The OASIS Integral Field Spectrograph





CONTENTS

A brief history

  • The TIGER prototype
  • OASIS at CFHT
  • OASIS at WHT

Overview

  • Observing modes
  • Detectors

Observing Modes



A BRIEF HISTORY

The TIGER prototype

The Integral Field Spectrograph TIGER (Traitement Intégral des Galaxies par l'Etude de leurs Raies), first described by Georges Courtès, was initiated by a collaboration between Observatoire de Lyon and Observatoire de Marseille. The prototype was initially dedicated to the study of the central regions of galaxies. The first observing run was conducted at CFHT in 1987 (photo from a 1995 run). Numerous important programs were then undertaken with TIGER 1 and it was rapidly demonstrated that the list of scientific targets for such an instrument should not be limited to galaxy nuclei only but should also include a large number of other astronomical phenomena. Nevertheless, the need for a better spatial resolution (1.2 arcsec in 1987 - 0.7 arcsec in the 1990s) and a larger amount of information (from 80 spectra in 1987 to 350 in 1990) lead to deep modifications of the instrument, and thus great efforts of the teams. Also, due to the very complex structure of the TIGER spectrographic exposures, data processing is a demanding task and had necessited a full-time engineer specially dedicated to the development of the software for several years. In spite of the clear success of TIGER 1, however, the development of the adaptative optics bonnette at CFHT would offer another opportunity for this kind of instrument: gathering a large number of spectra on a small field but with very high spatial resolution. A new integral field spectrograph (IFS) was needed...

OASIS at CFHT

This new IFS was been built by Observatoire de Lyon under the supervision of Roland Bacon. OASIS (Optically Adaptive System for Imaging Spectroscopy) was designed to make use of the superb image quality provided by the Adaptative Optics Bonnette (AOB). At the beginning of 1997, the instrument was completed (photo) and tested at the Observatoire de Haute Provence (photo). Excellent results were gathered and the instrument was then shipped to CFHT.
The first engineering run at CFHT (photo1, photo2) was completed in April 1997 although very bad weather conditions did not allow any astronomical observations. But on August 10th, OASIS saw its first light at CFHT! During this run, numerous mechanical and optical tests were performed for the f/20 mode and the software for data acquisition was implemented. In January 1998, two engineering nights, used to install a more elaborated user's interface, were followed by four nights of (final) commissioning by a team of external experts (photo). The data reduction software was locally implemented and a preliminary reduction performed by the observers was very successful. This software, entirely supported by Lyon, became available for different platforms worldwide in March 1998.
Many observing runs were scheduled for the end of March 1998. Although the observations were again affected by periods of very bad seeing, good science data have been obtained. OASIS was used in November 1998 (again during bad whether conditions) and in December 1998. During this last run, atmospheric conditions were finally very good and spectacular data were gathered on diverse objects (T Tauri stars, proto-stellar disks in the Orion nebula, M31 nucleus, AGNs, etc...). In 1999, many runs with OASIS were again compromised by bad weather but some nights were also extremely productive. At the beginning of 2000, a long 20 nights run proved very successful.
OASIS has been used at CFHT with both the LORAL 3 and the blue-enhanced thinned EEV2 CCDs.
A lot of people worked hard toward the success of this strange instrument, which impulsed the powerful rise of 3D spectroscopy.
In 2002, as the contract with CFHT ended, CFHT was considering specializing in high-resolution wide-field imagery (MEGACAM), and OASIS was brought back to Lyon.

OASIS at WHT

Meanwhile, the Sauron IFS (a derivative from OASIS, with lower sampling and wider field, specialized in galaxy dynamics) has been developped by the CRAL/TIGER team in Lyon, and successfully used at WHT by a consortium built around Lyon and Leiden observatories. This was the triggering fact which drove the NWO towards accepting to fund the implementation of OASIS at WHT, with ING and CRAL supporting partly the costs of NAOMI and OASIS modifications. ING is responsible for the regular exploitation of OASIS, the instrument itself remaining CRAL's property.
The instrument have been permanently installed at one of the Nasmyth foci, in a specially built controlled-environment enclosure (GRACE), where it stays with NAOMI, INGRID, OSCA... (Needless to say, it is very happy).
This very stable situation is new for OASIS, and the idea is to take advantage of that to use as far as possible a calibration files database, getting rid of the obligation to obtain a lot of service exposure during the night; it is expected that this will save a lot of on-the-sky time. More, the observations will be made as service observing by skilled WHT staff, and data reduction "pipelined" so that the observer will eventually be delivered completely processed datacubes, ready for physics.
The instrument was installed in june 2003 (photo 1, photo 2), and first light obtained during the same successfull technical run. As of summer 2004, work is still in progress on NAOMI performance in the visible region (a real challenge), on the WHT control software, and on XOasis, the reduction software. It is foreseen to open OASIS to the community, first on a shared-risk basis, for the second semester of 2004.

OVERVIEW

OASIS is a multi-mode spectro-imager, working in the 0.43 µm to 1 µm range and using a dedicated ING CCD. It is operated in service mode as a WHT instrument. OASIS make use of a corrected 38 arcsec (maximum) diameter field given by the NAOMI adaptative optics system.
OASIS can be operated in the following modes:
Both modes cover a number of different configurations (e.g. with different spatial samplings, fields of view,...), obtained by switching optical parts in the spatial stage of the instrument, which acts as an imaging relay. It is followed by the spectrographic stage, a classical focal reducer optical system.
The only parts which may have to be manipulated are the filters (not frequent). All other optical components are permanently installed and -of course- remotely controlled. On the observer's side, even the slightest change in one of the many available spectrographic parameters definitely requires a new full set of calibrations to be performed before and/or after the scientific observations. This is the key to achieve successful data reduction without too much trouble, and this is reminded by the control/acquisition software. It will, hopefully, provide an efficient "garde-fou" against observers jumping from one configuration to another, a sure recipe for disaster.
At WHT, and this is new from CFHT, a databank of calibration files for the various configurations is maintained by ING staff; the stability of the system (OASIS and NAOMI are installed in an environment-controlled room, GRACE, and never move) makes this new approach possible. Additionally, much less time is "lost" doing calibrations during clear nights.
Due to the high complexity of the instrument, a special user's interface has been implemented in the WHT control system. Instead of selecting the optical elements needed to achieve a specific optical configuration, the observer defines instead an observing "scenario" for which he/she specifies the specific instrumental requirements for the science program (e.g. spectral range, sky sampling, resolution). Different optical elements are then moved automatically by software to achieve an instrumental configuration fulfilling these specifications.

Detector

OASIS uses a dedicated CCD (MITLL3, 2kx4k, 15µm pixels). Its detailed characteristics can be found here on the ING WEB site.

OBSERVING MODES

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Last update: 13/04/2004. Send comments to A. Pecontal