Peter Luke

The VISTA IR camera

The VISTA IR Camera has now completed its detailed design phase and is on schedule for delivery to ESO’s Cerro Paranal Observatory in 2006. The camera consists of 16 Raytheon VIRGO 2048x2048 HgCdTe arrays in a sparse focal plane sampling a 1.65 degree field of view. A 1.4m diameter filter wheel provides slots for 7 distinct science filters, each comprising 16 individual filter panes. The camera also provides autoguiding and curvature sensing information for the VISTA telescope, and relies on tight tolerancing to meet the demanding requirements of the f/1 telescope design. The VISTA IR camera is unusual in that it contains no cold pupil-stop, but rather relies on a series of nested cold baffles to constrain the light reaching the focal plane to the science beam. In this paper we present a complete overview of the status of the final IR Camera design, its interaction with the VISTA telescope, and a summary of the predicted performance of the system.

The SALT HRS spectrograph : final design, instrument capabilities, and operational modes

The high-resolution échelle spectrograph, SALT HRS, is at an advanced stage of construction and will shortly become available to the user community of the Southern African Large Telescope (SALT). This paper presents a commentary on the construction progress to date and gives the instruments final specification with refined estimates for its performance based on the initial testing of the optics and the science-grade detectors. It also contributes a discussion of how the fibre input optics have been tailored to specific scientific aspirations to give four distinct operational modes. Finally, the use of the instrument is discussed in the context of the most common science cases.

Developments on the UK FMOS project for the Subaru Telescope

We describe the UK participation in the FMOS project to provide multi-object IR spectroscopy for the Subaru telescope. The UK is working on the design of an OH suppression IR spectrograph, this work comprises the optical design, the opto-mechanical layout, spectrograph thermal environment and cryogenics and detector control system. We give a progress report on the current design work.

Wavefront sensing within the VISTA infrared camera

VISTA is a 4-metre survey telescope currently being constructed on the NTT peak of ESO’s Cerro Paranal Observatory. The telescope will be equipped with a dedicated infrared camera providing images of a 1.65 degree field of view. The telescope and camera are of an innovative f/3.26 design with no intermediate focus and no cold stop. The mosaic of 16 IR detectors is located directly at Cassegrain focus and a novel baffle arrangement is used to suppress stray light within the cryostat. The pointing and alignment of the telescope and camera is monitored by wavefront sensing elements within the camera cryostat itself. This paper describes the optical, mechanical, electronic and thermal design of the combined curvature sensor and auto-guider units positioned at the periphery of the camera field of view. Centroid and image aberration data is provided to the telescope control system allowing real time correction of pointing and alignment of the actively positioned M2 unit. Also described are the custom optics, mounted in the camera filter wheel, which are used to perform near on-axis high order curvature sensing. Analysis of the corresponding defocused images allows calibration tables of M1 actuator positions to be constructed for varying telescope declination and temperature.

Cryogenic tests of volume-phase holographic gratings: results at 100 K

We present results from cryogenic tests of volume-phase holographic (VPH) gratings at ∼100 K. The aims of these tests are to see whether the diffraction efficiency as a function of wavelength is significantly different at a low temperature from that at room temperature and to see how the performance of a VPH grating is affected by a number of thermal cycles. We have completed ten cycles between room temperature and 100 K and find no clear evidence that the diffraction efficiency changes with temperature or with a successive thermal cycle.

Cryogenic tests of volume-phase holographic gratings

We present results from cryogenic tests of a Volume-Phase Holographic (VPH) grating at 200 K measured at near-infrared wavelengths. The aims of these tests were to see whether the diffraction efficiency and angular dispersion of a VPH grating are significantly different at a low temperature from those at a room temperature, and to see how many cooling and heating cycles the grating can withstand. We have completed 5 cycles between room temperature and 200 K, and find that the performance is nearly independent of temperature, at least over the temperature range which we are investigating. In future, we will not only try more cycles between these temperatures but also perform measurements at a much lower temperature (e.g., ~80 K).

New Microslice Technology for Hyperspectral Imaging

We present the results of a project to develop a proof of concept for a novel hyperspectral imager based on the use of advanced micro-optics technology. The technology gives considerably more spatial elements than a classic pushbroom which translates into far more light being integrated per unit of time. This permits us to observe at higher spatial and/or spectral resolution, darker targets and under lower illumination, as in the early morning. Observations of faint glow at night should also be possible but need further studies. A full instrument for laboratory demonstration and field tests has now been built and tested. It has about 10,000 spatial elements and spectra 150 pixel long. It is made of a set of cylindrical fore-optics followed by a new innovative optical system called a microslice Integral Field Unit (IFU) which is itself followed by a standard spectrograph. The fore-optics plus microslice IFU split the field into a large number of small slit-like images that are dispersed in the spectrograph. Our goal is to build instruments with at least hundreds of thousands of spatial elements.

A connectorized fiber downlink for FMOS

A consortium of UK, Australian and Japanese groups is designing a fibre-fed near IR (J & H band) multi-object spectrograph (FMOS) for the Subaru telescope. The prime focus of the telescope will support a 400-fibre multi-object positioning system, ECHIDNA. However, the IR spectrographs (of which there are two) are to be located close to the Nasmyth platform, so an interconnecting optical feed is required to deliver light from ECHIDNA. The Astronomical Instrumentation Group at the University of Durham is undertaking the design and construction of a suitable fibre-optic downlink. To allow the prime focus unit that houses ECHIDNA to be removed, the fibre cable is to include a connectorized break, located at the telescope top-end ring. The optical design also calls for a change in focal ratio from that delivered by ECHIDNA in order to couple light to the spectrograph with the greatest efficiency. This will be achieved in the connector coupling by means of an array of high-efficiency GRIN microlenses. The connector will additionally incorporate an integral back-illumination system for on-telescope testing and calibration. This paper describes the preliminary design of the fibre system that is to be constructed in Durham.

Cryogenic Tests of Volume-Phase Holographic Gratings: I. Results at 200 K

We present results from cryogenic tests of a Volume-Phase Holographic(VPH) grating at 200 K measured at near-infrared wavelengths. The aims of these tests were to see whether the diffraction efficiency and angular dispersion of a VPH grating are significantly different at a low temperature from those at a room temperature, and to see how many cooling and heating cycles the grating can withstand. We have completed 5 cycles between room temperature and 200 K, and find that the performance is nearly independent of temperature, at least over the temperature range which we are investigating. In future, we will not only try more cycles between these temperatures but also perform measurements at a much lower temperature (e.g., ∼80 K).

EMIR, cryogenic NIR multi-object spectrograph for GTC

EMIR is a near-IR, multi-slit camera-spectrograph under development for the 10m GTC on La Palma. It will deliver up to 45 independent R equals 3500-4000 spectra of sources over a field of view of 6 feet by 3 feet, and allow NIR imaging over a 6 foot by 6 foot FOV, with spatial sampling of 0.175 inch/pixel. The prime science goal of the instrument is to open K-band, wide field multi-object spectroscopy on 10m class telescopes. Science applications range from the study of star-forming galaxies beyond z equals 2, to observations of substellar objects and dust-enshrouded star formation regions. Main technological challenges include the large optics, the mechanical and thermal stability and the need to implement a mask exchange mechanism that does not require warming up the spectrograph. EMIR is begin developed by the Instituto de Astrofisica de Canarias, the Instituto Nacional de Tecnica Aeroespacial, the Universidad Complutense de Madrid, the Observatoire Midi-Pyrennees, and the University of Durham. Currently in its Preliminary Design phase, EMIR is expected to start science operation in 2004.