What are the current limitations to achieving high-quality speckle nulling on-sky?

The current limitations to achieving high-quality speckle nulling on-sky are wavefront correction, readout noise, and loop speed.

What is the method for removing speckles from the dark hole?

By iterating cycles of measurement and correction, starlight speckles that are sufficiently slow to last multiple cycles are removed from the dark hole area.

What is the camera used to capture the image?

The camera used to capture the image is not conjugated to the plane of the DM so that phase information gets recorded as amplitude information on the camera.

What are the free parameters that can be adjusted?

The amplitude of the RMS wavefront map, the magnitude of the low spatial frequency modes, and the speed of the subarray passing over the map (i.e., windspeed) are all free parameters that can be adjusted.

How did the nulling process reduce the average flux over the entire controlled area?

The nulling process reduced the average flux over the entire controlled area by 30% and by 58% in the region between 5–12λ=D, where the nulling was most effective.

What is the magnitude limit for speckle nulling?

As the speckles are ∼1000× fainter than the PSF core, a magnitude limit for speckle nulling of 3–4 in the H-band is imposed by the current cameras used due to the high readout noise (114 e ).

Why is the PIAA lens design used in SCExAO achromatic?

Due to the low material dispersion of CaF2, the conventional PIAA lens design used in SCExAO is achromatic across the NIR (y-K bands).

What is the optical fiber used to transport the light to the SCExAO bench?

The light is coupled into an endlessly single-mode photonic crystal fiber (NKT photonics - areoGUIDE8) which transports the light to the SCExAO bench.

How many fibers are used in the recombination bench?

Currently 18 of the 30 available fibers (2 sets of 9 fibers each) are used and they transport the light to a separate recombination bench (see Fig. 3) where the interferograms are formed and data collected.

What is the purpose of the speckle nulling algorithm?

This enables speckle nulling to be performed on fainter, more scientifically relevant targets, and for noncommon speckles due to chromatic dispersion in the atmosphere to be corrected for the first time, allowing for a significant improvement in detectability of faint companions.

What is the way to minimize the drift of the PSF?

the mounts for the OAP-based relay optics in the instrument were custom made from a single piece to minimize the drift of the PSF.

What is the difference between the turbulence simulator and the AO188?

Note that, due to the limited stroke of the DM (which is discussed in § 3.1.1), the turbulence simulator cannot be used to simulate full seeing conditions but provides a level of wavefront perturbation that is representative of post AO188 observing conditions.

(Open Access) The Subaru Coronagraphic Extreme Adaptive Optics System: Enabling High-Contrast Imaging on Solar-System Scales (2015) | Nemanja Jovanovic

Q: What technique has been used to make detections of planets?

Although angular differential imaging is the most commonly used technique for imaging planets thusfar (Marois et al. 2008), coronagraphy (Lafreniere et al. 2009; Serabyn et al. 2010a) and aperture-masking interferometry (Kraus & Ireland 2012) have also been used to make detections.

The Subaru Coronagraphic Extreme Adaptive Optics System:

Enabling High-Contrast Imaging on Solar-System Scales

N. J

OVANOVIC

1,2

F. M

ARTINACHE

O. G

UYON

1,4,5

C. C

LERGEON

G. S

INGH

1,6

T. K

UDO

V. G

ARREL

K. N

EWMAN

5,8

D. D

OUGHTY

1,5

J. L

OZI

J. M

ALES

4,9

Y. M

INOWA

Y. H

AYANO

N. T

AKATO

J. M

ORINO

J. K

UHN

E. S

ERABYN

B. N

ORRIS

P. T

UTHILL

G. S

CHWORER

6,12

P. S

TEWART

L. C

LOSE

E. H

UBY

6,13

G. P

ERRIN

S. L

ACOUR

L. G

AUCHET

S. V

IEVARD

N. M

URAKAMI

F. O

SHIYAMA

N. B

ABA

T. M

ATSUO

J. N

ISHIKAWA

M. T

AMURA

10,16

O. L

1,7

F. M

ARCHIS

G. D

UCHENE

18,19

T. K

OTANI

AND

J. W

OILLEZ

Received 2015 February 19; accepted 2015 June 23; published 2015 August 31

ABSTRACT. The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is a multipurpose

high-contrast imaging platform designed for the discovery and detailed characterization of exoplanetary systems

and serves as a testbed for high-contrast imaging technologies for ELTs. It is a multiband instrument which makes

use of light from 600 to 2500 nm, allowing for coronagraphic direct exoplanet imaging of the inner 3λ=D from the

stellar host. Wavefront sensing and control are key to the operation of SCExAO. A partial correction of low-order

modes is provided by Subaru’s facility adaptive optics system with the final correction, including high-order modes,

implemented downstream by a combination of a visible pyramid wavefront sensor and a 2000-element deformable

mirror. The well-corrected NIR (y-K bands) wavefronts can then be injected into any of the available coronagraphs,

including but not limited to the phase-induced amplitude apodization and the vector vortex coronagraphs, both of

which offer an inner working angle as low as 1λ=D. Noncommon path, low-order aberrations are sensed with a

coronagraphic low-order wavefront sensor in the infrared (IR). Low noise, high frame rate NIR detectors allow for

active speckle nulling and coherent differential imaging, while the HAWAII 2RG detector in the HiCIAO imager

and/or the CHARIS integral field spectrograph (from mid-2016) can take deeper exposures and/or perform angular,

spectral, and polarimetric differential imaging. Science in the visible is provided by two interferometric modules:

VAMPIRES and FIRST, which enable subdiffraction limited imaging in the visible region with polarimetric and

spectroscopic capabilities respectively. We describe the instrument in detail and present preliminary results both on-

sky and in the laboratory.

Online material: color figures

National Astronomical Observatory of Japan, Subaru Telescope, 650 North A’Ohoku Place, Hilo, HI, 96720; jovanovic.nem@gmail.com.

Department of Physics and Astronomy, Macquarie University, Sydney NSW 2109, Australia.

Observatoire de la Cote d’Azur, Boulevard de l’Observatoire, Nice 06304, France.

Steward Observatory, University of Arizona, Tucson, AZ 85721.

College of Optical Sciences, University of Arizona, Tucson, AZ 85721.

LESIA, Observatoire de Paris, 5 Place Jules Janssen, Meudon 92195, France.

Gemini Observatory, c/o AURA, Casilla 603, La Serena, Chile.

NASA Ames Research Center, Moffett Field, CA 94035.

NASA Sagan Fellow.

National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Japan.

Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109.

Sydney Institute for Astronomy (SIfA), Institute for Photonics and Optical Science (IPOS), School of Physics, University of Sydney, NSW 2006, Australia.

Département d'Astrophysique, Géophysique et Océanographie, Université de Liège, 17 Allée du Six Août, 4000 Liège, Belgium.

Division of Applied Physics, Faculty of Engineering, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan.

Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan.

Department of Astronomy, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan.

Carl Sagan Center at the SETI Institute, Mountain View, CA 94043.

Astronomy Department, University of California, Berkeley, CA 94720-3411.

University Grenoble Alpes & CNRS, Institut de Planetologie et d’Astrophysique de Grenoble (IPAG), Grenoble F-3800, France.

European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, Garching 85748, Germany.

890

UBLICATIONS OF THE

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OCIETY OF THE

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, 127:890–910, 2015 September

1. INTRODUCTION

The field of high-contrast imaging is advancing at a great

rate with several extreme adaptive optics systems having come

online in 2014, including the Gemini Planet Imager (GPI)

(Macintosh 2014), the Spectro-Polarimetric High-contrast Exo-

planet REsearch instrument (SPHERE) (Beuzit et al. 2008), and

the focus of this work, the Subaru Coronagraphic Extreme

Adaptive Optics (SCExAO) system which join the already run-

ning P1640 (Dekany et al. 2013). These systems all share a simi-

lar underlying architecture: they employ a high-order wavefront

sensor (WFS) and a deformable mirror (DM) to correct for atmo-

spheric perturbations enabling high Strehl ratios in the near-

infrared (NIR) (>90%), while a coronagraph is used to suppress

on-axis starlight downstream. The primary motivation for such

instrumentation is the direct detection of planetary mass compan-

ions at contrasts of 10

5

–10

6

with respect to the host star, at

small angular separations (down to 1–5λ=D) from the host star.

The era of exoplanetary detection has resulted in ∼1500 plan-

ets so far confirmed (Han et al. 2014). The majority of these

were detected via the transit technique with instruments such

as the Kepler Space Telescope (Borucki et al. 2010). The radial

velocity method (Mayor & Queloz 1995) has also been prolific

in detection yield. Both techniques are indirect in nature (the

presence of the planet is inferred by its effect on light from

the host star) and hence often deliver limited information about

the planets themselves. It has been shown that it is possible to

glean insights into atmospheric compositions via techniques

such as transit spectroscopy (Charbonneau 2001), whereby star

light from the host passes through the upper atmosphere of the

planet as it propagates to Earth, albeit with limited signal-to-

noise ratio. The ability to directly image planetary systems

and conduct detailed spectroscopic analysis is the next step to-

ward understanding the physical characteristics of their mem-

bers and refining planetary formation models.

To this end, so far <50 substellar companions have been di-

rectly imaged (see Fig. 3 in Pepe et al. [2014]). The challenge

lies in being able to see a companion, many orders of magnitude

fainter, at very small angular separations (<1″), from the blind-

ing glare of the host star. Indeed, the Earth would be >10

fainter than the sun if viewed from outside the solar system

in reflected light. Although these levels of contrast cannot be

overcome from ground-based observations at small angular sep-

arations (<0:5″), it is possible to circumvent this by imaging the

thermal signatures instead and targeting bigger objects. Indeed,

all planets imaged thus far were large Jupiter-like planets (which

are brightest) detected at longer wavelengths (in the near-IR H

and K-bands and the mid-IR L and M-bands) in thermal light (a

subset of detections include Kraus & I reland [2012]; Lagrange

et al. [2009]; Marois et al. [2008]). To overcome the glare from

the star which results in stellar photons swamping the signal

from the companion, adaptive optics systems (AO) are key

(Lagrange et al. 2009). Although angular differential imaging

is the most commonly used technique for imaging planets thus

far (Marois et al. 2008), coronagraphy (Lafreniere et al. 2009;

Serabyn et al. 2010a) and aperture-masking interferometry

(Kraus & Ireland 2012) have also been used to make detections.

With the direct detection of the light from the faint companion

itself, spectroscopy becomes a possibility and indeed prelimi-

nary spectra have been taken for some objects as well (Barman

et al. 2011; Oppenheimer et al. 2013).

In addition to planetary spectroscopy, how disks evolve

to form planetary systems is a key question that remains un-

answered. Thus far coronagraphic imagers like HiCIAO at

the Subaru Telescope have revealed intricate features of the in-

ner parts of circumstellar disks using polarization differential

imaging (under the SEEDS project [Tamura et al. 2009]). These

solar-system scale features include knots and spiral density

waves within disks like MWC758 and SAO 206462 (Grady et al.

2013; Muto et al. 2012). How such features are affected by or

lead to the formation and evolution of planets can only be ad-

dressed by high-contrast imaging of the inner parts (up to 15 AU

from the star) of such disks. To address the lack of information

in this region, high-contrast imaging platforms equipped with

advanced wavefront control and coronagraphs are pushing

for smaller inner working angles (IWA). In the limit of low

wavefront aberrations, currently achieved with AO systems op-

erating in the near-IR, coronagraphs are the ideal tool for im-

aging the surrounding structure/detail as they are unrivaled in

achievable contrast. Both the contrast and IWA are dependent

on the level of wavefront correction available. With wavefront

corrections typically offered by facility adaptive optics (AO)

systems on 5–10-m class telescopes (∼30–40% in H-band),

previous generations of coronagraphic imagers, such as the

Near-Infrared Coronagraphic Imager (NICI) on the Gemini

South telescope (Artigau 2008), were optimized for an IWA of

5–10λ=D. However, with extreme AO (ExAO) correc tion offer-

ing high Strehl ratio and stable pointing, GPI and SPHERE have

been optimized for imaging companions down to angular sep-

arations of ∼3λ=D (>120 mas in the H-band). SCExAO utilizes

several more sophisticated coronagraphs including the Phase -

Induced Amplitude Apodization (PIAA) (Guyon 2003) and

vector vortex (Mawet et al. 2010), which drive the IWA down

to just below 1λ=D. At a distance of 100 pc, the PIAA/vortex

coronagraphs on SCExAO would be able to image from 4 AU

outward (approximately the region beyond the orbit of Jupiter).

Further, in the case of the HR8799 system, the IWA would be

1.6 AU (the distance of Mars to our Sun) making it possible for

SCExAO to image the recently hypothesized fifth planet at

9 AU (mass between 1 and 9 Jupiter masses) (Gozdziewski &

Migaszewski 2014) if it is indeed present as predicted.

Despite the state-of-the-art IWA offered by these corona-

graphs in the near-IR, the structure of disks and the distribution

of planets at even closer separations than 1λ=D will remain in-

accessible with coronagraph technology alone. This scale is sci-

entifically very interesting as it corresponds to the inner parts of

the solar system where the majority of exoplanets have been

THE SCEXAO HIGH-CONTRAST IMAGER 891

2015 PASP, 127:890–910

found to date based on transit and radial velocity data (Han et al.

2014). To push into this regime, SCExAO uses two visible

wavelength interferometric imaging modules known as VAM-

PIRES (Norris et al. 2012a) and FIRST (Huby et al. 2012).

VAMPIRES is based on the powerful technique of aperture

masking interferometry (Tuthill et al. 2000), while FIRST is

based on an augmentation of that technique, known as pupil

remapping (Perrin et al. 2006). Operating in the visible part

of the spectrum, the angular resolution of these instruments

on an 8-m class telescope approaches a territory previously re-

served for long baseline interferometers (15 mas at λ ¼ 700 nm)

and expands the type of target that can be observed to include

massive stars. Although the modules operate at shorter wave-

lengths where th e wavefront correction is of a lower quality,

interferometric techniques allow for subdiffraction limited im-

aging (10 mas resolution using λ=2D criteria as conventionally

used in interferometry) even in this regime, albeit at lower con-

trasts (∼10

3

), making optimal use of the otherwise discarded

visible light. Despite the lower contrasts, aperture-masking in-

terferometry has already delivered faint companion detections at

unprecedented spatial scales (Kraus & Ireland 2012). Each

module additionally offers a unique capability. For example,

the polarimetric mode of VAMPIRES is designed to probe

the polarized signal from dusty structures such as disks around

young stars and shells around giant stars (Norris et al. 2012b,

2015) at a waveband where the signal is strongest. This is a visi-

ble analog of that offered by the SAMPol mode on the NACO

instrument at the VLT (Lenzen et al. 2003; Norris et al. 2012b;

Tuthill et al. 2010). FIRST, on the other hand, offers the poten-

tial for broadband spectroscopy and is tailored to imaging

binary systems and the surface features of large stars. Such ca-

pabilities greatly extend SCExAO beyond that of a regular ex-

AO facility.

Finally, with a diffraction-limited point-spread-function

(PSF) in the near-IR, and a large collecting area, SCExAO is

ideal for injecting light into fiber-fed spectrographs such as the

Infrared Doppler instrument (IRD) (Tamura et al. 2012). In ad-

dition, this forms the ideal platform for exploring photonic-

based technologies such as photonic lanterns (Leon-Saval et al.

2013) and integrated photonic spectrographs (Cvetojevic et al.

2012) for next-generation instrumentation.

The aim of this publication is to outline the SCExAO instru-

ment and its capabilities in detail and offer some preliminary

results produced by the system. In this vein, § 2 describes

the key components of SCExAO while § 3 highlights the func-

tionalities and limitations of the instrument. § 4 outlines plans

for future upgrades and the paper concludes with a summary

in § 5.

2. THE ELEMENTS OF SCEXAO

In order to understand the scientific possibilities and limita-

tions of the SCExAO instrument, it is important to first under-

stand the components and their functionalities. To aid the

discussion, a system level diagram of SCExAO is shown in

Figure 1, and an image of the instrument at the Nasmyth plat-

form is shown in Figure 2. A detailed schematic of the major

components is shown in Figure 3. The components and func-

tionalities of SCExAO have been undergoing commissioning

as will be outlined throughout this publication. A summary of

.1.—System level flow diagram of the SCExAO instrument. Thick purple and blue lines depict optical paths while thin green lines signify communication

channels. Dashed lines indicate that a connection does not currently exist but it is planned for the future. Filled boxes indicate commissioning status. Solid boxes:

commissioned; graded boxes: partially commissioned; white background: not commissioned. See the electronic edition of the PASP for a color version of this figure.

892 JOVANOVIC ET AL.

2015 PASP, 127:890 –910

the commissioning status of each mode of operation or module

of SCExAO can be found in the tables.

The main aim of SCExAO is to exploit the well-corrected

wavefront enabled by the high-order WFS to do high-contrast

imaging with light across a broad spectrum: from 600 to

2500 nm. As such, there are a number of instrument modules

within SCExAO that operate in different wavebands simulta-

neously while the coronagraph is collecting data. This hitchhik-

ing mode of operation enables maximum utilization of the

stellar flux, which allows for a more comprehensive study of

each target.

2.1. SCExAO at a Glance

The SCExAO instrument consists of two optical benches

mounted on top of one another, separated by ∼350 mm.

The bottom bench (IR bench) hosts the deformable mirror,

coronagraphs, and a Lyot-based low-order wavefront sensor

(LLOWFS) while the top bench (visible bench) hosts the pyra-

mid WFS, VAMPIRES, FIRST, and lucky imaging (see Fig. 3).

The benches are optically connected via a periscope.

The light from the facility adaptive optics system (AO188) is

injected into the IR bench of SCExAO and is incident on the

2000 element deformable mirror (2k DM) before it is split

by a dichroic into two distinct channels: light shorter than

940 nm is reflected up the periscope and onto the top bench

while light longer than 940 nm is transmitted. The visible light

is then split by spectral content by a range of long- and short-

pass dichroics which send the light to the pyramid WFS

(PyWFS) and visible light science instruments. The PyWFS

is used for the high-order wavefront correction and drives

the DM on the IR bench. The VAMPIRES and FIRST modules

utilize the light not used by the PyWFS. Lucky imaging/PSF

viewing makes use of light rejected by the aperture masks of

VAMPIRES.

The IR light that is transmitted by the dichroic on the IR

bench propagates through one of the available coronagraphs.

After the coronagraphs, the light reflected by the Lyot stop is

used to drive a LLOWFS in order to correct for the chromatic

and noncommon errors (such as tip/tilt) between the visible and

IR benches (Singh et al. 2014). The light transmitted by the co-

ronagraphs is th en incident on the science light beamsplitter

which determines the spectral content and exact amount of flux

to be sent to a high frame rate internal NIR camera as compared

to a science grade detector such as the HAWAII 2RG in the

HiCIAO instrument and soon to be commissioned CHARIS.

The internal NIR camera can then be used to drive various co-

herent differential imaging algorithms.

2.2. Detailed Optical Design

The instrument is designed to receive partially corrected

light from the facility adaptive optics system, AO188 (188 ac-

tuator deformable mirr or). The beam delivered from AO188

converges with a speed of f=14. Typical H-band Strehl ratios

are ∼30–40% in good seeing (Minowa et al. 2010). The beam

is collimated by an off-axis parabolic mirror (OAP1, f ¼

255 mm) creating an 18 mm beam. Details of the OAPs can

be found in the appendix. The reflected beam is incident upon

the 2k DM, details of which are in § 3.1.1. The surface of the

DM is placed one focal length from OAP1 which conjugates it

with the primary mirror of the telescope (i.e., it is in a pupil

plane). Once the beam has reflected off the DM, it is incident

upon a fixed pupil mask which replicates the central obstruction

and spiders of the telescope, albeit slightly oversized. This mask

is permane ntly in the beam (both on-sky and in the laboratory)

.2.—Image of SCExAO mounted at the Nasmyth IR platform at Subaru Telescope. To the left is AO188, which injects the light into SCExAO (center), and

HiCIAO is shown on the right. The FIRST recombination bench is not shown for visual clarity. See the electronic edition of the PASP for a color version of this figure.

THE SCEXAO HIGH-CONTRAST IMAGER 893

2015 PASP, 127:890–910

.3.—Schematic diagram of the SCExAO instrument. Top box (left): Portable calibration source layout. Top box (right): FIRST recombination bench. Middle box:

layout of the visible optical bench which is mounted on top of the IR bench. Bottom box: IR bench layout. Dual head green arrows indicate that a given optic can be

translated in/out of or along the beam. Orange arrows indicate light entering or leaving the designated bench at that location. See the electronic edition of the PASP for a

color version of this figure.

894 JOVANOVIC ET AL.

2015 PASP, 127:890 –910