-- InfraRed Doppler for the Subaru telescope --

IRD is an infrared high-dispersion fiber-fed spectrometer that covers the wavelengths from 0.97 to 1.75 microns with a maximum spectral resolution of 70,000. The instrument has a fiber injection module for star-light and laser frequency comb (LFC) light injection into fibers on the Nasmyth platform. Before the star-light is injected into fibers, its wavefront is corrected by the adaptive optics system, AO188. The spectrometer design incorporates a high-blaze angle Echelle grating, collimator mirrors, a cross disperser grating, a camera lens system, and two 2D array detectors. These are all enclosed in a vacuum chamber and are cooled to ~ 180 K (optics) and 80 K (detector) for thermal background and dark noise reduction. The vacuum chamber is placed in the coude room of the Subaru to stabilize the temperature environment of IRD. The IRD's LFC system implements the multi-gigahertz spaced comb generation technology that consists of a frequency stabilized laser diode, an optical pulse synthesizer, and a highly nonlinear fiber.
Spectral resolution ~70,000 max See also here
Wavelength coverage0.97-1.75 um
Fiber diameter0.48 arcsec
Wavelength calibratorsLaser frequency comb, ThAr lamp, UrNe lamp
Flat calibrationWhite lamp at NIR-Nas
ThroughputCheck here
Saturation level30,000 ADU (10% linearity error)
Temperature stability+/- 35 mK for 10 days (optics) +/-30 mK (Detector)
Sensitivity 1111 sec at 999 nm and 357 sec at 1620 nm to acheive S/N = 100 for dwarf stars with Teff = 3000 K and J = 10 mag.
769 sec at 999 nm and 385 sec at 1620 nm to acheive S/N = 100 for dwarf stars with Teff = 5500 K and J = 10 mag.
Sensitivity gain with LGS for fainter stars The use of the laser guide star (LGS) for AO188 will improve a fiber injection efficiency, if your target is very faint in the visible. From our engineering test, we found that a fiber injection efficiency was improved from 10% to 20% for R=15.2, I = 12.75 mag star (= R' mag is 14.1, see this figure ) in ~1".0 seeing condition. Please note that predicting the gain of the LGS mode for a different seeing condition and different brightness of a star is not easy because of a lack of actual observing data. In addition, the LGS mode requires large overhead, at least 15 minutes for AO adjustments. Please see the AO188 web page for more details.
DetectorsHAWAII2 RG x 2
Gain2.99 e-/ADU at YJ-band detector
2.78 e-/ADU at H-band detector
At the this stage, the gain measurements may have an uncertainty comparable to about +/- 0.2.
Dark current< ~0.01 e-/s for YJ-band and H-band detectors
Read-out noise~12 e- (10min. exposure)
Linearity 10% linearity error at 30,000 ADU
FIM acquisition CCD pixel scale 0.067 arcsec/pixel
Optics wavelength coverage 0.97-1.75 um
Fiber FOV 0".48
FIM acquisition CCD camera limiting magnitude J = 18.8 (S/N = 10, DIT = 10 sec, in case of a star not injected into a fiber)
J = 14.9 (S/N = 10, DIT = 60 sec, in case of a star light injected in to a fiber)
If you want to achieve high instrumental RV stability (~2 m/s), we recommend a target with J < 13 mag for a good image centering against a fiber core.
FIM acquisition CCD camera wavelength coverage0.97-1.1 um
FIM acquisition CCD camera FOV 30"
Requirement for a AO guide starIf a star R' mag is brighter than 13, a coupling efficiency into a fiber will be around 60%: R' = R-2.5log10(0.43*(R-I)^2-0.38*(R-I)+1.0). See also here
Wavelength coverage 1040-1750 nm
Mode spacing 12.5 GHz
Frequnecy stability 0.03 MHz (Standard deviation) for 140 hrs
Contrast>20 dB
Intensity stability 0.7 dB
Additional componetsSpectrum shaper, Depolarizer, Mode scramber
MethodSimultaneous wavelength reference
Fiber Two multi-mode fibers
Calibration sourcesLaser comb, ThAr Hollow Cathode Lamp, UrNe lamp
Scramblers Various (not necessary for users to specify)
RV instrument stability ~2 m/s in a short term ~ 1 week (the goal is 1m/s, and the stability demonstration is still ongoing for further improvement. Shared-risk operation in S2019A)
Min. exposure time1.5 sec.
Typical overhead time<600 sec. (if your target is faint, overhead will be increased)
Sensitivity calculationCheck this page
Data reductionto be updated

Contact mail address

Motohide Tamura, Takayuki Kotani (send mail to ird_tech (at) ird.mtk.nao.ac.jp)