We present the status of the Sardinia Radio Telescope (SRT) project, a new general purpose, fully steerable 64 m
diameter parabolic radiotelescope capable to operate with high efficiency in the 0.3-116 GHz frequency range. The
instrument is the result of a scientific and technical collaboration among three Structures of the Italian National Institute
for Astrophysics (INAF): the Institute of Radio Astronomy of Bologna, the Cagliari Astronomy Observatory (in
Sardinia,) and the Arcetri Astrophysical Observatory in Florence. Funding agencies are the Italian Ministry of Education
and Scientific Research, the Sardinia Regional Government, and the Italian Space Agency (ASI,) that has recently
rejoined the project. The telescope site is about 35 km North of Cagliari.
The radio telescope has a shaped Gregorian optical configuration with a 7.9 m diameter secondary mirror and
supplementary Beam-WaveGuide (BWG) mirrors. With four possible focal positions (primary, Gregorian, and two
BWGs), SRT will be able to allocate up to 20 remotely controllable receivers. One of the most advanced technical
features of the SRT is the active surface: the primary mirror will be composed by 1008 panels supported by electromechanical
actuators digitally controlled to compensate for gravitational deformations. With the completion of the
foundation on spring 2006 the SRT project entered its final construction phase. This paper reports on the latest advances
on the SRT project.
In this paper, the design, manufacturing and testing of the optical subassembly specifically tailored for the acousto-optical Wide Band Spectrometer (WBS), subsystem of HIFI (Heterodyne Instrument for Herschel), is presented.
The WBS optical sub assembly consists of a laser source module with two collimated lasers and a prism beamsplitter, and imaging optic modules. The light source is a near-infrared laserdiode operating at 785 nm. The outgoing beam from the collimating unit is elliptical with 8 mm width and splitted by a prism device into four beams. The quadruplets are focussed in the vertical direction by means of a cylindrical element thus achieving a four "sticks" like focussed pattern in an intermediate focus where the acoustic channels of a Bragg cell are positioned. A combination of scanoptic and cylindrical lens is used to image the deflected light on a four line linear CCD. The laser source unit has been designed to operate under paraxial working conditions.
Despite the conceptually simple optical configuration, the system has represented a technological challenge, being of the order of few micrometres the integration scale for the optics and for the tight tolerance set requested in terms of degree of collimation and for the alignment precisions and stabilities over a wide range of temperatures and other environmental conditions.
A compact sub-millimetre wavelength Nb superconducting tunnel junction receiver (TIRGO) has been installed on the UKIRT facility, Hawaii. The receiver, used in combination with an acousto-optic spectrometer, exhibited excellent noise performance, achieving a best noise equivalent temperature of 280K (DSB) at 808GHz. Despite unfavourable observing conditions, spectral observations of a variety of astronomical sources were made that effectively verified the sensitivity and usefulness of the instrument for astronomical research. The design, construction and performance of the receiver system are described and some of the astronomical data acquired during the observation period briefly presented.
The measure of the faint polarized signal of the Cosmic Microwave Background (few percent of the CMB Anisotropy) requires instruments with very low contamination from systematic effects, high stability and high sensitivity. The BaR-SPOrt experiment, in sharing with the SPOrt project on ISS, is based on analog correlation receivers with components custom designed to match all of these requirements. Here we present the architecture, the design analysis and the status of the realization of the 32 GHz receiver.
This contribution gives a description of the Sardinia Radio Telescope (SRT), a new general purpose, fully steerable antenna proposed by the Institute of Radio Astronomy (IRA) of the National Institute for Astrophysics. The radio telescope is under construction near Cagliari (Sardinia) and it will join the two existing antennas of Medicina (Bologna) and Noto (Siracusa) both operated by the IRA. With its large antenna size (64m diameter) and its active surface, SRT, capable of operations up to about 100GHz, will contribute significantly to VLBI networks and will represent a powerful single-dish radio telescope for many science fields. The radio telescope
has a Gregorian optical configuration with a supplementary beam-waveguide (BWG), which provides additional focal points. The Gregorian surfaces are shaped to minimize the spill-over and the standing wave between secondary mirror and feed. After the start of the contract for the radio telescope structural and mechanical fabrication in 2003, in the present year the foundation construction will be completed. The schedule foresees the radio telescope inauguration in late 2006.
BaR-SPOrt (Balloon-borne Radiometers for Sky Polarisation
Observations) is an experiment to measure the linearly polarized
emission of sky patches at 32 and 90 GHz with sub-degree angular
resolution. It is equipped with high sensitivity correlation
polarimeters for simultaneous detection of both the U and Q stokes
parameters of the incident radiation. On-axis telescope is used to
observe angular scales where the expected polarization of the
Cosmic Microwave Background (CMBP) peaks. This project shares most
of the know-how and sophisticated technology developed for the
SPOrt experiment onboard the International Space Station. The
payload is designed to flight onboard long duration stratospheric
balloons both in the Northern and Southern hemispheres where low
foreground emission sky patches are accessible. Due to the
weakness of the expected CMBP signal (in the range of microK),
much care has been spent to optimize the instrument design with
respect to the systematics generation, observing time efficiency
and long term stability. In this contribution we present the
instrument design, and first tests on some components of the 32
Large, greater than about 1 Ghz, bandwidth acousto-optical spectrometers are based on Bragg cells made by LiNbO<SUB>3</SUB> working in birefringent regime. It is well known that the frequency response of this kind of cells is nonlinear. The purpose of this paper is the study of possible solutions for the correction of this nonlinearity. We studied several different optical design. We analyzed 3, 4 and 5 lenses optical design with particular care to distortions and quality of the reimaging. About distortion we designed some lenses slightly off-axis in order to achieve a partial compensation of the asymmetric distortion in the spectra introduced by the non-linear frequency response of the Bragg cell. A solution involving all cylindrical lenses is also briefly shown along with some practical drawback leading to the rejection of such an option. Finally a trade-off is performed in order to balance difficulty and cost of realization, the optical quality, and the true residual distortion. For multichannel Bragg cells the differential distortion between the various channels is also analyzed.
The wideband acousto-optical spectrometer (WBS) for HIFI- FIRST is comprised of two array-AOS with 4 times 1 GHz bands each. There are some advantages to this design, the most important one is that relative frequency and amplitude variations between the 4 bands are rather unlikely. This is demonstrated by laboratory tests, which verify also that fairly slow beam-switching at 0.5 Hz may be a sufficient chop speed for HIFI. The performance of array-AOS has also been demonstrated during measurements at ground-based observatories. WBS consists of three independent units, one IF-, one optics-, and one electronics-unit. Some of the details of the WBS design are described, and the present performance estimates are given.
We describe the preliminary design of the proposed Heterodyne Instrument for FIRST (HIFI). The instrument will have a continuous frequency coverage over the range from 480 to 1250 GHz in five bands, while a sixth band will provide coverage for 1410 - 1910 GHz and 2400 - 2700 GHz. The first five bands will use SIS mixers and varactor frequency multipliers while in the sixth band a laser photomixer local oscillator will pump HEB mixers. HIFI will have an instantaneous bandwidth of 4 GHz, analyzed in parallel by two types of spectrometers: a pair of wide-band spectrometers (WBS), and a pair of high- resolution spectrometer (HRS). The wide-band spectrometer will use acousto-optic technology with a frequency resolution of 1 MHz and a bandwidth of 4 GHz for each of the two polarizations. The HRS will provide two combinations of bandwidth and resolution: 1 GHz bandwidth at 200 kHz resolution, and at least 500 MHz at 100 kHz resolution. The HRS will be divided into 4 or 5 sub-bands, each of which can be placed anywhere within the full 4 GHz IF band. The instrument will be able to perform rapid and complete spectral line surveys with resolving powers from 10<SUP>3</SUP> up to 10<SUP>7</SUP> (300 - 0.03 km/s) and deep line observations.