OVERVIEW

What you should first know about OASIS...

• Principles

OASIS is a complex multi-mode instrument whose description can be found on the specific instrument overview page. The present document deals only with the integral field spectrographic mode, alias the TIGER mode.

In this mode, the observed field is optically sampled into ~1100 spatial pixels via a micro-lens array, and all the corresponding light beams are fed into a classical grism spectrograph; this gives a set of ~1100 separate spectra, arranged in a so-called data cube. The TIGER reduction software developed and supported by the Observatoire de Lyon is specifically designed to reduce such data cubes. Both the sky and the spectral samplings can be set according to the astrophysical target from an elaborated user's interface implemented in a Pegasus session. The resulting spatial sampling is roughly twice the sky sampling selected (i.e. arcsecond/lens).

But do not forget, no miracle is to be expected : finer samplings means smaller fields, shorter spectra, and longer exposure times...



• OASIS and the CFHT adaptive optics

OASIS has been designed to take full advantage of the PUEO@ CFHT Adaptive Optics Bonnette f/20 focus, up to the highest possible resolution. In this configuration, the observer has to operate not only OASIS but also PUEO. All the normal control windows for PUEO have been implemented in the OASIS user's interface. It is then also very strongly recommended to familiarize yourself with the user's manual of PUEO before your observing run. It is also necessary to evaluate the possibilities (e.g. guiding on a bright nearby star) to close the loop for the AO correction for your objects in case these are too faint or diffuse for using a direct correction.

An important point about using OASIS in its f/20 mode is the possibility to change the AOB beamsplitters to optimize the amount of light going to the instrument. For OASIS observations, 4 different beamsplitters can be used, depending of the spectral range of interest. The recommended beamsplitter for each standard mode of OASIS are given in the Tables in the instrument overview. These beamsplitter can be changed during an observing night but about 15 minutes on the sky will be lost. Planning your observations ic crucial!

OASIS can also be used at the classical f/8 Cassegrain focus as a backup mode but also for science programs not depending on the finest spatial sampling. Spectral resolution will be lower but the throughput will be higher and the field of view larger.



• Detectors

OASIS uses the standard CFHT CCD@ chips, Dewars, controllers and acquisition system. Only 15µm pixels chips (or smaller) are allowed, to fit the optical characteristics. At the moment, only Loral3 can be used with OASIS but it is expected that a more sensitive CCD should become available sometime this year.



• Dispersers

OASIS employs the standard set of CFHT grisms@, the same used by MOS/OSIS. These grisms are installed in a standard MOS/OSIS cassette. The general instrument overview gives a comprehensive list of all the spectral resolutions offered in TIGER mode.



• Filters

The TIGER mode of OASIS must use interference filters to limit the spectrum length; a set of "standard" TIGER filters is provided, but you can use your own 2" custom filter if you do not find the right one into this set. You may choose one in the CFHT filter database@, if you are lucky to find a suitable one. On this point, you might read the general instrument overview, which presents the maximum filter bandwidth suitable for each spectral configuration. If a filter has to be ordered from a manufacturer to accommodate your needs, CFHT might be able to help financially (i.e. pay 50% of the filter) at condition that other members of the astronomical community manifest some interest in using this filter. It is very important that you communicate with the OASIS instrument scientist if you plan to use (or order) your own filter(s). All filters must be installed in the instrument well before the run, and the descriptive files updated accordingly by the CFHT staff.



• Spectral calibration

OASIS uses the upgraded CFHT Cassegrain bonnette built-in "GUMBALL" calibration system which reproduces the AOB beam characteristics. It offers a set of continuum (halogen bulbs) and standard spectral line lamps (Neon, Argon, Mercury). In addition, two fixed Fabry-Perot etalons for the blue and the red have been installed to provide channel spectra for the very short spectral ranges used by OASIS. Different combinations of lamps and individual exposure times are possible.



• Control system and user interface

The OASIS control system has been integrated into the CFHT standard PEGASUS@ observing interface, which already controls the CCD acquisition, the GUMBALL operation, the AOB operation, etc... The OASIS/TIGER part of this interface is fully described in the ad hoc chapter of the TIGER mode Observer's manual.



• Setups

Due to the complexity of OASIS, an extensive setup must be prepared by the CFHT staff before any observing run (e.g. focussing for each filter, mounting of grisms, etc). It is then essential that any observers specify their needs for their observing runs well in advance. A pre-run preparation form is available at the CFHT web site (here@). Please, fill this form and send it to us at least one month before your run.

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