MAROON-X The ﬁrst two years of EPRVs from Gemini North Andreas Seifahrta Jacob L. Beana David Kaspera Julian Stürmerb Madison Bradya Robert

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MAROON-X: The ﬁrst two years of EPRVs from Gemini

North

Andreas Seifahrta, Jacob L. Beana, David Kaspera, Julian Stürmerb, Madison Bradya, Robert

Liua, Mathias Zechmeisterc, Guðmundur Stefánssond, Ben Montete, John Whitef, Eduardo

Tapiaf, Teo Mocnikf, Siyi Xuf, and Christian Schwabg

aThe University of Chicago, Chicago, United States

bLandessternwarte Heidelberg, Heidelberg, Germany

cGeorg-August-Universität Göttingen, Göttingen, Germany

dPrinceton University, Princeton, United States

eUniversity of New South Wales (UNSW), Sydney, Australia

fGemini Observatory, Hilo, United States

gMacquarie University, Sydney, Australia

ABSTRACT

MAROON-X is a ﬁber-fed, optical EPRV spectrograph at the 8-m Gemini North Telescope on Mauna Kea,

Hawai’i. MAROON-X was commissioned as a visiting instrument in December 2019 and is in regular use since

May 2020. Originally designed for RV observations of M-dwarfs, the instrument is used for a broad range of

exoplanet and stellar science cases and has transitioned to be the second-most requested instrument on Gemini

North over a number of semesters. We report here on the ﬁrst two years of operations and radial velocity

observations. MAROON-X regularly achieves sub-m/s RV performance on sky with a short-term instrumental

noise ﬂoor at the 30 cm/s level. We will discuss various technical aspects in achieving this level of precision and

how to further improve long-term performance.

Keywords: Gemini Observatory, EPRV, Radial velocity, Exoplanets, Echelle spectrograph, Optical ﬁbers, Pupil

slicer

1. INTRODUCTION

Extreme precision radial velocity (EPRV) observations remain an important tool for exoplanet science, particu-

larly in conjunction with transit observations. From the combination of the radius and orbital period determined

from the transit light curve and the mass and orbital eccentricity determined from radial velocities, the planet

bulk density and insolation can be inferred. These are critical parameters both for putting statistical constraints

on the mass-radius relationship and mass function of extrasolar planets and for vetting the best targets for

atmospheric characterisation using ground- and space-based facilities, such as JWST.

Ultra-stabilized spectrometers such as HARPS and HARPS-N have paved the way to a whole new generation

of EPRV instruments on both hemispheres, covering both the visual and infrared wavelength regime and operating

on a wide range of telescope sizes. MAROON-X is part of this development, ﬁlling the gap of EPRV capabilities

on a large-sized telescope open to the entire US community.

While MAROON-X was speciﬁcally designed for following up small transiting planets around mid to late M

dwarfs with sub-m s−1RV precision, the instrument has turned into a versatile workhorse for a broad range of

exoplanet and stellar science cases. With a red-optical bandpass (500 – 920 nm), covered at a resolving power of

R'85,000, and simultaneous calibration and sky ﬁber, MAROON-X is now the second in-demand instrument

on Gemini North.

Further author information: E-mail: seifahrt@uchicago.edu

arXiv:2210.06563v1 [astro-ph.IM] 12 Oct 2022

MAROON-X was installed at the Gemini North Observatory in May 2019 and saw ﬁrst light in September

2019. We reported about the commissioning and science veriﬁcation results obtained in December 2019 in 1.

Additional details about the instrument design can be found in 2and 3.

In the following sections we provide a brief overview of the operational and RV performance found in the

ﬁrst two years of regular science observations, discuss the technical trade-oﬀs that limit the performance as well

as discuss ongoing upgrade projects.

2. MAROON-X OPERATIONAL PERFORMANCE

Although MAROON-X is currently still classiﬁed as a Visiting Instrument∗, it is permanently installed at Gemini

North. MAROON-X participates in Gemini’s queue observing scheme in three blocks of 1–5 weeks each during

a typical semester. When MAROON-X is scheduled for observations, the ﬁber injection unit (FIU, aka frontend

unit) is mounted at Gemini’s bottom instrument port, a space shared with Gemini’s facility instruments NIFS

and NIRI. The ﬁber conduit is pulled up through the telescope pier where it is stored when MAROON-X is

not scheduled for observations. The repeatability of the FIU boresight is within 100 on sky. Very little active

re-alignment is required to reach the same level of repeatability for re-seating of the ﬁber connector.

Over time we found smaller than expected ﬂexure between the telescope’s peripheral wavefront sensors

(PWFS) and the position of the science ﬁber even for large changes in elevation or cassegrain rotation angle. For

example, during transit spectroscopy programs where we stay on target for up to 7 hours, we found no need to

re-acquire the target. Initial (telescope-side) target acquisition places the science target well within a radius of

<200 around the science ﬁber when target positions and proper motions are correctly entered in the observing

tool (OT). The combination of pointing accuracy and low ﬂexure eliminated the need for oﬄoading oﬀsets from

the tip-tilt mirror in the FIU to the telescope guiding system. The software interface was subsequently re-written

to eliminate any active control of the telescope from the MAROON-X instrument control software.

Focus stability is equally better than expected. Small adjustments are sometimes made at the beginning of

a run but are only aﬀecting the coupling eﬃciency of calibration light into the science ﬁber by a few percent.

This is because science (downstream) and calibration (upstream) ﬁber have the same size and the image of the

calibration ﬁber is re-imaged 1:1 onto the science ﬁber after passing twice through camera, ADC, and tip-tilt

optics in the FIU, making this setup very sensitive to small amounts of de-focus. We have not found the need

for regular re-focusing to improve the coupling eﬃciency of stellar light, partially due to the independent (and

automatic) focus control of the telescope. During periods of exceptional seeing, we measure stellar FWHM down

to 0.300 on the science target with the FIU guide camera, another indication of adequate focus.

The guide camera in the frontend is operated with a typical integration time of 1–10 sec, depending on the

brightness of the source. Only targets as faint as V=16mag require exposure times of up to 15 sec for reliable

guiding on the 1% of light diverted from the target. The use of the three back-illuminated single-mode ﬁbers

to triangulate the position of the science ﬁber is working well. The triangulated science ﬁber position as well as

the actual target position and shape are logged at the guide rate in the telemetry stream. Thanks to the low

ﬂexure, FIU guiding losses can be tolerated for many minutes before systematic oﬀsets are notable.

Since we have never encountered cases of ﬁeld stars moving over the sky ﬁber location, the instrument is

now operated with the telescope cassegrain de-rotator disabled, i.e. with the frontend unit at a ﬁxed parallactic

angle. This also eliminates the need for fast ADC updates during meridian passage. After an initial operational

phase of manual ADC settings for each target, the ADC is now automatically tracking the telescope elevation

and requires no further input during the night.

Due to the constraints of Gemini’s queue scheduling operation, observations are taken with ﬁxed exposure

times, rather than a ﬁxed SNR. Depending on eﬀective seeing and cloud cover, variations in SNR between

exposures of the same target do occur, but seem to have no signiﬁcant impact on RV precision.

The ﬁber conduit entering the Pier Lab above the instrument’s thermal enclosure is dragged up and down

and rotated with the telescope motion.1Plexi-glass walls were installed in 2021 on two sides of the roof of the

enclosure to prevent the ﬁber from slipping oﬀ the enclosure. This also gives the ﬁber conduit more room to

∗See https://www.gemini.edu/instrumentation/current-instruments/maroon-x for more information.

uncoil, practically eliminating earlier incidents of loops in the ﬁber conduit being dragged up during telescope

motions.

Technical time loss during the ﬁrst two years of operation was near zero with no signiﬁcant failures during

observing runs. In contrast, a number of power failures and glycol cooling outages occurred when the instrument

was not scheduled for observations. This led to uncontrolled warm-ups of the detector cryostats and interrupted

the otherwise continuous operation of the instrument. In particular, the unexpected loss of observatory-side

glycol cooling led to an overheating of the waterjackets of the Sunpower Stirling cryocoolers which subsequently

caused an uncontrolled power on/oﬀ cycle which imparted a large amount of vibrations on the detector systems,

in one case for many hours. While this and other events did not lead to substantial misalignment or physical

damage to the instrument, it did interrupt the RV baseline of the instrument due to shifts in focus and thus color-

dependent changes in the instrumental proﬁle. The largest event caused a signiﬁcant increase in the resolving

power achieved with the red arm of the instrument, a rather fortuitous outcome, all things considered.

To prevent future cooling outages causing uncontrolled detector warm-ups, we installed a large coolant backup

Figure 1: Change in the resolving power of MAROON-X due to a cooling failure on 2021-05-09

and vibrations imparted by the cryocoolers. Shown are the values for three science ﬁbers. Thin points

show the raw values from each of the roughly 500 etalon lines per spectral order. Solid points mark the average

value within the FSR of each order. Variations with wavelength and between ﬁbers are driven by the complex

aberration pattern of the MAROON-X camera optics.

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MAROON-X:ThersttwoyearsofEPRVsfromGeminiNorthAndreasSeifahrta,JacobL.Beana,DavidKaspera,JulianStürmerb,MadisonBradya,RobertLiua,MathiasZechmeisterc,GuðmundurStefánssond,BenMontete,JohnWhitef,EduardoTapiaf,TeoMocnikf,SiyiXuf,andChristianSchwabgaTheUniversityofChicago,Chicago,UnitedStatesbLandesstern...

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MAROON-X The ﬁrst two years of EPRVs from Gemini North Andreas Seifahrta Jacob L. Beana David Kaspera Julian Stürmerb Madison Bradya Robert

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