One of the highlights of the European ELT Science Case book is the study of resolved stellar populations, potentially out to the Virgo Cluster of galaxies. A European ELT would enable such studies in a wide range of unexplored, distant environments, in terms of both galaxy morphology and metallicity. As part of a small study, a revised science case has been used to shape the conceptual design of a multi-object, multi-field spectrometer and imager (MOMSI). Here we present an overview of some key science drivers, and how to achieve these with elements such as multiplex, AO-correction, pick-off technology and spectral resolution.
The science case for the next generation of Extremely Large Telescopes (ELTs) covers a huge range of astronomical
topics and requires a wide range of capabilities. Here we describe top-level requirements on an ELT, which were derived
from some of the key science cases identified by European astronomers. After a brief summary of these science cases we
discuss the requirements on the ELT system in terms of several parameters, including wavelength range, field of view,
image quality etc. We discuss the science driver that sets the limits on each parameter. We also discuss specific
requirements on instrumentation, site and adaptive optics. In several cases, detailed simulated observations will be
required in order to set the requirements. While the example science cases provide a useful guide, we also note that an
important goal is to develop a facility that covers a broad parameter space, and maintains flexibility in order to adapt to
new scientific directions.
We present an overview of the science case for a 50-100m Extremely Large Telescope. This was the subject of a meeting in Marseilles, France in November 2003, attended by about 50 European astronomers. Four key scientific themes were identified by the participants: terrestrial planets in extra-solar systems; stellar populations across the Universe; building galaxies since the darkest ages; the first objects and re-ionisation structure of the Universe. Although by no means an exhaustive list of science areas in which ELT will have a great impact, these cases provide examples where an ELT can make a dramatic advance in our understanding of the Universe around us. This paper describes these highlighted science themes and the challenging demands they place on ELT performance. See http://www-astro.physics.ox.ac.uk/~imh/ELT/ for more information, including the full list of participants in this work.
Highlights from the science case for an extremely large telescope are presented. As an introduction, theoretical performance gains in terms of FWHM and depth achievable with an ideal ELT working at the diffraction limit are compared with those for current 8m class telescopes. Three example science cases for an ELT are then presented, all of which drive the desired telescope size towards the largest currently being discussed, i.e. up to 100m. The science topics chosen from many are (1) direct detection of extra-solar planets, (2) study of resolved stellar populations in the Virgo cluster and (3) detection of the first luminous sources and re-ionization of the Universe. Finally, work that is currently taking place in Europe towards development of the science case for an ELT is described.
The Gemini Multiobject Spectrographs (GMOS) were designed to take advantage of the exquisite image quality expected of the Gemini telescopes. To achieve this, two of the many requirements placed on the optical system was that it not degrade the best image quality
expected of the telescope by more than 10% while delivering a throughput of about 80% over the entire 0.4-1 μm waveband. In this paper, key components of the design and execution of this optical system are discussed and test results are presented demonstrating that it meets these requirements on Gemini today. Among other characteristics, we look at the image quality performance as a function of colour and field angle, the measured throughput, and the focalplane flatness on the detectors.
Of the Gemini Multi-Object Spectrograph's (GMOS) scientific requirements, one which led to technically interesting areas was the ability to measure velocities to an accuracy of 2km/s over the entire 5.5 arcminute square field. GMOS's design to meet this requirement includes a mechanical design for stiffness and without hysteresis or image rotation, and an open loop flexure control system which translates the detector position to compensate for flexure. The model used to predict the flexure is an empirical one developed from measured flexure results. In this paper we present the analysis of factors which enable meeting the 2km/s requirement, and the observing strategies needed to make those observations. We look in particular detail at the development and test of that flexure compensation system, including both lab results and on-telescope results.
The Gemini-North Multiobject Spectrograph (GMOS) includes a powerful capability for integral field spectroscopy - the first to be installed and used on an 8-10m telescope. GMOS is switched to this mode by the remote insertion of an integral field unit (IFU) into the focal plane in place of the masks used for multiobject spectroscopy. With 1500 lenslet-coupled fibres, it provides a total field of view exceeding 50 square arcseconds, including a separate field dedicated to background subtraction. We describe the design, construction and testing of the IFU and present performance results obtained during commissioning.
The first of two Gemini Multi Object Spectrographs (GMOS) has recently begun operation at the Gemini-North 8m telescope. In this presentation we give an overview of the instrument and describe the overall performance of GMOS-North both in the laboratory during integration, and at the telescope during commissioning. We describe the development process which led to meeting the demanding reliability and performance requirements on flexure, throughput and image quality. We then show examples of GMOS data and performance on the telescope in its imaging, long-slit and MOS modes. We also briefly highlight novel features in GMOS that are described in more detail in separate presentations, particularly the flexure compensation system and the on-instrument wavefront sensor. Finally we give an update of the current status of GMOS on Gemini-North and future plans.
The Gemini Multiobject Optical Spectrographs were designed to exploit the exceptional image quality anticipated form both the active and adaptive optics systems. High mechanical stability and repeatability and efficient reconfiguration and calibration were emphasized in the design, as well as the usual requirements of obtaining excellent image quality, high optical throughput and low optical distortion. In addition, an active flexure compensation system is used to assist in achieving a primary goal of attaining velocity accuracies of 2 km/s per spectrum in multiobject mode at the highest spectral resolution. Although the field is modest, small pixels are used to fully sample images as small as 0.2 inch and 28.3 million pixels will be recorded by the detector mosaic which consists of three 2048 * 4608 EEV CCDs. In this paper, results from extensive tests made during integration and testing of GMOS N components demonstrate that the design requirements are being met.
We present the characteristics of the new CCD imager, SUSI2, installed at the ESO 3.5 m NTT. The instrument shares the Nasmyth focus A with the new infrared imager-spectrograph SOFI. The focal plane array of USSI2 is a mosaic of 2 EEV44- 82, 2k X 4k, 15 micrometer pixels, thinned, anti-reflection coated CCDs, which are placed at the direct focus of the telescope (scale 0.08 arcsec/pixel, field of view 5.5 X 5.5 arcmin). The average QE for the two devices is 76, 90, 85, 80, 68, 49, 23% at 350, 400, 500, 600, 700, 800, 900 nm respectively. The overall instrument efficiency, including the three mirrors of the telescope and the detector but without filters, is computed to be 46, 55, 51, and 48% at the central wavelengths of the U, B, V and R bands. The CCDs are driven by the new ESO CCD controller FIERA. The system performance was measured during the commissioning of the instrument at the telescope in February 98. The mosaic is read in 16 seconds in the standard operating mode (2 X 2 binning of the CCDs) with a read-out-noise of 4.7 e-/pixel. The other CCD parameters such as CTE, dark current and linearity, were also found to comply with the requirements. The FWHM of stellar sources in images obtained in good seeing conditions were measured to be 0.49 arcsec, with no significant variation over the field of view.