Using the long-established Cardiff metal-mesh filter technology, we have exploited our ability to artificially manipulate the phase of a wavefront across a device in order to produce a dielectric-based Toraldo pupil working at millimeter wavelengths. The use of a Toraldo pupil to push the angular resolution of an optical imaging system beyond the classical diffraction limit is yet to be realized in the millimeter regime, but is an exciting prospect. Here we present the design and measured performance of a prototype Toraldo pupil, based on a 5 annuli design.
The concept of super-resolution refers to various methods for improving the angular resolution of an optical imaging system beyond the classical diffraction limit. In optical microscopy, several techniques have been developed with the aim of narrowing the central lobe of the illumination Point Spread Function (PSF). In Astronomy a few methods have been proposed to achieve reflector telescopes and antennas with resolution significantly better than the diffraction limit but, to our best knowledge, no working system is in operation. A possible practical approach consists of using the so-called "Toraldo Pupils" (TPs) or variable transmittance filters. These pupils were introduced by G. Toraldo di Francia in 1952,1 and consist of a series of discrete, concentric circular coronae providing specific optical transparency and dephasing in order to engineer the required PSF. The first successful laboratory test of TPs in the microwaves was achieved in 2003,2 and in the present work we build upon these initial measurements to perform electromagnetic (EM) numerical simulations of TPs, using a commercial full-wave software tool. These simulations were used to study various EM effects that can mask and/or affect the performance of the pupils and to analyze the near-field as well as the far-field response. Our EM analysis confirms that at 20 GHz the width of the central lobe in the far-field generated by a TP significantly decreases compared to a clear circular aperture with the same diameter.
The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (BLASTPol) is a suborbital mapping
experiment designed to study the role played by magnetic fields in the star formation process. BLASTPol uses
a total power instrument and an achromatic half-wave plate to modulate the polarization signal. During its first flight from Antarctica in December 2010, BLASTPol made degree scale maps of linearly polarized dust emission
from molecular clouds in three wavebands centered at 250, 350, and 500 μm. This unprecedented dataset in terms
of sky coverage, with sub-arcminute resolution, allows BLASTPol to trace magnetic fields in star-forming regions
at scales ranging from cores to entire molecular cloud complexes. A second long-duration flight is scheduled for
December 2012.
The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (BLAST-Pol) is a suborbital mapping
experiment designed to study the role played by magnetic fields in the star formation process. BLAST-Pol is
the reconstructed BLAST telescope, with the addition of linear polarization capability. Using a 1.8m Cassegrain
telescope, BLAST-Pol images the sky onto a focal plane that consists of 280 bolometric detectors in three arrays,
observing simultaneously at 250, 350, and 500μm. The diffraction-limited optical system provides a resolution of
30"at 250μm. The polarimeter consists of photolithographic polarizing grids mounted in front of each bolometer/
detector array. A rotating 4K achromatic half-wave plate provides additional polarization modulation. With
its unprecedented mapping speed and resolution, BLAST-Pol will produce three-color polarization maps for a
large number of molecular clouds. The instrument provides a much needed bridge in spatial coverage between larger-scale, coarse resolution surveys and narrow field of view, and high resolution observations of substructure
within molecular cloud cores. The first science flight will be from McMurdo Station, Antarctica in December
2010.
The Balloon-borne Large Aperture Submillimeter Telescope (BLAST) is a sub-orbital experiment designed to study the process of star formation in local galaxies (including the Milky Way) and in galaxies at cosmological distances. Using a 2m Cassegrain telescope, BLAST images the sky onto a focal plane, which consists of 270 bolometric detectors split between three arrays, observing simultaneously in 30% wide bands, centered at 250, 350, and 500 μm. The
diffraction-limited optical system provides a resolution of 30" at 250 μm. The pointing system enables raster-like scans with a positional accuracy of ~30", reconstructed to better than
5" rms in postflight analysis. BLAST had two successful flights, from the Arctic in 2005, and from Antarctica in 2006, which provided the first high-resolution and large-area (~0.8−200 deg2) submillimeter surveys at these wavelengths. As a pathfinder for the SPIRE instrument on Herschel, BLAST shares with the ESA satellite similar focal plane technology and scientific motivation. A third flight in 2009 will see the instrument modified to be polarization-sensitive (BLAST-pol). With its unprecedented mapping speed and resolution, BLAST-pol will provide insights into Galactic star-forming nurseries, and give the necessary link between the larger, coarse resolution surveys and the narrow, resolved observations of star-forming structures from space and ground based instruments being commissioned in the next 5 years.
KEYWORDS: Receivers, Antennas, Mirrors, Telescopes, Radio astronomy, Control systems, Space telescopes, Radio telescopes, Astronomy, Electromagnetic coupling
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.
Advances in bolometric detector technology over the past decade have
allowed submillimeter wavelength measurements to contribute important
data to some of the most challenging questions in observational
cosmology. The availability of large format bolometer arrays will
provide observations with unprecedented image fidelity. The
Balloon-borne Large Aperture Submillimeter Telescope (BLAST) will be
one of the first experiments to make full use of this new capability.
The high altitude (~35$ km) of the balloon platform allows for
high-sensitivity measurements in the 250, 350 and 500 micron bands
with a total of 260 detectors.
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.
We discuss the conceptual and practical guidelines of a method to
calculate the cross-polarization of a telescope, including its relay optics, using a commercial optical design software, without the need to use complex, slow and expensive Physical Optics programs.
These effects are usually negligible at visible and infrared
wavelengths but may be of considerable importance at radio wavelengths. Offset reflector antenna configurations, common in the telecommunication industry, and antenna relay optics consisting of offset mirrors, common in millimeter and submillimeter-wave telescopes, result in an increased contribution to the cross-polarization. Polarization measurements are also becoming very important in Radio Astronomy. In fact, dust emission polarimetry and the study of linearly polarized, nonmasing, rotational lines (e.g., CO) with submm telescopes are both powerful diagnostic of magnetic fields in molecular clouds. However, the low average source polarization requires a careful optimization of the optical design to keep any instrumental polarization contribution from both telescope and relay optics as low as possible in astronomical polarimetry experiments.
Likewise, in telecommunications applications polarization separation
can be used to effectively double the available bandwidth provided the
isolation between the two orthogonal polarization states is sufficient.
In the troposphere water vapor plays a fundamental role in radio propagation. The refractivity of water vapor is about 20 times greater in the radio range than in near-infrared or optical regimes. As a consequence, phase fluctuations at frequencies higher than about 1 GHz are predominantly caused by fluctuations in the distribution of water vapor. On filled-aperture telescopes radio seeing shows up as an anomalous refraction (AR), i.e. an apparent displacement of a radio source from its true position. The magnitude of this effect, as a fraction of the beam width, is bigger on larger telescopes. I will thus present a model study of AR effects, obtained producing numerical simulations of two-dimensional phase screens. I will finally discuss the basic concept and requirements of a tip-tilt compensation system at millimeter wavelengths, and will also describe a proposed design based on a scanning microwave radiometer as a wave front sensing device.
Balloon borne instruments offer opportunities for astronomical observation in an environment that is superior to many ground observatories and less expensive than space borne observatories. BLAST is an infrared astronomical telescope that will use a high altitude balloon as an observation platform. The BLAST telescope has a unique set of requirements: lightweight, low cost, zenith to horizon pointing, 10 arc-second pointing accuracy, high Strehl ratio far infrared/sub-milllimeter observation. The design of the BLAST telescope takes advantage of a pre-existing 2-m experimental lightweight carbon fiber mirror. Using this spherical primary, an aspheric secondary was designed to produce a diffraction limited telescope at 250 microns. The telescope metering structure was designed and fabricated using high stiffness, low mass, thermally stable carbon fiber. The secondary mirror was aggressively lightweighted and fabricated using state of the art diamond turning technology. Design was verified prior to manufacturing using finite element structural analyses in order to demonstrate compliance with the deflection requirements of the secondary mirror when pointing zenith and near horizon. This paper will review the optical and opto-mechanical design, fabrication, integration and alignment of the BLAST telescope.
I discuss the guidelines to the optical design for (sub)millimeter telescopes employing a spherical primary and large-format bolometer arrays. Although various optical solutions for ground visible/IR telescopes using a spherical primary are discussed in the literature, these configurations are usually not acceptable for submillimeter-wave telescopes on orbital or sub-orbital platforms. I thus analyse alternative optical solutions that might be used on spherical-primary antennas, employing either on-axis or (cooled) off-axis optical elements to correct the spherical aberration introduced by the primary, that might be also suitable for ground-based telescopes. These configurations are discussed in relation to optical performance and various operational design constraints, using as a baseline spherical-primary telescope BLAST, the "Balloon-borne Large-Aperture Sub-millimeter Telescope", currently under construction. Modern (sub)millimeter telescopes also require optimized auxiliary optics to efficiently couple the telescope to the receiver over large fields of view. I will thus discuss how to analyze and optimize the design using both the Strehl ratio and the coupling efficiency to evaluate the quality of the off-axis wavefronts and the loss in the power coupling with the horn, using a customized optimization algorithm.
Radio seeing shows up on filled-aperture telescopes as an apparent displacement of a radio source from its true position, known as anomalous refraction (AR). The magnitude of this effect, as a fraction of the beam width, is bigger on larger telescopes. Here we report the partial results of systematic AR measurements conducted with the 14 m telescope of the Five College Radio Astronomy Observatory. The measured values range from approximately equals 2' (winter) to approximately equals 20' (summer). These data indicate that the pointing accuracy of large telescopes will be limited by tropospheric turbulence. We therefore discuss the basic concept and preliminary design of a tip-tilt compensation system at millimeter wavelengths that would use a 183 GHz radiometer as a wave front sensing device, capable of recovering most of the turbulence-induced pointing error.
The 'Large Millimeter Telescope' (LMT), or 'Gran Telescopio Milimetrico' (GTM), is a joint USA/Mexico Project and will be the worlds's largest short millimeter-wavelength ((lambda) equals 1 to 3.4 mm) radio telescope. The large collecting area, equivalent to a circular aperture of 50 m diameter, and the high altitude (4600 m) site will allow the LMT/GTM to achieve unprecedented astronomical performance. The telescope will be equipped with state-of-the-art heterodyne ((lambda) equals 3 mm) and bolometric ((lambda) equals 1 mm) focal plane arrays which will make it a powerful high angular resolution mapping instrument. Here, we present the current status of the LMT/GTM Project and its primary astronomical performance specifications. We then describe the optical design of the telescope, including some important aspects related to the wobbling subreflector and to the reimaging optics. We also analyze some of the electromagnetic characteristics of the alternative telescope configurations, enclosed or open-air, that were considered before the final selection of an open-air telescope took place.
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