CFHT, Instruments, Spectroscopy, OASIS, Instrument Overview
 



 



 
 
 
 



 

Welcome to the OASIS Web Page !

This page presents an overview of OASIS, the integral field spectrograph for the visible at CFHT. A more complete description of the instrument, an exposure time simulator and an extensive user's manual are included in the other chapters. Information about how and what to prepare for an observing run can also be found in these pages. If you have any immediate requests, please contact the OASIS instrument scientist at CFHT, Pierre Martin.

Welcome to the world of OASIS and... Enjoy!


 

CONTENTS



 

A BRIEF HISTORY

OVERVIEW


OASIS is a multi-mode spectro-imager, working in the 0.43 µm to 1 µm range and using a CFHT 2kx2k 15µm pixels CCD (i.e. Loral3) or 2kx4k 13.5µm pixels (EEV1). It is operated as a CFHT "guest" instrument. OASIS normally make use of the corrected 90 arcsec diameter F/20 field given by the CFHT Adaptative Optics Bonnette. It can also be used at the direct F/8 Cassegrain focus as a backup mode and for science programs necessiting integral field spectroscopy but with a coarser spatial sampling defined by the natural seeing. (Note: This mode has now been fully tested. It is then VERY important to bring a backup program in case of seeing worse than 1.2", value for which the AO correction becomes inoperative. ).

For now, 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.

This multi-mode/configuration approach has been chosen:

To keep the CFH staff/observers workload manageable, the only parts which may have to be manipulated are the filters (not frequent) and the grisms in their MOS/SIS cassette. 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 configurations 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. This is reminded by the control/acquisition software and will, hopefully, provide an efficient "garde-fou" against observers jumping from one configuration to another, a sure recipe for disaster...

Due to the high complexity of the instrument, a special user's interface has been implemented in the current Pegasus sessions at CFHT. 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)(form1, form2). Different optical elements are then moved automatically by software to achieve an instrumental configuration fulfilling these specifications.

Pointing Limits: OASIS being a long instrument, there are some pointing limits to be taken into account while planning your observations. These limits are implemented in the TCS software so collisions with the telescope pier will be automatically avoided but some objects in your program might not be accessible at all with OASIS! With Loral3 as the detector, the pointing limits in declination are +67d59'15" and -35d54'49". With the EEV1 detector, the limits are more severe because the dewar is longer: +62d and -28d.

Detectors: Two detectors, Loral 3 and EEV1, are available for OASIS, depending of the spectral range required for a science program. Loral3 is a thick 2k x 2k device with a very moderate quantum efficiency but remains the detector to be used for programs requesting the red spectral domain (that is, over 7000 Ang). Its characteristics can be found here. The EEV1 is a thinned 2k x 4.5k device with very good quantum efficiency and low readout-noise, ideal for the visible range of OASIS (i.e. 4200 - 7000 Ang). It should not be used for the red domain because fringing is very important at about 35% at 8500 Ang. Its characteristics can be found here.

Another significant difference between both detectors is the size of the pixels. Loral3 has 15µm pixels while EEV1 has 13.5µm pixels, that is, 10% smaller. This results in a different spectral resolution in the spectroscopic configurations, smaller field in one direction for the EEV1, and different pixel scales for the imaging mode. These are noted in the configuration tables below. The readout time for Loral3 is about 165 seconds. For EEV1, it is about 50 seconds (raster of 2k x 2.5k, useful for OASIS).
 


 


OBSERVING MODES

  • IMAGERY

  • This mode can be used as a complement to the spectrogaphic mode but also for pointing purposes. These configurations are available:

    Spatial resolution
    F/20 AOB
    Scale(1)Field
    0.060"/pixel90" dia.
    0.025"/pixel45" dia.
    F/8 CASSEGRAIN
    ScaleField
    0.062"/pixel110" dia.
    (1): with a 15µm pixel size; 10% smaller with EEV1



  • TIGER

  • This is the main mode in OASIS. The micro-lens array splits the input image into small micro-pupils. Each pupil is then dispersed in the spectrograph stage and imaged on the detector. In this mode, the spatial sampling is defined by the lenslet size and is independent of the CCD pixel size. The spectral coverage is defined by a filter to avoid overlapping between the different spectra. Total transmission efficiency was measured to vary between 5 to 10% depending of the configuration used. However, these measurements were done with the 50-50 AOB beamsplitter and should be significantly improved with the new AOB beamsplitters. The optical arrangement is outlined in this picture.

    Using the different enlargers, grisms and filters mounted in OASIS, the TIGER configurations below are currently available. In these tables, the "useful wavelength limits" refer to 50% of the transmission of the filters. These values are approximate (± 5Å); further information on the transmission curves of the filters is available here.

    Spatial resolution
    F/20 AOB
    Sky samplingField
    0.04"1.6" * 1.2"
    0.11"4.1" * 3.3"
    0.16"6.2" * 5.0"
    0.30"11" * 9"
    F/8 CASSEGRAIN
    Sky samplingField
    0.27"10.4" * 8.3"
    0.41"15" * 12"

    1) Note that the longest direction is oriented East-West and it is smaller by 10% with EEV1.
    Low spectral resolution configurations (f/20)
    Name Grism Useful wavelength limits Dispersion
    Å/pixel
    Resolving
    power
    AOB
    beamsplitter
    Lambda inf (Å) Lambda sup (Å)
    LR2 R150 8650 10370 4.74 1000 I


    Medium spectral resolution configurations (f/20)
    Name Grism Useful wavelength limits Dispersion
    Å/pixel
    Resolving
    power
    AOB
    beamsplitter
    Lambda inf (Å) Lambda sup (Å)
    MR0 O300 4150 4930 2.13 1059 50-50
    MR1 O300 4760 5558 2.15 1210 V
    MR2 R300 6210 7008 2.17 1525 R
    MR3 R300 8346 9152 2.23 1965 I
    MR4 O300 5502 6324 2.16 1375 R
    MR5 R300 6940 7764 2.19 1710 R
    MR6 R300 7685 8455 2.21 1880 I
    MR7 R300 9150 9896 2.25 2220 I


    High spectral resolution configurations (f/20)
    Name Grism Useful wavelength limits Dispersion
    Å/pixel
    Resolving
    power
    AOB
    beamsplitter
    Lambda inf (Å) Lambda sup (Å)
    HR1 B600 4852 5196 0.99 2530 V
    HR2 B600 5118 5484 1.00 2650 V
    HR3 O600 6209 6549 0.99 3215 R
    HR4 O600 6492 6838 1.00 3335 R
    HR5 R600 8435 8749 0.87 4295 I


    1) Note that the dispersion is smaller by 10% if the EEV1 is used.

    2) Note that the resolution is lowered by about 10% if the f/8 focus - 0.4" sampling is used.

    Back to top of the page. Back to OASIS home page.
    Last update: 09/01/1998. Send comments to martin@cfht.hawaii.edu