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
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