1. Instrument and Data

1.1. General Description

MAROON-X is a fiber-fed, cross-dispersed echelle spectrograph installed at the Gemini-North Observatory. Its primary science goal is the detection of Earth-sized planets in the habitable zones of mid to late M dwarf stars through high-precision radial velocity measurements, with a target precision of 1 m/s.

The instrument has no movable parts and operates in a single fixed configuration. Incoming light is split into two arms that are recorded simultaneously on independent CCD detectors:

  • Blue arm: 491-670 nm

  • Red arm: 649-920 nm

The blue detector is a 4400×4400 pixel, 16-bit array read through four amplifiers (quadrants Q1–Q4), with a gain of 2.72 e⁻/DN and read noise of 1.14 (variance units). The red detector is also 4400×4400 pixels, read through two amplifiers (R1–R2), with a gain of 2.74 e⁻/DN and read noise of 1.63 (variance units).

Raw data delivered from the Gemini Observatory Archive (GOA) arrives as a bundle — a single Multi-Extension FITS (MEF) file containing both arms (NYYYYMMDDMnnnn.fits). The first pipeline step is to split this bundle into separate blue and red arm files for independent processing.

1.2. Description of the Modes

MAROON-X has only one observing mode. At each wavelength, five fibers are arranged along the cross-dispersion direction on the detector:

  • Fiber 1: Off-target sky background fiber

  • Fibers 2, 3, 4: Three pupil-sliced fractions of the on-sky science target

  • Fiber 5: Simultaneous calibration fiber (fed calibration light during science observations)

Each fiber traces a stripe across the detector for each echelle order. The blue arm covers 34 orders per fiber and the red arm covers 28 orders per fiber.

The fiber illumination pattern for each frame is encoded as a five-character string — one character per fiber — in the file header and used by the pipeline for frame classification:

Code

Illumination source

D

Dark (no illumination)

F

Flat field lamp

O

Object (stellar target)

S

Sky background (fiber 1 only)

E

Fabry-Perot etalon comb

T

ThAr hollow-cathode arc lamp

I

Iodine cell

L

Laser Frequency Comb (LFC)

Common frame types and their fiber patterns are summarised in the table below:

Frame type

Pattern

Purpose

Science

SOOOE

Standard science: sky in fiber 1, target in fibers 2–4, etalon in fiber 5

Dark

DDDDE

Dark calibration with etalon in simultaneous calibration fiber

Flat (science fibers)

DFFFD

Flat field for science fibers 2, 3, 4

Flat (outer fibers)

FDDDF

Flat field for fibers 1 and 5

Etalon wavecal

DEEEE

Dynamic (nightly) wavelength calibration

ThAr wavecal

DTTTT

Static wavelength calibration

LFC wavecal

DLLLL

Alternative wavelength calibration with Laser Frequency Comb

1.3. Required Calibration and Associated Observations

The following calibration frames are required to reduce MAROON-X data:

Master Darks (DDDDE)

Dark frames are taken with all science fibers dark and the etalon illuminating fiber 5. This is necessary because during science observations the etalon wings contribute a few tens of counts into adjacent fibers. Dark frames must be matched to the science frame by exposure time and by the ND filter position of the simultaneous calibration fiber, and should be taken within one to two days of the science frames due to variability in the etalon source brightness.

Standard exposure times are 60, 120, 300, 600, 900, 1200, and 1800 seconds. Multiple master darks at different exposure times can be combined into pixel-by-pixel coefficient files (z0, z1), parameterising the relationship F = z1 + z0 × log10(Texp). These coefficient files allow synthetic master darks to be generated for any required exposure time.

Master Flats (DFFFD + FDDDF)

Two complementary sets of flat frames are required, because no single illumination pattern can expose all five fibers simultaneously without the wings of illuminated fibers contaminating adjacent dark fibers for straylight removal:

  • DFFFD: illuminates science fibers 2, 3, 4 only.

  • FDDDF: illuminates the outer fibers 1 and 5 only.

Each partial-illumination flat has its 2D straylight background removed before the two sets are combined into a single five-fiber (FFFFF) master flat. The master flat provides the fiber/order trace polynomials used by all subsequent extractions and the blaze function for flux normalisation.

Due to the spectrograph’s stability, one processed master flat is typically valid for at least two weeks.

Static Wavelength Calibration (ThAr, DTTTT)

ThAr hollow-cathode lamp frames establish an absolute wavelength scale accurate to approximately 500 m/s. This calibration has been performed only once in the instrument’s lifetime. The resulting solution is stored as precomputed lookup files (WLSTAT_b.fits and WLSTAT_r.fits) and is not regenerated through the pipeline.

Dynamic Wavelength Calibration (Etalon, DEEEE)

Fabry-Perot etalon frames are taken every night of observing to track instrumental drift relative to the static wavelength solution. Fitting etalon peak positions to a cubic spline restores wavelength accuracy from ~500 m/s (static) to 10-20 cm/s.

Simultaneous Wavelength Calibration (Etalon in Fiber 5)

During standard science observations (SOOOE), fiber 5 is simultaneously illuminated by the etalon. This provides a continuous record of instrumental drift at the time of each science exposure and is used in the applyWavelengthSolution step to correct the science fibers (2, 3, 4).

1.4. Important Instrument Characteristics or Issues

Sky fiber (fiber 1) illumination failure

The original flat-field design called for two complementary illumination patterns — DFFFD (science fibers 2, 3, 4) and FDDDF (outer fibers 1 and 5) — so that all five fibers could be traced and the combined master flat would be a fully illuminated FFFFF frame. In practice, there have been issues illuminating the sky fiber (fiber 1), and FDDDF flats have been replaced by DDDDF frames that illuminate only fiber 5. The pipeline therefore accepts DDDDF as a substitute for FDDDF when building the master flat.