Colin R. Cunningham

SCUBA: A common-user submillimetre camera operating on the James Clerk Maxwell Telescope

SCUBA, the Submillimetre Common-User Bolometer Array, built by the Royal Observatory Edinburgh for the James Clerk Maxwell Telescope, is the most versatile and powerful of a new generation of submillimetre cameras. It combines a sensitive dual-waveband imaging array with a three-band photometer, and is sky-background-limited by the emission from the Mauna Kea atmosphere at all observing wavelengths from 350 μμto 2 mm. The increased sensitivity and array size mean that SCUBA maps close to 10 000 times faster than its single-pixel predecessor (UKT14). SCUBA is a facility instrument, open to the world community of users, and is provided with a high level of user support. We give an overview of the instrument, describe the observing modes, user interface and performance figures on the telescope, and present a sample of the exciting new results that have revolutionized submillimetre astronomy.

SCUBA: a submillimeter camera operating on the James Clerk Maxwell Telescope

The Submillimeter Common-User Bolometer Array (SCUBA) is one of a new generation of cameras designed to operate in the submillimeter waveband. The instrument has a wide wavelength range covering all the atmospheric transmission windows between 300 and 2000 micrometer. In the heart of the instrument are two arrays of bolometers optimized for the short (350/450 micrometer) and long (750/850 micrometer) wavelength ends of the submillimeter spectrum. The two arrays can be used simultaneously, giving a unique dual-wavelength capability, and have a 2.3 arc-minute field of view on the sky. Background-limited performance is achieved by cooling the arrays to below 100 mK. SCUBA has now been in active service for over a year, and has already made substantial breakthroughs in many areas of astronomy. In this paper we present an overview of the performance of SCUBA during the commissioning phase on the James Clerk Maxwell Telescope (JCMT).

SPIRE beam steering mirror: a cryogenic 2 axis mechanism for the Herschel Space Observatory

The Beam Steering Mirror (BSM) subsystem is a critical part of the SPIRE Instrument for the ESA Herschel Space Observatory. It is used to steer the beam of the telescope on the photometer and spectrometer arrays in 2 orthogonal directions, for purposes of fully sampling the image, fine pointing and signal modulation. The UK Astronomy Technology Centre (ATC) is part of a consortium of 15 institutes in Europe and the USA which was formed to build SPIRE and which is lead by Dr M. Griffin of the University of Wales, Cardiff.

Mid-Infrared Volume Phase Gratings Manufactured using Ultrafast Laser Inscription

We report on the ultrafast laser inscription (ULI) of volume phase gratings inside gallium lanthanum sulphide (GLS) chalcogenide glass substrates. The effect of laser pulse energy and grating thickness on the dispersive properties of the gratings is investigated, with the aim of improving the performance of the gratings in the mid-infrared. The grating with the optimum performance in the mid-infrared exhibited a 1st order absolute diffraction efficiency of 61% at 1300 nm and 24% at 2640 nm. Based on the work reported here, we conclude that ULI is promising for the fabrication of mid-infrared volume phase gratings, with potential applications including astronomical instrumentation and remote sensing.

100 mK bolometers for the submillimetre common-user bolometer array (scuba) I. Design and construction

We describe the design and construction of bolometric detectors for SCUBA - the Submillimetre Common-User Bolometer Array for the James Clerk Maxwell Telescope in Hawaii. The instrument contains 131 individual detectors, in two arrays, optimized for the submillimetre atmospheric transmission windows. The detectors are cooled by dilution refrigeration to a temperature of 100 mK, so that the receiver performance will be limited by photon noise from the sky and telescope background in all wavebands. A future paper will describe the performance of the detectors with reference to typical data obtained during the laboratory commissioning period.

Performance of volume phase gratings manufactured using ultrafast laser inscription

Ultrafast laser inscription (ULI) is a rapidly maturing technique which uses focused ultrashort laser pulses to locally modify the refractive index of dielectric materials in three-dimensions (3D). Recently, ULI has been applied to the fabrication of astrophotonic devices such as integrated beam combiners, 3D integrated waveguide fan-outs and multimode-to-single mode convertors (photonic lanterns). Here, we outline our work on applying ULI to the fabrication of volume phase gratings (VPGs) in fused silica and gallium lanthanum sulphide (GLS) glasses. The VPGs we fabricated had a spatial frequency of 333 lines/mm. The optimum fused silica grating was found to exhibit a first order diffraction efficiency of 40 % at 633 nm, but exhibited approximately 40 % integrated scattered light. The optimum GLS grating was found to exhibit a first order diffraction efficiency of 71 % at 633 nm and less than 5 % integrated scattered light. Importantly for future astronomy applications, both gratings survived cooling to 20 K. This paper summarises the grating design and ULI manufacturing process, and provides details of the diffraction efficiency performance and blaze curves for the VPGs. In contrast to conventional fabrication technologies, ULI can be used to fabricate VPGs in almost any dielectric material, including mid-IR transmitting materials such as the GLS glass used here. Furthermore, ULI potentially provides the freedom to produce complex groove patterns or blazed gratings. For these reasons, we believe that ULI opens the way towards the development of novel VPGs for future astronomy related applications.

Multiple Integral Field Spectroscopy Using Image Slicers

We present the results of a detailed technical study of the use of image slicers for multiple integral field spectroscopy at infrared wavelengths. Our solution uses independently controlled robotic arms to relay selected portions of the focal plane to fixed positions where they are dissected using a set of advanced image slicers. We discuss the technical requirements of this approach and describe a feasibility study to examine the risks and technical challenges.

Long wavelength (SUB-mm) telescope simulator

An automated long wavelength (submillimetre) telescope simulator is described which produces a diffraction limited image in the simulator focal plane for laboratory measurement of the matching of receivers of photometers to the telescope being simulated.

Mid-infrared transmission gratings in chalcogenide glass manufactured using ultrafast laser inscription

Ultrafast laser inscription is a versatile manufacturing technique which can be used to modify the refractive index of various glasses on a microscopic scale. This enables the production of a number of photonic devices such as waveguides, beam-splitters, photonic lanterns, and diffraction gratings. In this paper, we report on the use of ultrafast laser inscription to fabricate volume phase transmission gratings in mid-infrared transmitting chalcogenide glass. We describe the optimisation of the laser inscription process parameters enhancing grating performances via the combination of spectrally resolved grating transmission measurements and theoretical analysis models. The first order diffraction efficiency of the gratings was measured at mid-infrared wavelengths (3-5 μm), and found to exceed 60% at the Littrow blaze wavelength, compared to a substrate external transmittance of 67%. This impressive result implies the diffraction efficiency should exceed 90% for a grating substrate treated with an anti-reflection coating. There is excellent agreement between the modelled grating efficiency and the measured data, and from a least squares fit to the measured data the refractive index modulation achieved during the inscription process is inferred. These encouraging initial results demonstrate that ultrafast laser inscription of chalcogenide glass may provide a potential new and alternative technology for the manufacture of astronomical diffraction gratings for use at near-infrared and mid-infrared wavelengths.

Towards freeform microlens arrays for near infrared astronomical instruments

A key requirement for astronomical instruments in next generation Extremely Large Telescopes (ELTs) is the development of large-aperture Integral Field Units (IFUs) that enable the efficient and spatially contiguous sampling of the telescope image plane for coupling stellar light onto a spectrometer. Current IFUs are complex to fabricate and suffer from stray light issues, which limits their application in high-contrast studies such as exoplanet imaging. In this paper, we present our work on the development of freeform microlens arrays using the rapidly maturing technique of ultrafast laser inscription and selective wet chemical etching. Using the focus spot from a femtosecond laser source as a tool with an essentially unrestricted “tool-path”, we demonstrate that it is possible to directly write the surface of a lenslet in three dimensions within the volume of a transparent material. We further show that high surface quality of the lenses can be achieved by using an oxy-natural gas flame to polish the lens surface roughness that is characteristic of the post-etched structures. Using our technique, the shape and position of each lenslet can be tailored to match the spatial positioning of a two-dimensional multimode fiber array, which can be monolithically integrated with the microlens array.