (CNR) and now belong-ing to INAF. The project is funded by theItalian Ministry of University and of Scientificand Technological Research (MURST) and bythe Sardinian Regional Government. The SRTproject aims to construct a general purpose,fully steerable, 64-m diameter “shaped” ra-dio telescope capable of operating with highefficiency from 300 MHz to over 100 GHz.When the active surface will become fully op-erational, the expected antenna efficiency willrange from the maximum value of about 63%(at 10 GHz) to about 35% (at 100 GHz).

The selected site (Lat 39◦ 29′ 50′′ N, Lon09◦ 14′ 40′′ E) is the area named “PranuSanguni” close to the town of San Basilio,about 35 km North of Cagliari, and is at about700 meters above sea level. This site is charac-terized by low radio frequency interference, it

is reasonably dry during the winter season and,finally, it is located in an orographic depressionthat acts as a natural wind-screen (the recordedaverage wind speed is about 4 m/s).

The SRT will be used as single-dish andalso as a fundamental component of the en-hanced VLBI network. The combination ofmultiple focal positions and a wide frequencyrange will offer the users of the SRT a greatflexibility. The main antenna geometry is ashaped reflector system pair, based on the clas-sical Gregorian configuration. The shaped sur-faces eliminate the multiple reflections be-tween the secondary and the feed, while opti-mizing field-of-view and antenna efficiency.

2. Mechanical structure

The antenna design is based on the clas-sical wheel-and-track configuration (Fig. 1).The main reflector consists of a backstructurewhich, through several actuators, supports thesurface composed of rings of reflecting panels.The primary reflector surface consists of 1008

20 Olmi & Grueff: SRT: design and technical specifications

Fig. 1. Schematic view of the SRT.

individual aluminum panels (with a panel man-ufacturing RMS < 70 µm), with a surface areabetween 2.4 m2 and 5.3 m2, which are dividedinto 14 concentric rings. The actuators systemis composed of 1120 precision actuators (max-imum error 15 µm) under computer control,to compensate gravitational and thermal distor-tions of the backstructure. The subreflector sur-face consists of 49 individual aluminum panels(with a panel manufacturing RMS < 50µm),having an average area of about 1 m2, with onecentral panel and the other 48 panels assem-bled into 3 rings. The position of the subreflec-tor relative to the primary mirror is constantlycontrolled in real time by means of six largeprecision actuators and an elaborate metrologysystem.

A quadrupod connected to the backstruc-ture supports the subrefector, the prime focuspositioner and instrumentation. The SRT ali-dade is a welded steel structure on a large con-crete tower that forms the bulk of the antennafoundation. A pintle-bearing at the top of thetower provides support against lateral loads.The mechanical design provides a nearly uni-form loading condition on the reflector in or-der to maintain symmetry (homology) in thesurface deflections. The reflector backstructure

is attached to the elevation wheel through amassive pyramidal structure. This design al-lows the construction of a large room withinthe alidade for installing instrumentation at theGregorian and Beam Wave Guide (BWG) fo-cal positions.

One advantage of the Gregorian configura-tion is its capability to have prime focus in-struments that can be positioned and movedaway when necessary for observations fromthe Gregorian or BWG focal positions. To ac-cess the prime focus on the SRT a rotating armis eccentrically mounted near the secondarymirror. It may house several feed horns and as-sociated cryogenic receiving systems for oper-ating over a range of radio frequencies from300 MHz to 20 GHz. A drive system can rotatethe arm and then two servo-motors can movethe selected feed assembly into the position ofthe prime focus.

3. Optics

As mentioned earlier, the SRT optical design isbased on the classical symmetrical Gregorianconfiguration (Fig. 2); however, the primaryand secondary surfaces are “shaped”, i.e. theycannot be described with a classical conical or

Olmi & Grueff: SRT: design and technical specifications 21

Fig. 2. Optical configuration and ray tracing of the SRT.

aspherical surface and are given in terms of atable of sags, with rotational symmetry, as afunction of the radial coordinate. In the past,the introduction of shaped surfaces in antennadesign had the purpose to provide the maxi-mum antenna efficiency for tracking or observ-ing point sources. The condition of maximumantenna efficiency naturally leads to havinguniform illumination over the antenna aper-ture, which cannot be achieved with any stan-dard feed-horn. As a consequence, design tech-niques have been developed where special sur-faces, which can be calculated numerically, ac-cept the non-uniform illumination of the feedand redistribute it uniformly over the antennaaperture (Galindo 1964; Collins 1973). Theconservation of energy flow along the ray tra-jectories, the condition of uniform illuminationover the antenna aperture and the application ofSnell’s law are sufficient to completely specifythe antenna surfaces.

However, antenna systems shaped for op-timal axial performance fail to meet the basiccriteria required for reasonable off-axis perfor-mance, because they violate the Abbe sine con-dition (Hudson 1989). The predominant off-axis aberrations are severe coma and field cur-

vature. For this reason, the SRT optics weredesigned to increase the field-of-view (FOV)for off-axis imaging scanning, by using re-flector shapes close to the classical Gregoriansystem, but with reduced center illuminationof the subreflector (Cortes-Medellin 2002)to minimize multiple reflections between thefeed and the secondary mirror, which haveadverse effects on spectroscopic observations.In addition, the SRT shaping design also re-duces the main reflector edge illumination re-sulting in low sidelobes and low antenna noisetemperatures (Fig. 3). Thus, the SRT shapingachieves a trade-off between antenna efficiencyand extent of the FOV, which will not pre-vent the use of heterodyne focal plane arrays(Olmi, these Proceedings). The resulting FOVat the Gregorian focus is, e.g., more than 4′ at22 GHz and about 1′ at 100 GHz.

4. Active surface

At high frequencies, and particularly in the 3-mm band, the SRT becomes electrically verylarge (D/λ > 2×104) and thus an actively con-trolled surface (ACS) is required to adjust thesurface sections in (quasi-) real time to main-

22 Olmi & Grueff: SRT: design and technical specifications

Fig. 3. SRT aperture illumination.

Table 1. Focal positions and frequency rangesof the SRT

Focal Frequency F/D ratioposition range (GHz)Prime 0.3< ν <20 0.33Gregorian 7.5< ν <∼100 2.35BWG 1.4< ν <35 1.37 & 2.84

tain the nominal shape of the primary reflector.The ACS mainly consists of a series of sensorsand an active control system to compensaterepeatable gravitational deformations, thermaldeformations with long (>∼ 0.5 hr) time-scales,and possibly quasi-static wind loads. In theSRT the ACS will also be used to convert theshaped primary reflector into a paraboloid foroperations at the prime focus.

The development of the ACS and metrol-ogy system for the SRT is still ongoing, but itis likely to be very similar to the Flexible BodyCompensation (FBC) system proposed for theLarge Millimeter Telescope (LMT). This tech-nique uses panel actuators, thermal sensors,inclinometers and a focus-center device forcorrecting the subreflector position. Therefore,this method relies on a correct understand-ing and modeling of the telescope structure,which is different from, e.g., the Green Bank

Telescope (GBT) approach, where the geomet-rical shape of the deformed surface is mea-sured directly, with a huge sensing effort (Olmi2003 for a review).

5. Receivers and focal positions

One of the special characteristics of the SRTis the availability of three different focal posi-tions, summarized in Table 1, to operate in dif-ferent frequency ranges. The use of the primefocus position is possible also at relatively highfrequencies, thanks to the ability of the activesurface to convert the shaped primary reflectorinto a paraboloid, and can be accessed throughthe Prime Focus positioner assembly describedabove. It will be used mainly for P- and L-bandreceivers. The Gregorian focal position is lo-cated on the upper level of the tilting receivercabin, and can be accessed by up to eight re-ceivers, for operations from 7.5 GHz to the 3-mm band, housed on a rotating turret 3.7 m indiameter (Gregorian positioner assembly; seeFig. 4). The turret is also provided with a circu-lar hole to illuminate the BWG optics locatedon the lower level of the receiver cabin.

The BWG optical system consists of anaxial rotating tertiary mirror and fixed relaymirrors to provide up to four additional focal

Olmi & Grueff: SRT: design and technical specifications 23

Fig. 4. Receiver cabin of the SRT with theGregorian positioner assembly and the BWG sys-tem.

points. The two BWG layouts currently de-signed provide two different focal ratio trans-formations: the BWG-I layout converts theGregorian focal ratio, from F = 2.35 toF = 1.37, whereas the BWG-II layout convertsfrom F = 2.35 to F = 2.84 and was designedsuch that the output focal point lies beneath theelevation axis of the antenna. The BWG focalpositions will be used at frequencies betweenabout 1.4 and 35 GHz. The BWG focus canalso host transmitting equipment, suitable forSpace Radio Science and the active tracking ofspacecraft.

6. Current status of work

The foundation of the telescope is a circularring of reinforced concrete with a diameter ofabout 40 m. Its design is based on an octago-nal structure, consisting of concrete arms thatconnect the central room to the perimeter wallthat will support the antenna track: this struc-ture is almost completed (Fig. 5). The contractfor engineering and fabrication of the me-

chanical structure has been awarded to MANTechnologie AG (Mainz, Germany), and thebeams and nodes components of the alidadeare currently under construction (Figs. 6 and7). Shipping to Sardinia and installation onsite should be completed by the end of 2006.The contracts for the fabrication of the pri-mary and secondary reflector panels have beenawarded to Cospal (Bergamo, Italy). The con-tract for the fabrication of the panel actuatorssystem has been awarded to Vitrociset (Rome,Italy). All the mechanical parts (AZ track andwheels, EL-gears, bearings, etc.) have alreadybeen fabricated at several factories in Germanyand Italy (e.g., Fig 8).

Fig. 5. Aerial view of the current status of the SRTfoundation.

Fig. 6. A series of alidade beams already com-pleted.

24 Olmi & Grueff: SRT: design and technical specifications

Fig. 7. One of the structural nodes of the alidade.

Fig. 8. One of the wheels that will allow the AZmovement of the SRT.

7. Conclusions

The SRT is one of the next-generation cm- andmm-wave single-dish telescopes currently un-der construction and will constitute a highlyflexible, sensitive antenna that will also signif-icantly contribute to the Italian, European andinternational VLBI networks. Its novel, shapeddesign will allow high-sensitivity spectral ob-servations and will also have a field-of-viewlarge enough for populating the Gregorian fo-cal plane with arrays of receivers. The SRTwill be capable of operating with high effi-ciency in a wide frequency range, and partic-ularly in the astronomically important 3-mmwavelength band. Commissioning of the tele-scope and “early science” observations are ex-pected to take place during the year 2008.

References

Collins, G.W. 1973, IEEE Trans. AntennasPropag., AP-21, 309

Cortes-Medellin, G. 2002, Antennas andPropag. Soc. Intern. Symp., IEEE Vol. 4,136

Galindo, V. 1964, IEEE Trans. AntennasPropag., AP-12, 403

Hudson, J.A. 1989, Radio Science, Vol. 24, N.4, 417

Olmi, L. 2003, in ”Recent research devel-opments in Astronomy and Astrophysics”,Vol. 1, 433 (Research Signpost, Kerala,India)