SPIRou Early Performances
Last update: August 2018.

The numbers below show the early-performance measurements after several months of operating SPIRou in the lab and on sky. See the footnotes below for explanations.

Throughput in JHK (1) 5, 8, 8%
Peak SNR in 1h on H=11, 3100K star 100 per pix at 1.65micron
Magnitude for S/N=10 in 1h (2) mH=14
Magnitude for 2m/s precision in 900s (3) mH~7.0
Thermal background at 2.35 microns 100 ph/s/A between at 5C
Sensitivity of the guiding (4) mH=11
On-sky aperture 1.29"
Wavelength Domain (5) 0.955-2.515 microns
Spectral Resolution from calibration lines (6) 65,000
Spectral Resolution from the broadening of stellar spectra (6) 70,000
Pixel size 2.28 km/s
Radial-velocity accuracy (7) 12 cm/s (internal)
~2 m/s (on quiet M RV standards)
Method spectral calibration: use of simultaneous FP
Polarimetric sensitivity in stellar lines (8) < 0.003%
Polarimetric crosstalk (9) few %
Type H4RG
Dimensions 4k x 4k
Pixel size 15 microns
Red cut-off (50% QE) 2.45
Quantum Efficiency over JHK > 80%
Cosmetics (clusters) < 0.1% bad pixels
Readout mode Ramp fitting
Dark current 0.018 e-/s
Readout Noise (CDS) 12 e-
Readout Noise (ramps) (10) 6.5 e-
Overheads Slew and start guiding (not charged): <90 s
charged time per exposure: 28s (spectroscopy) or 35s (polarimetry)
Number of available hours per night (11) 7 h (estimate)
Precipitable water vapour 50% of time (observing airmass) (12) 2.85 mm
Precipitable water vapour 80% of time (observing airmass) (12) 5.40 mm
Calibrations RV and telluric standards: ~20 min per night
Pipeline processing


1. SPIRou throughput will be revised in the coming months, with the insertion of new optical elements. The quoted values reflects the instrument performances as in August 2018.

2. Limiting magnitude in 1h shows the H magnitude of a 3100K star where a SNR per pixel of 10 is achieved in median conditions (0.6" seeing in H) in a total exposure time of 1 hour.

3. Limiting magnitude for a 2m/s accuracy in 900s is the typical H magnitude of a 3100K non-rotating star where simulations predict an RV uncertainty not limited by photon noise under median conditions (0.6" seeing in H). RV uncertainty is expected on the same star and same exposure time to be up to 20 m/s if the rotation profile is 10km/s and telluric lines are just masked out.

4. Sensitivity of the guiding is the limiting magnitude below which guiding delivers an rms stability of the input image averaged over a typical exposure better than 0.05". Note that offset guiding is also possible.

5. The wavelength domain is continuous without gaps.

6. The spectral resolution relevant for radial velocity work, Rrv, relates to the gaussian broadening that the spectrograph induces onto the spectrum of a typical M dwarf. Rcal, the resolution estimated from calibration lines, has been measured on lamp exposures. Both resolutions are not equal when the spectrograph point-spread function (PSF) is not Gaussian. For SPIRou, Zemax predicts Rcal = 63.8K and Rrv =73.4K.

7. Internal RV accuracy has been measured in the lab on continuous series of Fabry-Perot exposures. They have shown internal relative stability at a level of 12 cm/s over 24h, measured on the science fibers and corrected for the absolute drift as measured on the reference fiber. Measuring the stellar RV after correcting for telluric absorption shows a precision of about 2 m/s on a quiet, non-rotating, M star.

8. The sensitivity of polarization in stellar lines is within specifications.

9. A residual polarization crosstalk is observed, especially at low external temperatures, of the order of 1-4%.

10. The readout noise in the ramp sampling decreases with the number of exposures, until it reaches a minimum, around 6.5 electrons.

11. The observing efficiency is based on the SPIRou commissioning and takes into account specific known overheads.

12. The amount of precipitable water vapour is expected to have an impact on the spectra and radial velocity measurement. The fraction of time which minimizes the precipitable water vapour may be better adapted for extreme PRV requirements.