CFHT Information Bulletin, number 38, First Semester 1998

OASIS: An Integral Field Spectrograph at CFHT

Pierre Martin, Resident Astronomer, (

OASIS is a multi-mode spectro imager for the visible (400nm to 1000nm) built by Observatoire de Lyon and designed to make use of the superb image quality offered by the adaptive optics bonnette (AOB). OASIS is the successor of the TIGER integral field spectrograph operated at CFHT since 1987. Numerous optical configurations are available for all kind of scientific programs requesting high spatial resolution. OASIS is mounted directly below the AOB at the Cassegrain environment and is used with CCDs with pixels of 15 microns. For a bigger field but a lower spatial resolution, it can also be employed at the f/8 focus but this configuration has not been tested yet. Due to the complexity of the instrument, a special Pegasus user interface has been developed for data acquisition and will be fully tested in January 1998. For configuring OASIS, the observer selects a "scenario", that is, specific instrumental requirements for the scientific program (e.g. spectral range and resolution). Different optical elements are then moved by software to achieve an instrumental configuration fulfilling these specifications. OASIS saw its first light at CFHT in August 1997 during a very successful engineering run. Final commissioning is planned for January and several observing missions are scheduled for March.

For the coming year, three modes will be available:

1) The Imaging Mode: When coupled with the f/20 AOB, OASIS offers two different fields of view for imaging: 90" (with 0.060"/pixel) and 45" (0.025"/pixel). Image quality as good as 0.1'' can be expected in I band. The imaging mode can be used for pointing, PSF sampling and as a complement for the spectroscopic modes.

2) The TIGER mode: The main mode in OASIS, TIGER offers a 2D spectroscopy capability using an array of 1100 hexagonal micro-lenses which split the imput image into small micro-pupils. Each pupil is then dispersed in the spectrograph stage and imaged on the detector. The lens array can be used with four different enlargers and the CFHT set of grisms for the visible (see optical layout at ). A large range of spectral samplings is available for low (~ 1000) to medium resolutions (~3000). Depending of the enlarger used, the field of view varies from about 1.5" to 11''. Sky samplings range from 0.04"/pixel to 0.30"/pixel. Tests during the engineering run revealed that the total efficiency is about 5 to 10% in this mode. The calibration unit at CFHT ("Gumball") has been recently upgraded to provide more light for the calibration frames. Since TIGER can produce up to 1100 spectra on a single CCD frame, special data reduction software (distributed and supported by Lyon Observatory) is being developed and should be available at the beginning of 1998.

3) The long-slit mode: Allows spectroscopy on the whole field offered by the AOB with 0.1'' sampling and low spectral resolution (< 1400).

More information about the different modes of OASIS can be obtained at: . An OASIS web page including detailed description of the instrument, manuals for data acquisition and reduction as well as other material related to the instrument will be available soon at the CFHT site.

Editor: Dr. T. M. C. Abbott,
Copyright © 1998, CFHT Corporation. All rights reserved.