GeminiFocus January 2018 | Page 19

MAROON-X To meet the opportunities and challenges described above and push towards eventu- ally identifying other Earth-like planets, the MAROON-X team has carried out detailed simulations to identify the optimum wave- length range to observe low-mass M dwarfs for radial velocity measurements. We find that the red part of the optical spectrum contains as much radial velocity informa- tion as the near-infrared for stars down to masses of 0.10 M B (T eff  ≈ 2,600 K), if not more, because radial velocity measurements depend not just on the number of collected photons, but also on the spectral line den- sity. Although M dwarfs are brighter around 1 micron (μm), the very high line density at shorter wavelengths more than compen- sates for the difference. This means that the optimum wavelength intervals for radial ve- locity measurements of solar-type and low- mass stars are not very different, and they could be spanned by a single spectrograph. We have therefore designed MAROON-X as a red-optical (500-900 nanometers (nm)), high-resolution (R = 80,000) spectrograph capable of delivering high-precision radial velocities with an intrinsic instrument sta- bility of < 0.5 m/s. The instrument’s core spectrograph is fiber-fed (including a fiber for simultaneous calibration), enclosed in a vacuum chamber, and thermally and me- chanically isolated from its environment (see also Seifahrt et al., 2016). We based the spectrograph’s design on an asymmetric white-pupil approach, which re-images and then re-collimates all dispersed beams after the echelle grating into a common pupil to minimize the diameter of the cross-dispers- er and camera. The asymmetry arises from compressing the beam before entering the cross-disperser without sacrificing the ab- erration compensation of the classical sym- metric white-pupil design. This design varia- tion has been used successfully on other instruments, for example, on the High-Res- olution Spectrograph (HRS) at the Southern Table 1. Spectral resolution R = 80,000 Acceptance angle FOV = 0.77” at the 8 m Gemini Telescope Wavelength range 500 nm – 900 nm (in 56 orders) Number and reach of arms Two (500-670 nm and 650-900 nm) Cross-disperser Anamorphic VPH grisms Beam diameter 100 mm (at echelle grating), 33 mm (at cross-disperser) Main fiber 100 μm octagonal (CeramOptec) Number and type of slicer 3x pupil slicer Slit forming fibers Five 50 x 150 μm rectangular (CeramOptec), incl. sky and calibration Inter-order and inter-slice spacing ≥ 10 pixel Average sampling 3.5 pixel per FWHM Blue detector Standard 30 μm thick 4k × 4k STA 4850 CCD (15 μm pixel size) Red detector Deep-depletion 100 μm thick 4k × 4k STA 4850 CCD (15 μm pixel size) Calibration Fabry-Perot etalon for simultaneous reference (fed by 2nd fiber) Exposure meter Chromatic Environment for main optics Vacuum operation, 1 mK temperature stability Environment for camera optics Pressure sealed operation, 20 mK temperature stability Long-term instrument stability 0.7 m/s (requirement), 0.5 m/s (goal) Total efficiency 11% (requirement) to 15% (goal) at 700 nm (at 70th percentile seeing) Observational efficiency S/N = 100 at 750 nm for a V = 16.5 late M dwarf in 30 minutes January 2018 MAROON-X main characteristics. GeminiFocus 17