1908.03623.pdf

Astro2020 APC White Paper: Optical and Infrared Observations from the Ground

10 July 2019

High-resolution Infrared Spectrograph for Exoplanet

Characterization with the Keck and Thirty Meter Telescopes

Dimitri Mawet

1,2

**, Michael Fitzgerald

, Quinn Konopacky

, Charles Beichman

1,2,5

, Nemanja

Jovanovic

, Richard Dekany

, David Hover

, Eric Chisholm

, David Ciardi

Etienne Artigau

Ravinder Banyal

, Thomas Beatty

, Bj

orn Benneke

, Geoffrey A. Blake

, Adam Burgasser

Gabriela Canalizo

, Guo Chen

, Tuan Do

, Greg Doppmann

, Ren

e Doyon

, Courtney

Dressing

, Min Fang

, Thomas Greene

, Lynne Hillenbrand

, Andrew Howard

, Stephen

Kane

, Tiffany Kataria

, Eliza Kempton

, Heather Knutson

, Takayuki Kotani

, David

Lafreni

ere

, Chao Liu

, Shogo Nishiyama

, Gajendra Pandey

, Peter Plavchan

, Lisa Prato

S.P. Rajaguru

, Paul Robertson

, Colette Salyk

, Bun’ei Sato

, Everett Schlawin

, Sujan

Sengupta

, Thirupathi Sivarani

, Warren Skidmore

, Motohide Tamura

, Hiroshi Terada

6, 26

Gautam Vasisht

, Ji Wang

, Hui Zhang

Abstract

HISPEC (High-resolution Infrared Spectrograph for Exoplanet Characterization) is a proposed diffraction-

limited spectrograph for the W.M. Keck Observatory, and a pathfinder for the MODHIS facility project

(Multi-Object Diffraction-limited High-resolution Infrared Spectrograph) on the Thirty Meter Telescope.

HISPEC/MODHIS builds on diffraction-limited spectrograph designs such as Palomar-PARVI and LBT-

iLocator, both of which rely on adaptively corrected single-mode fiber feeds. Seeing-limited high-

resolution spectrographs, by virtue of the conservation of beam etendue, grow in volume following a

power law (

is the telescope diameter), and are subject to daunting challenges associated with their

large size (e.g. mechanical and thermal stability). Diffraction-limited spectrographs fed by single mode

fibers are decoupled from the telescope input, and are orders of magnitude more compact and have

intrinsically more stable line spread functions. On the flip side, their efficiency is directly proportional to

the performance of the adaptive optics (AO) system. AO technologies have matured rapidly over the

past two decades, becoming mainstream on current large ground-based telescopes and baselined for

future extremely large telescopes. HISPEC/MODHIS will take R

100,000 spectra of a few objects in a

10” field-of-view sampled at the diffraction limit (10 mas scale), simultaneously from 0.95 to 2.4

m (y

band to K band). The scientific scope ranges from exoplanet infrared precision radial velocities, transit

and close-in exoplanet spectroscopy (atmospheric composition and dynamics, RM effect), spectroscopy

of directly imaged planets (atmospheric composition, spin measurements, Doppler imaging), brown

dwarf characterization, stellar physics/chemistry, proto-planetary disk kinematics/composition, Solar

system (e.g. comets), extragalactic science, and cosmology. HISPEC/MODHIS features a compact

and cost-effective design optimized to fully exploit the existing Keck-AO and future TMT-NFIRAOS

infrastructures and boost the scientific reach of both Keck Observatory and TMT soon after first light.

California Institute of Technology, CA, USA.

Jet Propulsion Laboratory, CA, USA.

University of California, Los

Angeles, CA, USA.

University of California, San Diego, CA, USA.

NASA Exoplanet Science Center, CA, USA.

Thirty Meter Telescope, CA, USA.

Universit

e de Montr

eal, QC, Canada.

Indian Institute of Astrophysics,

Bangalore, India.

University of Arizona, AZ, USA.

University of California, Riverside, CA, USA.

Purple

Mountain Observatory, Nanjing, China.

W.M. Keck Observatory, HI, USA.

University of California, Berkeley, CA,

USA.

Xiamen University, Fujian China.

NASA Ames Research Center, CA, USA.

University of Maryland, MD,

USA.

Astrobiology Center, National Institutes of Natural Sciences, Japan.

National Astronomical Observatories,

Chinese Academy of Sciences, Beijing, China.

Miyagi University of Education, Sendai, Japan.

George Mason

University, VA, USA.

Lowell Observatory, AZ, USA.

University of California, Irvine, CA, USA.

Vassar College,

NY, USA.

Tokyo Institute of Technology, Tokyo, Japan.

University of Tokyo, Tokyo, Japan.

National

Astronomical Observatory of Japan, Tokyo, Japan.

Ohio State University, OH, USA.

Nanjing University, Nanjing,

China.

E-mail, phone

: dmawet@astro.caltech.edu, 626-395-1452

arXiv:1908.03623v1 [astro-ph.IM] 9 Aug 2019

1. Key Science Goals and

Objectives

HISPEC/MODHIS will offer a powerful new win-

dow into a range of science topics, from exoplan-

ets to distant galaxies. Moreover, because of its

compact, diffraction-limited design, MODHIS is tar-

geted as a first light instrument for TMT which will

provide a high spectral resolution capability that

would not otherwise be possible for many years.

1.1 High Spectral Resolution Characteriza-

tion of Exoplanets

Using high-dispersion spectroscopy as a way of spec-

tral filtering has been successfully demonstrated in

a few studies. The improved resolution, sensitivity,

and Line Spread Function (LSF) stability provided

by the Single Mode Fiber (SMF) fed diffraction-

limited spectrograph designs will help mitigate sys-

tematic noise introduced when isolating signatures

of the planet from those of the star and terrestrial

atmosphere.

Characterization of Close-In Planets.

For transit-

ing and non-transiting hot Jupiters and warm Nep-

tunes, high-resolution transmission spectroscopy has

been used to detect molecular gas (Snellen et al.,

2010; Birkby et al., 2013; de Kok et al., 2014) and

to study day to night side wind velocity (Snellen

et al., 2010), providing an ultimate test for 3D exo-

planet atmosphere models (Miller-Ricci Kempton &

Rauscher, 2012). For planets detected with the RV

method, spectral lines owing to the planet and star

may be separated via their radial velocities (

km/s). The RV of a planet can thus be measured

to break the degeneracy between the true planet

mass and orbital inclination intrinsic to RV detec-

tion (Brogi et al., 2012, 2013, 2014; Lockwood et al.,

2014). HISPEC will provide the sensitivity, spec-

tral resolution, and spectral coverage necessary for

follow-up opportunities in the TESS era. MOD-

HIS on TMT will easily push the sensitivity bound-

aries to sub-Neptunes and super-Earths (Dragomir

et al., 2019)

†

, including the possibility of detecting

Earth-like biosignatures on rocky exoplanets around

nearby stars (Lopez-Morales et al., 2019)

†

Characterization of Long-Period Imaged Planets.

Coupling a high-resolution spectrograph with a high-

†

Denotes an Astro2020 Science White Paper

contrast imaging instrument will enable the direct

characterization of exoplanet atmospheres (Snellen

et al. 2015; Bowler et al. 2019

†

). In this scheme,

the AO system serves as a spatial filter, separating

the light from the star and the planet, and the spec-

trograph serves as the spectral filter, which differ-

entiates between features in the stellar and plane-

tary spectra (Wang et al., 2017; Mawet et al., 2017).

High-resolution spectroscopy has three game chang-

ing benefits: 1- Detailed species-by-species molecu-

lar characterization, abundance ratios such as [C/O].

2- Doppler measurements of the planet’s spin (Snellen

et al., 2014), orbital velocity, plus mapping of atmo-

spheric and/or surface features (Crossfield, 2014). 3-

Improved detection capability (Wang et al., 2017;

Mawet et al., 2017) by side-stepping speckle noise

calibration issues affecting low-resolution spectro-

scopic data from current integral field spectrographs

such as SPHERE (Beuzit et al., 2008) and GPI (Mac-

intosh et al., 2007). HISPEC on Keck will address

the detailed spectroscopic characterization of young

giant planets, but using the same technique with

MODHIS on TMT will enable the direct detection

and characterization of Neptune-size and possibly

Earth-size exoplanets (Wang et al., 2019)

†

, although

the latter case will generally require an extreme AO

front-end instrument such as PSI (see white paper

from Fitzgerald et al. 2019).

1.2 Detection of Exoplanets at or Within the

Diffraction Limit

Fiber nulling as a means to detect and character-

ize exoplanets and circumstellar disks at or within

the diffraction limit was first introduced and demon-

strated by Haguenauer & Serabyn (2006). This con-

cept is largely based on the Bracewell nulling in-

terferometer (Bracewell & MacPhie, 1979). The

first fiber nuller was demonstrated on sky at Palo-

mar observatory by Hanot et al. (2011). Ruane et al.

(2018) recently introduced the concept of vortex

fiber nulling (VFN), which circumvents the need of

a rotating baseline and greatly simplifies the design

and operation of the fiber nuller. The VFN con-

cept was demonstrated in the laboratory by Echev-

erri et al. (2019), and will be the subject of an on-

sky science demonstration at Keck observatory in

2020. A VFN mode is currently baselined for HIS-

PEC/MODHIS. The combination of VFN starlight

High-resolution Infrared Spectrograph for Exoplanet Characterization with Keck and TMT — 2/14

Spectral resolution

100,000

Wavelength coverage

0.95-2.4

m (yJHK) simultaneous

Multiplexing

1-9 channels, including object, sky, calibration

Angular resolution at y band

7 mas (TMT) - 20 mas (Keck)

Angular resolution at K band

15 mas (TMT) - 44 mas (Keck)

Field of regard

10” patrol diameter

High contrast capabilities

−

raw contrast at the diffraction limit

Point-source limiting mag (1 hr, S/N=10 per sp. ch.)

17 (Keck) – 19 (TMT) mag

Calibration

Laser Frequency Comb, Etalon, Gas cells

Instrumental stability

30 cm/s

Table 1.

HISPEC/MODHIS specifications.

suppression at the

−

raw contrast level (limited

by AO residuals and finite stellar size) and high-

resolution spectroscopy will enable the detection

and high-resolution spectroscopic characterization

of planets at or within the diffraction limit (down to

mas on TMT). Using the latest giant planet

occurrence rates from Nielsen et al. (2019), we

predict the discovery and simultaneous character-

ization of dozens of new young giant planets in

nearby young associations and star forming regions

with both Keck-HISPEC, and even more with TMT-

MODHIS. Benefiting from the giant aperture and

angular resolution of TMT, Ruane et al. (2018) pre-

dicts the detection of Ross 128 b in reflected light in

30 hours. Less challenging configurations, which in-

clude giant planet, mini-Neptunes and super-Earths

will also be accessible. The most challenging cases

of temperate Earth-size planets in less favorable con-

figurations than Ross 128 b (e.g. more distant) will

require PSI (Fitzgerald et al. 2019).

1.3 Exoplanet detection and masses with

PRV

Though we know of a multitude of planetary sys-

tems from transit missions, planetary masses are

essential for constraining their properties, including

densities and atmospheric properties (Batalha et al.,

2017). Thus, the need for new precision radial ve-

locity (PRV) instrumentation extends not only to

the current 8-10 meter telescope facilities, but be-

yond into the ELT era, where large collecting areas

will allow for the collection of high SNR spectra in

short enough time to obtain information about stellar

activity (Ciardi et al., 2019)

†

1.3.1 Stellar Jitter and the NIR Advantage

Doppler monitoring at NIR wavelengths offers two

enormous assists to the goal of measuring the masses

of exoplanets orbiting young and/or cool host stars.

First, both cool stars and the majority of young

stars are brighter in the infrared than in the op-

tical yielding higher SNR at longer wavelengths.

Second, there are theoretical and observational rea-

sons to expect that RV noise is reduced in the NIR

(Plavchan et al., 2015). Recent studies of M stars

with CARMENES show this tendency clearly in the

y-band and future work is likely to extend this trend

further into the NIR (Tal-Or et al., 2018).

Studies of very young stars have established that

the amplitude of RV variability is a factor of 2-4

lower in the NIR, where spot-to-photosphere tem-

perature contrasts are lower (Prato et al., 2008; Mah-

mud et al., 2011; Crockett et al., 2012; Carleo et al.,

2018). Robertson et al. (2016) found that the K I

doublet (7665 and 7699

A) is a good indicator of stel-

lar activity levels, leading us to investigate the utility

of the

-band K I doublet at 12,400

A and 12,500

for assessing correlations between chromospheric

activity and RV variability. HISPEC/MODHIS’s

high spectral resolution will also aid in characteriz-

ing Zeeman splitting as an indicator of stellar jitter

(Moutou et al., 2017).

1.3.2 NIR PRV science cases

The radius and ephemeris of transiting planets are

known

a priori

and the expected RV amplitude can

also be predicted to within a factor of a few. Fur-

ther, the rotation periods and characteristic lifetimes

of the surface features of host stars can often be

High-resolution Infrared Spectrograph for Exoplanet Characterization with Keck and TMT — 3/14

Figure 1.

HISPEC/MODHIS potential exoplanet

discovery and characterization space in a Mass

(Earth masses) vs Separation (SMA in AU) diagram.

The various markers denote simulated planet

populations within 27 pc using the occurrence rates

from Kopparapu et al. (2018) extrapolated up to a

semi-major axis of 30 AU using an exponential

cutoff. Various cutoffs in magnitude were applied to

reflect TMT photon noise limits in 10 hours. The

marker size is proportional to the planet size: red for

giant planets (radius

⊕

); orange for Neptunes (6

⊕

3.5); yellow for mini-Neptunes (3.5

⊕

1.75); dark green for super-Earth and Earth-size

planets (1.75

⊕

0.5); and light green for

temperate ([0

√

(

)

, 1

√

(

)

] AU)

super-Earth and Earth-size planets. The round

markers are for planets around cool stars (T

e f f

4000 K), while the square markers denote planets

around warmer stars (T

e f f

4000 K).

determined from the high precision time series pho-

tometry, critical information for the modeling of

time series RV measurements to remove stellar ac-

tivity (Aigrain et al., 2012; Haywood et al., 2014;

Dai et al., 2017).

Young Planets.

Planets transiting young host stars

have now been discovered from K2 observations of

young moving groups (

∼

50-90 Myr) (David et al.,

2016, 2018) and open clusters (

∼

600-800 Myr) (Mann

et al., 2016, 2017; Ciardi et al., 2018), with doubt-

less many more to be discovered by TESS. Among

these are ”Hot Jupiters” with large predicted Doppler

semi-amplitudes (

∼

10-100 m/s). With the expected

reduction of stellar jitter in the NIR, the intrinsic

stellar variability at HISPEC/MODHIS wavelengths

will be comparable to or at worst a factor of 2 larger

than the reflex motion due to the planets. By combin-

ing transit radii and PRV masses it will be possible

to determine the density of these still contracting

planets and thus to open a new era in the study of

the formation of gas giant planets.

Transiting Habitable Zone (HZ) Planets.

The

transiting HZ planets most favorable for follow-up

spectroscopy will orbit cool stars, e.g. Trappist-1

with 7 orbiting planets (Grimm et al., 2018), and

the nearest of these will be prime targets for spec-

troscopic studies of their atmospheres. TESS radii

combined with HISPEC/MODHIS masses will yield

planet density to determine whether a planet is rocky,

icy, or gaseous, as well as the surface gravity which

is needed for the interpretation of transit spectroscopy.

Orbital Architectures for Small Transiting Plan-

ets.

The angle between the stellar spin axis and

the angular momentum vector of the planet orbit,

or orbital obliquity, provides rich information on

the history of planet formation and evolution (Winn

& Fabrycky, 2015). Obliquity is measured via the

Rossiter-McLaughlin (RM) effect (Rossiter, 1924;

McLaughlin, 1924) by monitoring the change in

radial velocity for the duration of a transit, which

usually lasts for only a few hours, or less than half an

hour for planets in the habitable zones of the coolest

M dwarfs. HISPEC/MODHIS will provide the sen-

sitivity, spectral resolution and temporal resolution

needed to expand the current sample of small planets

whose RM effect has been measured to date (Hirano

et al., 2012; Albrecht et al., 2013; Huber et al., 2013;

Sanchis-Ojeda et al., 2015). With MODHIS on TMT,

hundreds of small planets will become accessible

resulting in unique insights into how these planets

form and migrate (Johnson et al., 2019)

†

Surveying the Coolest Mature Stars.

HISPEC /

MODHIS will have an important niche for the coolest

stars requiring the highest sensitivity (Reiners et al.,

2018) and for binary systems requiring AO to sep-

arate the components. For example, there are over

600 stars with spectral types between M9 and L9

with

−

◦

and H

14 mag (Best et al., 2018),

which Keck-HISPEC could study with

instrument-

High-resolution Infrared Spectrograph for Exoplanet Characterization with Keck and TMT — 4/14

limited

precision of

30 cm/s. With just a few

observations per object, HISPEC could screen these

systems for RV variations and then look for planets

where the problem of rapid rotation (

20 km/s)

would be at least partially offset by the large signals

expected for Uranus-mass objects (e.g. 20 m/s for a

Uranus orbiting a 50 M

Jup

brown dwarf in a 30 day

orbit). MODHIS on TMT will provide improved an-

gular resolution and sensitivity so closer separation

and fainter binaries can be addressed.

1.4 Stellar and Planetary System Formation

Proto-planetary Disks.

The evolution, and ultimately

the dispersal of proto-planetary (PP) disks, holds the

key to many open questions related to planet for-

mation. High-resolution infrared spectroscopy is

instrumental in understanding PP disk gas dynamics

(e.g. the study of the emission and absorption of

CO ro-vibrational lines). Spatially resolved high-

resolution spectro-astrometry (at mas scales) of the

molecular gas allows one to measure its distribution

in space and velocity, and to correlate these measure-

ments with disk geometries and accretion activity.

Infrared spectroscopy also enables detailed charac-

terization of the molecular composition of young

disks, including organic molecules (Jang-Condell

et al., 2019)

†

Detecting/Characterizing Young Forming Plan-

ets.

Answering the key question ”How do giant

planets form?” cannot be achieved by only observ-

ing older, dynamically evolved systems. Imaging

extrasolar giant planets near the epoch of their forma-

tion is much easier, due to their higher luminosities.

This requires the full angular resolution of a large

telescope such as Keck or TMT, in order to separate

giant planets at solar system scales from their host

stars (Sallum et al., 2019)

†

. Most of the luminosity

of these forming planets is expected to be emitted

at NIR wavelengths (Zhu et al., 2015) from a cir-

cumplanetary disk. It has so far proved difficult to

uniquely separate circumplanetary emission from

circumstellar disk emission, as this requires resolv-

ing line emission at high spectral dispersion. The

kinematic signatures of these warm disks will occur

in the range of 3-30 km/s (similar to the velocities

of Jupiter’s moons), with strong line emission from

CO and H

O in the bandpass of HISPEC. These

data might even lead to measurements of the dynam-

ical masses of these forming exoplanets, in much

the same way as one can derive masses of T Tauri

stars from the velocity curves of circumstellar disks.

Spatially resolved, high-resolution spectroscopy of

young giant planets will also enable searches for

accretion signatures, if the planet is still growing. Fi-

nally, Doppler monitoring of directly-imaged planets

of any age could lead to discoveries of exomoons

(Vanderburg et al., 2018).

1.5 Physics of Very Low Mass Objects

HISPEC/MODHIS have the potential to revolution-

ize our understanding of the detailed properties of

brown dwarfs, the cousins of gas giant planets. The

simultaneous wavelength coverage provided by these

instruments will allow for detailed weather monitor-

ing on L and T dwarfs, while the high spectral res-

olution offers the opportunity for Doppler imaging

of their surfaces (Crossfield, 2014). The strength

of their magnetic fields can be probed with high

resolution spectroscopy, and kinematic information

gleaned from measurements of the RVs of the lowest

mass cluster members (Burgasser et al., 2019)

†

Furthermore, PRV measurements of binary sys-

tems, enabled by the high spatial resolution of the

Laser Guide Star (LGS) AO feed, will advance our

understanding of the physics of very low mass ob-

jects in a number of ways: establishing highly pre-

cise 3D orbits of short period sub-stellar objects

to establish mass benchmarks for testing evolution-

ary/spectral models (Konopacky et al., 2010; Bur-

gasser et al., 2012); examining low mass compan-

ions of more massive stars with well determined

ages, metallicity, etc such as the T dwarf companion

to G3 star, HD 19467 (Crepp et al., 2015); using

PRV and Gaia data to constrain the stellar mass-

radius relationship for metal-poor, low mass objects

using transiting, low mass halo stars discovered by

K2 (Saylor et al., 2018).

1.6 Solar System Science

A large wavelength coverage, high-dispersion in-

frared spectrograph is key to characterizing the molec-

ular composition of planets and small bodies in the

Solar system, including distant Kuiper Belt objects

(KBOs) and comets. For the well-known Jovian and

Saturnian moons, the diffraction limit of the adap-

tively corrected 10-30 meter telescopes allows spa-

tially resolving surface features (e.g. volcanoes on

High-resolution Infrared Spectrograph for Exoplanet Characterization with Keck and TMT — 5/14

Io or cryovolcanoes on Europa) and performing de-

tailed remote molecular characterization (Chanover

et al., 2019)

†

. HISPEC/MODHIS will enable new

science on the planets in our Solar System. High

resolution, AO-fed spectroscopy can shed light on

the methane abundance and variability across the

Martian surface, an area of great interest for astro-

biology (Wong et al., 2019)

†

. It can also be used

to investigate cloud features on the ice giant worlds

like Neptune and Uranus.

1.7 Galactic and Extra-galactic Science

HISPEC/MODHIS will be workhorse instruments

for a wide range of Galactic science. This includes

abundance and kinematic characterization of individ-

ual stars in the field, in clusters and in other unique

areas of the Milky Way, such as the Galactic Center

(Do et al., 2019)

†

. In the Galactic Center, there is

also the opportunity to test fundamental physics such

as the constancy of fundamental constants in the

presence of an extreme gravitational field. The spa-

tial resolution of HISPEC/MODHIS plus the spec-

tral resolution are required to look at variations of

the fine structure constant, for example. Additional

effects of general relativity from the measurement

of orbits of stars near the SgrA* will be more easily

detected with high resolution spectroscopy than with

continued astrometric monitoring, offering a unique

niche for both HISPEC and MODHIS in this field.

HISPEC/MODHIS will also perform extragalac-

tic science, including follow-ups of ultra-luminous

infrared transients (e.g. supernovae in nearby galax-

ies) from ZTF and LSST, studies of stars in ultra-

compact dwarf galaxies, and spectro-astrometry of

rotating gas disks for the detection of supermassive

black holes in galactic nuclei. The instruments can

be used to detect intermediate mass black holes,

characterize AGN mergers, and investigate stellar

feedback in nearby galaxies. Both HISPEC and

MODHIS are being designed with laser guide star

AO systems, which opens up a wide range of faint

object science previously unavailable to current in-

strumentation.

2. Technical Overview

In this section, we describe HISPEC, which is the

diffraction-limited high-resolution infrared spectro-

graph concept common to both Keck and the TMT-

MODHIS facility. At Keck, HISPEC is to be fed

by the front-end fiber injection unit of the Keck

Planet Imager and Characterizer (KPIC, see Mawet

et al., 2016, 2018; Delorme et al., 2018). TMT-

MODHIS includes the HISPEC spectrograph and

a new front-end instrument interfacing the spectro-

graph to the first-light AO system of TMT, NFI-

RAOS (Narrow Field InfraRed Adaptive Optics Sys-

tem, Herriot et al., 2014).

2.1 Front-end instrument

The front-end instrument (FEI) is the essential link

between the AO system and the single-mode spec-

trograph. Its purpose is to inject the diffraction-

limited beam of the target into one or several SMFs

and maintain accurate alignment throughout long-

exposure observations. The pointing accuracy and

stability is achieved through active sensing and con-

trol of the target and fiber positions using a scheme

similar to Colavita et al. (1999).

An actuated tip-tilt mirror (TTM) is used to align

the target image position with the tip of the SMF,

whose relative locations are determined by simul-

taneously imaging the scene and the SMF on to a

acquisition/tracking imaging camera (Figure 2). A

beamsplitter (BS) or dichroic reflects part of the sci-

ence beam to the tracking camera directly after the

TTM. To locate the SMF, a light source is retro-fed

through a set of reference alignment fibers located

close to the science fiber. Ideally the reference fibers

are part of the same bundle as the science fibers, en-

suring mechanical stability of their relative positions.

The BS reflects light from the SMF towards a corner

cube (CC) retroflector, which sends the beam back

through the BS and towards the tracking camera. A

beacon image is formed on the tracking camera at

the location of the SMF. The beacon is used to deter-

mine the TTM settings to co-align the object image

and the SMF.

The FEI is also designed to include high contrast

capabilities for faint off-axis sources (e.g., exoplan-

ets) using various pupil plane apodizers and masks,

and provide feedback mechanisms for starlight sup-

pression using the upstream AO systems (Keck AO

or NFIRAOS on TMT). Optimized Phase Induced

Amplitude Apodization (PIAA) lenses are used to

remap the input beam from the Keck and TMT aper-

tures into quasi-Gaussian beams matched to the fun-

High-resolution Infrared Spectrograph for Exoplanet Characterization with Keck and TMT — 6/14

Figure 2.

HISPEC/MODHIS Front-End Instrument (FEI) notional concept.

damental mode of the SMF as in Jovanovic et al.

(2017), enabling close to ideal injection efficiency’s

in the diffraction limit (

90%).

Multiplexing can be ensured by using image slic-

ing techniques, where the focal plane is divided in

sectors, which can all be addressed with an inde-

pendent fiber injection unit (Figure 2). Each fiber

injection unit includes a dedicated TTM that can

patrol the field of view on the fixed fiber bundle.

2.2 Spectrograph

The HISPEC notional optical design (see Figure 3) is

a diffraction-limited echellette spectrometer with an

almost all reflective design with a spectral resolving

power of R

∼

180,000 and R

∼

110,000 in the yJ and

HK passbands, respectively. The full wavelength

range from 9600

A - 23,850

A is broken up into

two wavelength channels from 9600

A - 13,270

(i.e. yJ-bands) and 14,760

A - 23,850

A (i.e. HK-

bands). The wavelength gap from 13,270

A - 14,760

A is not covered by either channel, but is also where

there is strong absorption from the atmosphere. The

main reasons to have two wavelength channels in

the spectrograph are: light does not propagate as

a single mode in a SM fiber for much more than

a 1 octave spectral range and secondly to achieve

close to Nyquist sampling over the full operating

wavelength range where there is a 2.5

factor from

the shortest to the longest wavelength.

Light enters the spectrograph through a fiber ar-

ray (2 columns). Each individual wavelength chan-

nel sees 1 column of several science, sky, and cal-

ibration fibers. The fiber array is collimated by an

F/4 Three Mirror Anastigmat (TMA). The collima-

tor forms a circular pupil with a 25 mm diameter

where a physical aperture stop is placed.

The relatively fast TMA can collimate a fiber

ferrule diameter up to 2 mm in size and therefore

is a optical design that is flexible enough to sup-

port any potential upgrades in wavelength coverage

and fiber channel multiplicity. A single Off-Axis

Parabola (OAP) collimator does not provide a suf-

ficiently large enough diffraction-limited FOV at a

F/4 focal ratio for the HISPEC fiber array and is lim-

ited to a field diameter size of

∼

0.4 mm. The TMA

collimator design will also make centering the fiber

array less sensitive to mis-alignments compared to

an OAP collimator design.

The collimated light then gets dispersed in the

spectral direction by reflecting off of a 7 lines per

mm R4 echelle grating. The grating is used in a

quasi-Littrow condition where an out-of-plane angle

of 3.5

◦

is needed to separate the incoming and

outgoing light from the grating surface sufficiently

enough to package the remaining optical elements

without interference. The echelle grating splits the

yJ-bands into 81 total spectral orders ranging from

209 - 289 and the HK-bands into 72 total spectral

orders ranging from 116 - 187.

High-resolution Infrared Spectrograph for Exoplanet Characterization with Keck and TMT — 7/14

Figure 3.

The HISPEC conceptual optical layout for the design incorporating a TMA F/4 collimator, a

reflection grating ruled with 7 lines mm

−

, a VPH cross-disperser, and a TMA F/22 camera. The

spectrograph gives a fixed spectral format on the detector array in an overall compact design form.

The dispersed light from the grating is then sepa-

rated into its two wavelength channels via a dichroic

mirror. The yJ-band is in reflection and the HK-

band is in transmission, where the dichroic 50/50

transition edge is at 14,000

A. The light then travels

into the individual cameras after passing through a

Volume Phase Holographic (VPH) cross-dispersing

transmission grating. The VPH cross-disperser al-

lows for a simpler and almost symmetric layout. To

first order a VPH grating has a constant angular

dispersion, which allows for a more uniform sepa-

ration of the orders in the cross-dispersed direction

and therefore allow for higher multiplicity of fibers

to be re-imaged between the orders. With a cross-

dispersing prism the non-linear change of index of

refraction with wavelength yields a closer spacing

of the spectral orders containing the longest wave-

lengths therefore limiting the allowed fiber multi-

plexing in the spectrograph. During the next design

phase further study will have to be made about the

cryogenic optical quality and stability of a VPH grat-

ing. Otherwise cross-dispersing prisms will have

to be used. Most likely two ZnSe prisms with a

refractive index of

∼

2.4 will have to be used in the

yJ channel and a single Silicon prism with a refrac-

tive index of

∼

3.4 for the HK channel in order to

get enough cross-dispersing power with reasonable

prism apex angles to spread the spectral orders over

the full height of the detector array. The dichroic and

cross-disperser are the only two refractive elements

in the spectrograph, where changes in the index of re-

fraction caused by thermal variations constrains any

spectrum shifts at the detector to the cross-dispersed

direction.

The individual cameras then re-images 1 column

of fibers onto their respective detectors. The HK

channel will use a Teledyne H2RG 2.5

m cut-off

detector with a 18

m per pixel size (upgrade option:

H4RG 2.5

m cut-off detector), while the yJ channel

will use a Teledyne H4RG with a 10

m per pixel

size. This roughly 2

factor in pixel size between

the detector arrays means the two channels can uti-

High-resolution Infrared Spectrograph for Exoplanet Characterization with Keck and TMT — 8/14

Figure 4.

The HISPEC spectrograph conceptual

mechanical layout.

lize an identical camera design, while maintaining

proper sampling. This will help in lowering the cost

of the cameras since they can be fabricated from the

same parent optic. The camera design is also a TMA

with a focal ratio of F/22, which gives diffraction-

limited image quality (i.e.,

95% Strehl) over the

entire detector area in both channels. The yJ chan-

nel has a spectral resolution of R

∼

180,000 with a

pixel sampling ranging from 2.1 - 2.9 pixels and a

spectral sampling of

∼

0.03

A/pix. The HK channel

has a spectral resolution of R

∼

110,000 with a pixel

sampling ranging from 1.8 - 2.9 pixels and a spectral

sampling of

∼

0.07

A/pix.

A notional mechanical design of the spectro-

graph is shown in Figure 4, featuring a compact and

simple layout, optimized for thermal and mechanical

stability.

3. Technology Drivers

Although the front-end instrument is based on mostly

mature technologies, there are some aspects that will

require further development to determine the most

efficient and robust way to realize the instrument in

time for TMT first light.

Beam shaping optics:

the PIAA lenses have

been successfully used in the past (Jovanovic et al.,

2017) but require further development to optimize

them for the two discrete wavebands of the HISPEC

spectrograph (yJ and HK).

Image slicing optics:

the choice of how to slice

the image will be partially driven by the layout of sci-

ence fields to be observed and partially by the types

and quality of optics available. Further development

may be needed here.

Optimal tracking strategies:

there are very

few SMF injections operating on-sky. Maintaining

alignment between the science target and the SMF

to within

≈

will be required to maintain ef-

ficient coupling. Optimal strategies to provide this

level of alignment are being proof-tested by KPIC.

3.1 Low loss infrared single-mode fibers

HISPEC/MODHIS will exploit SMFs because their

output beam profile does not evolve with time and is

independent of the input illumination profile. This

will provide superior stability for the line spread

function (LSF) for Doppler applications. In addi-

tion, as outlined above, a diffraction-limited feed

decouples the spectrograph performance from that

of the telescopes aperture allowing for a highly com-

pact yet very high resolution instrument to be real-

ized, substantially reducing footprint, mechanical

deflection and cost. It also means that a single spec-

trograph could be built and used at both Keck then

TMT.

However, SMFs have a limited range of wave-

lengths over which they are single-mode. There are

no SMFs covering the full y-K wavelength range.

Therefore the FIU will split the light in two pass-

bands and inject the light into two separate fiber

bundles that each span narrower wavelength ranges

(yJ and HK). There are many suitable options yJ

SMFs, including OFS BF05635-02 fiber (core and

cladding diameters of 4.4

m and 125

m, a Nu-

merical Aperture (NA) of 0.16, and a single mode

cut-off wavelength of 9600

A). For the HK SMF,

one option is offered by Le Verre Fluore (LVF), who

produce some of the highest performance fluoride

fibers. The best fit stock fiber is #3051 (core and

cladding diameters of 7.0

m and 125

m, a NA of

0.17, and a single mode cut-off wavelength of 15,500

A), which is not single-mode across the entire H-

band. A custom fiber will need to be developed with

LVF that operates across HK with minimal losses.

Alternatively, one could consider shifting the split

from between the J and H bands to in the middle

of the H band, but then the location of the dichroic

edge would need to be carefully considered to not

coincide with important spectral features.

High-resolution Infrared Spectrograph for Exoplanet Characterization with Keck and TMT — 9/14

3.2 Few mode fibers and photonic lanterns

Although the priority is to utilize SMFs, it may be

favorable to use fibers with higher collection effi-

ciency’s in the shorter wavebands (yJ) where the

Strehl ratio and hence coupling efficiency will be

lower. For this there are two options: few mode

fibers and photonic lanterns. Few mode fibers lay

at the boundary between multimode fibers (MMF)

and SMFs and support as their name implies, a few

modes (typically 2-8). By supporting more modes

these fibers typically allow for higher coupling ef-

ficiency than a SMF (Horton & Bland-Hawthorn,

2007). Owing to the fact that they support several

modes, ensuring the output beam is homogenized

and hence stable with time will require active scram-

bling. Alternatively, a photonic lantern consists of

a MMF at the input end and series of SMFs at the

other end. The light makes a gradual transition from

the MMF end, which is trivial to couple into to a

series of SMFs which offer diffraction limited perfor-

mance to the spectrograph (Leon-Saval et al., 2013;

Birks et al., 2015). In this way the flux will be

split amongst fibers reducing the flux/channel, but

the total flux will be much higher than for a SMF,

especially at shorter wavelengths where the AS per-

formance is reducing. We will source commercially

available versions of both fiber types and conduct

laboratory experiments to assess their applicability

for HISPEC/MODHIS. We will then work with ven-

dors and other research institutes to customize these

devices for our application. This development is

expected to occur over the next 5 years.

3.3 Infrared Laser Frequency Combs

Infrared laser frequency combs (LFCs) will pro-

vide long term, sub-m/s precision across the HIS-

PEC band. Devices now in hand or in development

use Electro-Opical Modulation (EOM) to generate

combs with intrinsic spacings of 10-30 GHZ which

are readily resolvable with R

∼

100

000

spectro-

graphs without the need for extensive pre-filtering.

These combs have been demonstrated at Keck (Yi

et al., 2016) and with HPF at the Hobby-Eberly tele-

scope (Metcalf et al., 2019). They can either be

referenced to a stable laser line such as acetylene (

30 cm/s), to a second, 100 MHz comb (

10 cm/s),

or via

−

self-referencing (sub cm/s stability tied

to an SI frequency standard). New LFC technolo-

Phase

Cal.

Years

Labor

(wk yrs)

Total

Cost

Conceptual

Design

1.00

6.1

$1.714M

Preliminary

Design

1.00

8.5

$3.190M

Final

Design

1.25

11.0

$3.265M

Fabrication

1.50

9.2

$10.311M

Integration

1.25

16.5

$4.474M

AIV

1.00

6.9

$1.854M

Total

7.00

58.2

$24.808M

Table 2.

Development schedule for MODHIS,

broken down by phase, duration, and labor.

gies based on micro-resonator microcombs promise

to greatly reduce the size, cost and complexity of

NIR LFCs with a prototype already demonstrated

at Keck (Suh et al., 2019). Future LFC’s will be

developed with extended wavelength range to cover

y-K. We plan to further develop this new technology

with significant investment over the next 5-7 years.

4. Organization, Partnerships, and

Current Status

HISPEC/MODHIS is currently led by Caltech, UC

Los Angeles, and UC San Diego, with science team

members from the entire US community, and TMT

International Observatory (see author list). Partic-

ipation in the instrument construction is currently

being discussed with partners from Canada (NRC,

University of Montreal) and Japan. We expect to

extend the instrument team to TMT international

partners, including China and India.

5. Schedule and Cost Estimates

In this section, we focus on the schedule and cost

estimate for the TMT-MODHIS facility, which in-

cludes the full cost of the HISPEC spectrograph. As

mentioned before, the fabrication and deployment

of HISPEC is currently being fast-tracked for Keck

Observatory as a pathfinder to TMT-MODHIS.

5.1 Schedule

The MODHIS schedule and costing draws credibil-

ity from similarly scoped high-resolution spectro-

graphs (e.g., PARVI), FEI (e.g., KPIC), and the other

High-resolution Infrared Spectrograph for Exoplanet Characterization with Keck and TMT — 10/14

TMT first light instruments (IRIS and WFOS). We

also expect to draw upon ongoing development of

Keck-HISPEC to help accelerate the design phases

of MODHIS. The development of MODHIS follows

a 7 year schedule outlined in Table 2. We anticipate

the three design phases (conceptual, preliminary,

and final) to last 2.25 years, followed by a 1.5 year

fabrication phase, a 1.25 year integration phase, and

finally a 1 year assembly, integration, and validation

(AIV) phase at the telescope.

5.2 Cost Estimates

The development of MODHIS is costed at just un-

der $25M, placing it at the low-end of the medium

cost category for ground-based projects evaluated by

ASTRO2020: “Funding for this effort is expected to

come from TMT and its Members”. A full cost esti-

mate of developing MODHIS, including sufficient

contingency, was performed in June 2019 by Cal-

tech and TMT. Table 3 itemizes MODHIS costs by

work breakdown structure tasks – the methodology

is consistent with other TMT first light instruments.

We highlight specific deliverables below.

The “Structure & OIWFS” element contains the

MODHIS interface and support structure that en-

ables attachment to NFIRAOS (TMT first light AO

system), the MODHIS front-end instrument rotator,

and the on-instrument wavefront sensor (OIWFS)

assembly, including the OIWFS detector cryostat

and controllers.

The “Front End Instrument” element includes

the subsystem that acquires, directs, collects, and

maintains alignment of the light at the output of

NFIRAOS, delivering the light into the single mode

fiber feeding the spectrograph. The cost includes all

elements plotted in Figure 2.

The costing of the “Spectrograph” is directly in-

formed by a detailed cost analysis of both PARVI

and Keck-HISPEC. This subsystem includes the

spectrograph cryostat and optics, two infrared H4RG

detectors and associated readout electronics, the in-

ternal calibration system (including two stable Laser

Frequency Combs), and all equipment needed for

integration and testing, including summit AIV.

The costing of the “Instrument Control Software”

element contains all software related activities re-

quired to support observing and testing, including

the instrument sequencer, hardware control modules,

Task

Labor

(wk yrs)

Total

Cost

($)

Total

Cost

(%)

Project

Management

3.7

$1.908M

Systems

Engineering

6.2

$2.197M

Science

Team

5.5

$1.065M

Structure &

OIWFS

11.1

$4.875M

Front End

Instrument

7.4

$2.924M

Fiber

Management

1.8

$0.586M

Spectrograph

8.1

$7.347M

Instrument

Control SW

5.4

$1.383M

Data Reduction

Pipeline

1.5

$0.389M

Integration

& AIV

7.5

$2.134M

Total

58.2

$24.808M

100

Table 3.

Development cost for MODHIS broken

down by task, labor, and cost.

interfaces to NFIRAOS, and hardware necessary for

the development and prototyping of the software

deliverables.

The “Integration & AIV” element includes the

integration of the MODHIS subsystems to form the

overall MODHIS science instrument, including in-

tegrating the OIWFS with the front end instrument,

the front end instrument with the fiber system, and

the fiber system with the spectrograph. All of these

integration steps will involve execution of progres-

sive acceptance test plans culminating in a pre-ship

readiness review. AIV includes hardware integra-

tion with NFIRAOS and the telescope, along with

integrating the control software and data reduction

pipeline to the observatory software environment.

High-resolution Infrared Spectrograph for Exoplanet Characterization with Keck and TMT — 11/14

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