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SPIRou - instrument specifications & basic concept

Par Jean-Francois Donati - 12/09/2008


SPIRou : instrument specifications & basic concept

The science requirements are as follows:
spectral domain: 0.98-2.4µm (w/ full coverage up to at least 2µm)
spectral resolution > 50,000 (70,000 if possible)
radial velocity accuracy < 1m/s
S/N=150 per 3km/s pixel in 1hr @ J=12 & K=11
thermal instrument background in the K band < telescope thermal emission, ie K>13.5
all polarisation states accessible with >99% efficiency and <1% crosstalk over full spectral domain

The corresponding instrument specifications are:
peak throughput > 10% (telescope & detector included)
stellar spectra recorded as 2 orthogonal polarisation channels (interleaved spectra) at all times, with possibility of exchanging polarisation channels between successive exposures
reference calibration spectrum recorded as third (interleaved) channel at all times
retarders achromatic to < 1% over full spectral domain
transmission tip/tilt module proving image stability (1Hz) at entrance aperture ~ 0.02" and image quality ~ 0.5"
atmospheric dispersion corrector (ADC) providing correction up to airmass 2.7
entrance pinhole within mirror and camera to provide viewing/guiding in a 1' field
usual calibration facilities (flat field & thorium lamps) plus wavelength reference with high spectral line density (>100 lines/order) and <1m/s stability
fibre-fed bench-mounted cryogenic spectrograph cooled down to LN2 temperature (77K) with pressure (1mb) and temperature (0.01K) control
spot diagrams from spectrograph optics < 1 ccd pxl (~3km/s) throughout full spectral domain

To meet these specifications, we propose that SPIRou consists of:
a Cassegrain module collecting stellar light from a small (200μm=1.4") circular pinhole and containing all optical components (Wollaston prism, rhomb retarders and achromats) needed for an achromatic polarisation analysis and an adequate focal reduction (f/8 to f/4). Interface with the telescope is achieved through another module including an atmospheric dispersion corrector, a transmission tip-tilt image stabilisation unit and a viewing/guiding camera looking at the entrance aperture; it also provides the usual spectral calibration facilities, as well as a stable wavelength reference module.
a 20m triple fiber feed (2 object & 1 reference fibers) conveying the light to the spectrograph. Using ultra-low OH silica fibers (eg made of Suprasil 300, 100/110μm core/clad diameters) ensures a transmission of 85% at 2μm, 60% at 2.2μm and 5% at 2.4μm. A 3slice/fiber Bowen/Walraven image slicer coupled to a focal reducer (f/4 to f/8), both cooled down by a Peltier unit, provides the entrance slit for the spectrograph (1.85x0.07mm). Light from all 3 fibers is recorded at all times.
a cryogenic spectrograph (dual pupil design) featuring a 15cm pupil, 2 parabolic off-axis collimators (having 1.2m focal length), a R2 diffraction grating (with 23.2gr/mm), a prism-train cross-disperser (providing 0.55 mm minimal inter-order separation at detector level) and a f/2 fully dioptric camera (32cm focal length). With a 2kx2k hawaii detector (18μm pixels), orders #85 (0.98μm) to #32 (2.4μm) can be recorded simultaneously, ensuring 50K spectral resolution with almost complete spectral coverage (full order coverage at 2μm, 84% order coverage at 2.4μm).

Being similar to ESPaDOnS, this design should provide a total throughput of 15% (atmosphere and detector included) - at J=12 and K=11, it should yield S/N=150 for a 1hr observation. Pressure and thermal control should ensure RV accuracies similar to those achieved with HARPS.
The spectrograph cryostat can be copied from GIANO (see image on the right), ie featuring a stainless steel cylinder (2m long & 1.3m wide) and containing an optical bench/tank filled with LN2 and on which all the spectrograph optics is mounted.
To improve its RV stability, SPIRou will feature a fixed instrumental configuration (as opposed to ESPaDOnS offering 3 different setups). In other words, all frames recorded with SPIRou will include 2 interleaved stellar spectra (one for each orthogonal state of the selected polarisation) and one spectrum of the RV reference cell.

As such, SPIRou fills a scientific niche and represent a unique opportunity. All existing nIR spectrographs have very narrow spectral domains:
instrumentresolutionslit widthdomaincoverage
while very few similar instruments are in construction or planned:
instrumentresolutionslit widthdomaincoveragestatus
GIANO/TNG50,0000.50"0.9-2.5µmfullin construction (op in 2010)
PRVS/GEMINI50,000fibre feed0.9-1.8µmfullabandonned
NAHUAL/GTC50,0000.175"0.9-2.4µmfullnot funded yet

Only GIANO is being constructed at the moment. In addition to GIANO, SPIRou implements polarimetric capabilities - directly inherited from the ESPaDOnS/NARVAL experience - and therefore appears as a unique opportunity for carrying out the selected science goals



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