A multimode horn differs from a single mode horn in that it has a larger sized waveguide feeding it. Multimode horns can therefore be utilized as high efficiency feeds for bolometric detectors, providing increased throughput and sensitivity over single mode feeds, while also ensuring good control of the beam pattern characteristics. Although a cavity mounted bolometer can be modelled as a perfect black body radiator (using reciprocity in order to calculate beam patterns), nevertheless, this is an approximation. In this paper we present how this approach can be improved to actually include the cavity coupled bolometer, now modelled as a thin absorbing film. Generally, this is a big challenge for finite element software, in that the structures are typically electrically large. However, the radiation pattern of multimode horns can be more efficiently simulated using mode matching, typically with smooth-walled waveguide modes as the basis and computing an overall scattering matrix for the horn-waveguide-cavity system. Another issue on the optical efficiency of the detectors is the presence of any free space gaps, through which power can escape. This is best dealt with treating the system as an absorber. Appropriate reflection and transmission matrices can be determined for the cavity using the natural eigenfields of the bolometer cavity system. We discuss how the approach can be applied to proposed terahertz systems, and also present results on how the approach was applied to improve beam pattern predictions on the sky for the multi-mode HFI 857GHz channel on Planck.
In this paper we describe the optical modelling of astronomical telescopes that exploit bolometric detectors fed by multimoded horn antennas. In cases where the horn shape is profiled rather than being a simple cone, we determine the beam at the horn aperture using an electromagnetic mode-matching technique. Bolometers, usually placed in an integrating cavity, can excite many hybrid modes in a corrugated horn; we usually assume they excite all modes equally. If the waveguide section feeding the horn is oversized these modes can propagate independently, thereby increasing the throughput of the system. We use an SVD analysis on the matrix that describes the scattering between waveguide (TE/TM) modes to recover the independent orthogonal fields (hybrid modes) and then propagate these to the sky independently where they are added in quadrature. Beam patterns at many frequencies across the band are then added with a weighting appropriate to the source spectrum. Here we describe simulations carried out on the highest-frequency (857-GHz) channel of the Planck HFI instrument. We concentrate in particular on the use of multimode feedhorns and consider the effects of possible manufacturing tolerances on the beam on the sky. We also investigate the feasibility of modelling far-out sidelobes across a wide band for electrically large structures and bolometers fed by multi-mode feedhorns. Our optical simulations are carried out using the industry-standard GRASP software package.
The robotic 2m Liverpool Telescope, based on the Canary island of La Palma, has a diverse instrument suite and a strong track record in time domain science, with highlights including early time photometry and spectra of supernovae, measurements of the polarization of gamma-ray burst afterglows, and high cadence light curves of transiting extrasolar planets. In the next decade the time domain will become an increasingly prominent part of the astronomical agenda with new facilities such as LSST, SKA, CTA and Gaia, and promised detections of astrophysical gravitational wave and neutrino sources opening new windows on the transient universe. To capitalise on this exciting new era we intend to build Liverpool Telescope 2: a new robotic facility on La Palma dedicated to time domain science. The next generation of survey facilities will discover large numbers of new transient sources, but there will be a pressing need for follow-up observations for scientific exploitation, in particular spectroscopic follow-up. Liverpool Telescope 2 will have a 4-metre aperture, enabling optical/infrared spectroscopy of faint objects. Robotic telescopes are capable of rapid reaction to unpredictable phenomena, and for fast-fading transients like gamma-ray burst afterglows. This rapid reaction enables observations which would be impossible on less agile telescopes of much larger aperture. We intend Liverpool Telescope 2 to have a world-leading response time, with the aim that we will be taking data with a few tens of seconds of receipt of a trigger from a ground- or space-based transient detection facility. We outline here our scientific goals and present the results of our preliminary optical design studies.
New developments in waveguide mode matching techniques are considered, in particular the efficient modeling of
waveguide cavity coupled detectors. This approach is useful in far-infrared astronomical instrumentation and cosmic
microwave background experiments in which bolometers feeding horn antennas or Winston cones are often employed
for high sensitivity, good control of stray light and well behaved beam patterns. While such systems can, in theory, be
modeled using full wave FEM techniques it would be desirable, especially for large structures in terms of the
wavelength, to exploit more efficient mode matching techniques, particularly for initial design optimization. This would
also be especially useful for cavities feeding partially coherent multi-mode horns or Winston cones. The mode matching
approach also allows for straightforward modeling of the complete coupling structure including the horn, waveguide
cavity and absorbing layer of the bolometer, thus marking a significant advance in the ability to predict the optical
efficiencies of cavity coupled bolometers. We consider typical single mode and multi-mode examples that illustrate the
power of the technique.
Efficient optical modelling in the far infrared is challenging because of the dominance of diffraction effects in typical
astronomical instruments. With the development of the next generation of array imagers and multi-moded feed systems
the necessity for computational efficiency has become critical to ensure an optimised design, comprehensive system and
telescope analysis and image deconvolution. A multi-technique capability is necessary to simulate both efficiently and
accurately the propagation of the signal collected by the telescope through the quasi-optical beam guide and feed
structures using an appropriate combination of modelling tools, seamlessly passing from one regime to the next from
detector to sky. Physical optics for example, although computationally intensive, is useful tool when detailed telescope
beam analysis is required, particularly for providing cross-polarisation information. Modal analysis is often appropriate
for modelling beam guide structures while analysing the detector feed coupling may rely on a more complete
electromagnetic analysis because of the small sizes involved and the use of waveguide and planar structures. Image
recovery ideally requires a deconvolution technique based on a modal approach and precise knowledge of the beams on
the sky. In this paper we report on our work in the continued development of such appropriate techniques with the
particular goal of prototyping powerful efficient computational tools for imaging arrays and partially coherent systems.
In the presentation, we will discuss these issues and present examples from real instrumentation.
Special approaches unique to the waveband are required for the modelling of terahertz optical systems. Ray tracing is
inadequate because in typical instruments the propagating beams are not very many wavelengths in diameter and a
"quasi-optical" approach is required in which Fresnel diffraction effects can be efficiently and accurately simulated.
Typically, it is also necessary to be able to simulate the coupling of quasi-optical beams to feed antenna structures to
predict optical performance. In many systems the beams can be considered to be coherent and their propagation through
a beam guide consisting of re-focussing elements can be efficiently modelled using modal analysis, especially useful for
quick design purposes, beam control and optimisation. This modal approach has been extended to allow for aberrations
and truncation particularly relevant in compact mirror based systems. At the same time physical optics, although
computationally intensive, is also a useful tool when detailed analysis is required, particularly for providing crosspolarisation
information and high accuracy far-field beam patterns from large reflecting antennas, for example. However,
modal analysis in general is a very powerful tool, which enables one also to understand issues associated with throughput
when partially coherent systems are being considered. This is important for the efficient optical modelling of large arrays
systems now being developed for next generation astronomical instrumentation. In the presentation, we will discuss
these issues and present examples from real instrumentation. We also summarise our continuing work on the
development of computationally efficient modelling tools for fast quasi-optical design and analysis.
The modeling of millimeter and sub-millimeter-wave optical systems requires special approaches. In many systems the
beams can be considered to be coherent and their propagation can be efficiently modeled using modal analysis,
especially useful for quick design purposes. Physical optics is also a useful tool when detailed analysis is required.
Modal analysis in general, however, is a very powerful technique, which enables one also to understand issues associated
with throughput when partially coherent systems and arrays are being considered. In the paper we discuss these issues
and present some examples from millimeter and submillimeter wave astronomical instrumentation.
A stochastic process of structural finite element simulation of a two mirror telescope has been combined with
vector electromagnetic analysis of the thermally stressed structure to determine the influence of material properties
on performance at terahertz frequencies. The output gives a reliable understanding of the correlation
between properties of the materials used in the building of the telescope, on a component by component basis,
and the aberrations of the optics under operating conditions. In this study the only stresses considered were
thermo-elastic and the optics were simple, but the method is generally applicable to any optical system and can
take account of both stress induced deformation and assembly tolerances.
This paper describes the opto-mechanical design of a large instrument for sub-mm, SCUBA-2, to be commissioned at JCMT. The scientific requirements, specially the large fov and the constraints of the telescope mechanical structure, lead to a complex optical design using freeform aluminium mirrors . The mechanical design is also challenging with large modules to be mounted and aligned in the telescope as well as the cryogenic instrument containing the mirrors, the filters, the dichroics and the detector modules. The cryogenic isostatic mounting, the structural and thermal designs are presented. This includes details of the fabrication of the structure and design of a shutter mechanism for operation at 4K. The results of the first AIV cool-down are also presented.
All current proposals for the construction of Extremely Large (30-
to 100-m) ground-based Telescopes (ELTs) assume segmentation of (at
least) the primary mirror. This has implications for the low-level
structure of their PSFs which are likely to be significant for a
number of important scientific applications, including the potential
detection and characterization of terrestrial planets orbiting
nearby solar-type stars. Several studies of these effects have been
carried out; all rely on Fourier methods applied to an approximate
model of the telescope entrance pupil. Concerned that these methods
may be prone to quite significant errors we have undertaken a
physical optics analysis. This numerical simulation uses methods
well-established in long-wave optics, with well-defined convergence
criteria, to model the PSF of a 50-m segmented-primary telescope.
Our results are in very close agreement with those from Fourier
methods when an F/16 system is modelled, but significant differences
are seen when a more realistic F/1 system is modelled. We also
present preliminary results of simulations of an "imperfect
telescope". The regular diffraction pattern seen from the perfect
mirror is almost completely destroyed even when the form errors and
alignment errors of the segments are sufficiently small to maintain
a Strehl ratio of 90%.
The optical modeling of large segmented telescopes presents an interesting technical challenge. Up to now the approach to the problem has been based upon Fourier optics. Analytic work and numerical simulations have been presented in referenced papers. So far as the author is aware, all numerical methods presented to date center upon an FFT on a planar phae mask that represents a perfectly regular segmented pupil. An alternaive approah based upon physcal optic has been investigated at the UK Astronomy Technology Center. The segmentation of the telescope primary mirror makes the physical optics computation an ideal candidate for distributed computing. This poster describes the program of work that has been followed to date.
MIRI, the mid-IR instrument for NGST is being provided by a collaboration between a consortium of European institutes, ESA, NASA, JPL and US scientists, with the Europeans responsible for the optics module. The instrument will provide diffraction limited imaging and spectroscopic capability over the 5-28μm region with unprecedented sensitivity. In this paper we describe the current optical design of the medium resolution spectroscopy channel (MIRI-S). This uses a novel arrangement of dichroics, image slicers and spectrometers to optimise the division of a limited number of detector pixels between spatial and spectral information whilst working within the tight mass and volume constraints imposed by a space mission. We also present our design for reflective image slicers that are adapted properly for diffraction limited performance to provide a high throughput over the full wavelength range of the instrument.
SCUBA-2 is a second generation, wide-field submillimeter camera under development for the James Clerk Maxwell Telescope. With over 12,000 pixels, in two arrays, SCUBA-2 will map the submillimeter sky ~1000 times faster than the current SCUBA instrument to the same signal-to-noise. Many areas of astronomy will benefit from such a highly sensitive survey instrument: from studies of galaxy formation and evolution in the early Universe to understanding star and planet formation in our own Galaxy. Due to be operational in 2006, SCUBA-2 will also act as a "pathfinder" for the new generation of submillimeter interferometers (such as ALMA) by performing large-area surveys to an unprecedented depth. The challenge of developing the detectors and multiplexer is discussed in this paper.
NAOMI (Nasmyth Adaptive Optics for Multi-purpose Instrumentation) is a recently completed and commissioned astronomical facility on the 4.2m William Herschel Telescope. The system is designed to work initially with Natural Guide Stars and also to be upgradeable for use with a single laser guide star. It has been designed to work with both near infrared and optical instrumentation (both imagers and spectrographs). The system uses a linearised segmented adaptive mirror and dual-CCD Shack-Hartmann wavefront sensor together with a multiple-DSP real-time processing system. Control system parameters can be updated on-the-fly by monitoring processes and the system can self-optimize its base optical figure to compensate for the optical characteristics of attached scientific instrumentation. The scientific motivation, consequent specification and implementation of NAOMI are described, together with example performance data and information on future upgrades and instrumentation.
A conceptual design for the WHT Natural Guide Star Adaptive Optics system, now called NAOMI (Nasmyth Adaptive Optics for Multi-purpose Instrumentation) was presented at the 1995 SPIE meeting1. Although the general principle of using off-axis paraboloidal mirrors (OAP) as a collimator- camera combination, with a deformable mirror located between them in the collimated beam, remains the same many features of the design have been improved.