NIRPS is an infrared precision Radial Velocity (pRV) spectrograph covering the range 950 nm-1800 nm. NIRPS uses a high-order Adaptive Optics (AO) system to couple the starlight into a fiber corresponding to 0.4" on the sky as efficiently or better than HARPS or ESPRESSO couple the light in a 1.0" fiber. This allows the spectrograph to be very compact, more thermally stable, and less costly. Using a custom tan(θ)=4 dispersion grating in combination with a start-of-the-art Hawaii4RG detector makes NIRPS very efficient with complete coverage of the YJH bands at just under 100 000 resolution. On the ESO 3.6-m telescope, NIRPS and HARPS are working simultaneously on the same target, building a single powerful high-resolution, high-fidelity spectrograph covering the 0.37-1.8 µm domain. NIRPS will complement HARPS in validating Earth-like planets found around G and K-type stars whose signal is at the same order of magnitude than the stellar noise. While the telescope-side AO system was installed on the ESO 3.6-m telescope in 2019, the infrared cryogenic spectrograph has been integrated at the telescope in early-2022 and has had first light in June 2022. Results from the first light mission show that NIRPS performs very nicely, that the AO system works up to magnitude I=14.5, that the transmission matches requirements and that the RV stability of 1 m/s is within reach While performance assessment is ongoing, NIRPS has demonstrated on-sky m/s-level stability over a night and <3 m/s level over two weeks. Limitations on the RV performances arise from modal noise that can be mitigated through better scrambling strategies. Better performances are also expected following a grating upgrade in July 2022; these will be tested in late-2022.
NIRPS (Near Infra-Red Planet Searcher) is an AO-assisted and fiber-fed spectrograph for high precision radial velocity measurements in the YJH-bands. NIRPS also has the specificity to be an SCAO assisted instrument, enabling the use of few-mode fibers for the first time. This choice offers an excellent trade-off by allowing to design a compact cryogenic spectrograph, while maintaining a high coupling efficiency under bad seeing conditions and for faint stars. The main drawback resides in a much more important modal-noise, a problem that has to be tackled for allowing 1m/s precision radial velocity measurements. In this paper, we present the NIRPS Front-End: an overview of its design (opto-mechanics, control), its performance on-sky, as well as a few lessons learned along the way.
Since 1st light in 2002, HARPS has been setting the standard in the exo-planet detection by radial velocity (RV) measurements[1]. Based on this experience, our consortium is developing a high accuracy near-infrared RV spectrograph covering YJH bands to detect and characterize low-mass planets in the habitable zone of M dwarfs. It will allow RV measurements at the 1-m/s level and will look for habitable planets around M- type stars by following up the candidates found by the upcoming space missions TESS, CHEOPS and later PLATO. NIRPS and HARPS, working simultaneously on the ESO 3.6m are bound to become a single powerful high-resolution, high-fidelity spectrograph covering from 0.4 to 1.8 micron. NIRPS will complement HARPS in validating earth-like planets found around G and K-type stars whose signal is at the same order of magnitude than the stellar noise. Because at equal resolving power the overall dimensions of a spectrograph vary linearly with the input beam étendue, spectrograph designed for seeing-limited observations are large and expensive. NIRPS will use a high order adaptive optics system to couple the starlight into a fiber corresponding to 0.4” on the sky as efficiently or better than HARPS or ESPRESSO couple the light 0.9” fiber. This allows the spectrograph to be very compact, more thermally stable and less costly. Using a custom tan(θ)=4 dispersion grating in combination with a start-of-the-art Hawaii4RG detector makes NIRPS very efficient with complete coverage of the YJH bands at 110’000 resolution. NIRPS works in a regime that is in-between the usual multi-mode (MM) where 1000’s of modes propagates in the fiber and the single mode well suited for perfect optical systems. This regime called few-modes regime is prone to modal noise- Results from a significant R and D effort made to characterize and circumvent the modal noise show that this contribution to the performance budget shall not preclude the RV performance to be achieved.
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