Emerging Age of Precision Spectroscopy using
Astro Combs: From OAO/HIDES to TMTMalte Schramm AIST
Sendai Dec 2015

Team: H. Inaba, S. Okubo, K. Nakamura, A. Onae, K. Hosaka, , T.
Kobayashi (AIST)
K. Minoshima (UEC), F.L. Hong (YNU),Hiroki Yamamoto (YNU)
Collaborators: H. Izumiura, E. Kambe (OAO)

Astro-comb facilities

HET
FLWO/TRES
Subaru/IRD (2016)& IRTF/CSHELL
Keck(2019)
ESO/HARPS& ESPRESSO (2016?)

TNG/HARPS-N& VTT

CAHA/PMAS

OAO/HIDES
2015/16

Why Astro-comb?
The “comb” cosnists of evenly spaced lines who’s frequencies are known apriori to better than 1 in 1015
●●

●

●

Homogenous
wavelength coverageHigh intensity lines overthe whole range
Simultaneous reference
but no overlap spectrum(e.g. Iodine Cell)
Ultra stable(Repeatability)-4.2 m/s/yr

Ar2Th line drift

et al.

What is an Astro-comb
Reference Laser

Defines overall accuracy
Pulse Formation in Oscillator

Optical Continuum
Using HNLF after
Er-doped Fiber Amplifier
Stellar light

Yi et al. 2015

OAO/HIDES Comb Character Sheet●●●●●Optical Comb 380-540 nm (300-2500 nmpossible)
homogenous line spacing 100 Mhz → 20 Ghz
narrow comb lines with a FWHM of few kHz(determined by the reference laser)
Extremely low frequency (wavelength)
uncertainty of the comb itself at the sub-mm/s
level
Final accuracy ~<m/s level (current level 30 m/swith Th-Ar)

OAO HIDES Comb – Main
Components I

Things to do●

Ensure similar line intensities (maybe
additional filter necessary)●

Ensure the system is robust and easy to use●

Optimize final RV accuracy affected by- Thermo-mechanical Stability of the spectrograph- Thermo-mechanical Stability of the detector- Stability of the light injection

Probably a few more ...

How does a comb signal look like
Test data from ESO 3.6m/HARPS (2010)

EchelleOrder

Zoom infrom ESO archiveAstro-comb vs. Th-Ar
Reference ModeCOMBTh-Ar

The Comb Spectrum

et .alExample of LFC calibrated stellar spectrum
Comb

Normalized flux

Star

Wavelength

Yi et al. 2015

A comb can reveal CCD inhomogeneities

Most important application for now: Exoplanets
Jupiter@ 1 AU: 28.4 m s-1Jupiter@ 5 AU: 12.7 m s-1 Possible targets for HIDES

Neptune@ 0.1 AU : 4.8 m s-1

Neptune@ 1 AU

RV Stability: 1.5 m s-1

Super-Earth (5 M⊕)@ 0.1 AU: 1.4 m s-1

Super-Earth (5 M⊕)@ 1 AU: 0.45 m s-1

Earth●@ 1 AU: 9 cm s-1

Planet Detectability with radialvelocities
Typical issues:–

Telescope guiding (30 cm/s)–

Detector instabilities and wavelength reference
precisionDetecting low-mass planets
2 cm/s10 cm/s
1 m/s

MassJupiter

NeptuneEartha [AU]Pasquini et al.

Binary BHs on sub pc – pc scales●Here SUBARU (HSC+PFS) can contribute: Only very
few candidates known

MJD

Our ownmeasurementsfrom 2014

Log Orbital Period

Magnitude

We need spectroscopic monitoringwith accurate wavelength solution toconfirm the binary nature andprovide targets for gravitational
wave detections
Graham et al.2015 Nature

Monitoring AGN to identify potential
candidates

BH massCosmological variation of the fine structure
constant
Variation of 1 ppm in α or μ leads to a velocity shift of 20 m/s●●●●

Typically use Quasar
absorption line systems
e.g. Lya forest
Currently extremely
challenging task
Good test for next
generation telescopes
Precision can finallycompete with clock
measurements
CODEX like instruments aim at even better
presicion

Most challenging task:
Direct measurements of the dynamics of the universe”It should be possible to choose between various models of the expanding
universe if the deceleration of a given galaxy could be measured. Precise
predictions of the expected change in z=d/ 0 for reasonable observing
times (say 100 years) is exceedingly small. Nevertheless, the predictions
are interesting, since they form part of the available theory for theevolution of the universe” Sandage 1962 ApJ 136,319

RV: 2.8 cm/s

Most challenging task:
Direct measurements of the dynamics of the universe”It should be possible to choose between various models of the expanding
universe if the deceleration of a given galaxy could be measured. Precise
predictions of the expected change in z=d/ 0 for reasonable observing
times (say 100 years) is exceedingly small. Nevertheless, the predictions
are interesting, since they form part of the available theory for theevolution of the universe” Sandage 1962 ApJ 136,319
Requirements:

CODEX simul.

* TMT or E-ELT class telescope*Absolute wavelength accuracy
at 1-2 cm/s (~25 kHz) given 20 yrtime scale*Stability 1-2 cm/s/night
LONG TERM STABILITY IS
CRUCIAL

Liske et al. 2009

Science Requirements or the need for LFCs

Haehnelt 2010

Summary●●●

Astro-combs provide a reproducible, (long-term) stablewavelength calibrator of evenly spaced lines with known
frequencies at 1 in 1015
Currently main application for exoplanet detection (evenwith small telescopes)
TMT and E-ELT will enable new science ()– Variation of fine structure constant comparable toatomic clocks– Sandage test (Cosmological probe)– Primordial nucleosythesis