Download Adaptive Optics at the Keck Observatory: Goals and Near

Adaptive Optics at the Keck Observatory: Goals and
Near-term Priorities
AO Working Group Report; 13 March 2003
1. Overview
The Keck Adaptive Optics Working Group (AOWG) recommends an aggressive path
forward to maintain the leadership of the Keck Observatory at the forefront of
astronomical adaptive optics. Our recommendations have three thrusts: (1) improved
performance of the present AO systems, (2) completion of the laser guide star system and
associated instrumentation, and (3) next generation, advanced AO systems.
Our primary recommendation remains the development of a next-generation, highperformance, general-use AO system, which would deliver stable, high Strehl ratio,
infrared images in moderate field-of-view areas throughout the sky. This system, named
Keck Precision Adaptive Optics (KPAO), would allow Keck to take the lead in generaluse high-precision adaptive optics where many of the scientific advances in highresolution imaging are expected.
The development of a precision AO system at Keck is predicated on the assumption that
the current AO system will be operating at full ability, including the deployment of a
laser guide star and the integral field spectrograph OSIRIS. In several cases, significant
work still needs to be carried out to achieve this beginning point for future systems. Most
critically, before any serious work on new AO systems begin, the under-performance of
the current AO system needs to be understood and fixed. Work is currently underway
which could answer these questions. This work needs to be given extremely high priority
and support.
Near-term opportunities also exist for performing important clearly needed upgrades to
the current system. One such upgrade presently identified is to replace the firstgeneration wavefront control system with a new state-of-the-art system including new
lower noise cameras, new tip/tilt sensors, and new real-time processors allowing critically
needed real-time diagnostics and telemetry. Implementation of these types of important
upgrades will be critical to keeping the current AO system on the cutting edge while
work begins on the next generation of systems; in addition with careful planning many of
these improvements should be directly applicable to the development of the next
generation systems.
We also continue to strongly advocate the support of the external development of a
powerful adaptive optics systems designed solely for high contrast observations of the
brightest stars. While limited in its uses, such a system would allow the observatory to
command an undisputable lead in the extremely important field of direct detection of
extra-solar planets. Owing to the relatively small investment for such a high scientific
payback, the AOWG strongly recommends that the observatory pursue this opportunity.
For FY04, we have worked with Peter Wizinowich, David LeMignant and the Keck AO
team to define a set of priorities that will maximize science capabilities in the near-term
and that will work towards the long term goal of an advanced system. These priorities
are:
1. AO operations support.
2. K2 LGS AO facility available for science operation.
3. Performance characterization and optimization for the existing AO systems.
(including an upgrade to PCS)
4. OSIRIS implementation with K2 AO.
5. Interferometer-required AO modifications (Interferometer-funded).
6. New wavefront controller implementation (externally-funded).
7. KPAO studies.
8. XAOPI interface design support and design review (externally-funded).
9. NSF-funded laser progress monitoring.
The next two sections discuss the next generation adaptive optics systems: KPAO (Keck
Precision Adaptive Optics), and XAOPI (Extreme AO) along with the most important
preliminary tasks that need to be taken.
2. Keck Precision Adaptive Optics (KPAO)
The AOWG strategic plan of November 2002 recommended the initiation of the
development stages of a new high-precision adaptive optics system, recently dubbed the
Keck Precision Adaptive Optics (KPAO). Such a system is envisioned to be one which
would deliver extremely high infrared Strehl ratios, high stability, near-complete sky
coverage, a good knowledge of the delivered PSF, and the ability to observe at high
resolution into the visible (albeit with lower Strehl ratios). The AOWG believes that the
largest opportunity in a future AO system at Keck will be to overcome the single most
important set of limitations in all current AO systems: imperfect Strehl, PSF variability,
and a lack of concrete PSF knowledge (note that we are implicitly assuming that the
current limitation on sky coverage will be solved in the short term with laser guide stars
thus we do not list that here). Quantitative measurements of many types currently suffer
in the face of these problems: large wings to the PSF make precise spatially resolved
spectroscopy unreliable and searches for faint companions more difficult, PSF variability
and lack of PSF knowledge make deconvolution of sources suspect, and imperfect Strehl
ratios make precision photometry impossible.
The current Keck AO observers, and those who will become AO users with the advent of
the laser guide star system, have learned (or will learn) to work around these limitations
and find the scientific problems that are best addressed within these constraints, but the
full power of high resolution observations at the diffraction limit of the Keck telescopes
will not be met until these limitations can be largely overcome.
2.1 Summary Scientific Justification
The development of a high precision adaptive optics system would enable currently
impossible observations in all fields of astronomy. One of our strongest recommendations
is that in the FY04 timescale a project scientist for KPAO should be selected to work
with the AO team to ensure that the maximum scientific capabilities are achieved. We
anticipate that the development of a thorough science case will be one of the project
scientist’s first roles, but here we summarize a few important scientific examples in the
areas of solar system, galactic, and extragalactic astronomy.
2.1.1 Solar system astronomy: global change on Pluto
Much of the AO work in the solar system has been done on the large satellites and outer
planets which are in the 1-4 arcsecond size range and thus easily resolved with current
AO. Results have been generally spectacular, including the discovery of clouds and the
highest-resolution ever maps of Titan, volcanic outbursts on Io, and atmospheric structure
on Neptune. Another important class of solar system objects, the smaller icy planets and
satellites, are just out of range of current AO systems. With sizes of a couple of tenths of
an arcsecond or smaller, these objects are only marginally resolved and with low and
variable Strehl no reliable results on surface structures, shapes, and compositional
gradients can be obtained. As an example, one of the current outstanding questions on the
climate of Pluto – when and if the nitrogen atmosphere is going to freeze out – would
best be answered by even a crude compositional map of the surface of Pluto determining
the amount, state, and location of nitrogen, methane, and water ices on the surface. Pluto
subtends only 0.1 arcseconds, so current AO systems, while able to show that Pluto is
resolved, are incapable of reliably mapping spatial resolved spectra across the surface.
With high-resolution stable PSFs (coupled with planetary rotation), however, the problem
becomes tractable; HST UV observations have allowed the construction of a crude map
which shows that Pluto has more contrast than almost any other body in the solar system,
but we have no current idea what the extremely bright and extremely dark areas on the
surface are composed of and how these change with Pluto’s seasons. Similar types of
research on other solar system bodies will be made possible by such a system.
2.1.2 Galactic astronomy: low mass stars in binaries and clusters
Moderate Strehl AO systems are particularly good at resolving pairs of point sources, so
much of the galactic AO work has focused on companion searches. The current limitation
to much of this work is the contrast achievable by the current AO system, which roughly
scales as Strehl/(1-Strehl). The development of the XAOPI system will address the need
for an extremely high contrast instrument to be used on the brightest targets, but many
interesting targets will be too faint for this NGS system even in its faint target mode. For
example, studies of large numbers of stars in clusters of various ages and studies of
companions to low-mass stars will be difficult with the modest Strehls expected from the
current AO system with LGS. XAOPI is expected to allow ultra-high contrast
observations of extremely bright targets, but only the creation of an all sky high Strehl
system will allow high contrast observations of any target in the sky.
One well known limitation of moderate and variable Strehl is the inability to perform
precise photometry. Infrared surveys of clusters are finding increasing numbers of free
floating planetary-mass objects, and observations at the limits of adaptive optics promise
to be able to answer questions of whether or not these objects extend to Jupiter masses
and below. Unfortunately, the observations require the types of precise photometry which
are impossible with moderate Strehl systems. A next generation stable high Strehl system
would be the first to allow the detection and photometric measurement of these faint
objects. Precision adaptive optics on faint stellar populations would be possible at last.
2.1.3 Extragalactic astronomy: star formation in quasar host galaxies
The deployment of the laser guide star should usher in a new era of extragalactic AO
studies, but these studies will face the same or worse Strehl and stability limitations
already faced by NGS observers. The development of a new generation adaptive optics
system would lead to the ability to perform the first precision adaptive optics work on
extragalactic sources. As an example, current AO work on the host galaxies of quasars
has already shown the presence of young stellar populations and suggested that mergers
may be playing a crucial role. One serious limitation of this work is precisely and
believably separating the quasar from the host galaxy. With moderate and variably Strehl
the problem is largely intractable except in a qualitative way. High stable Strehl ratios
achieved by a next generation Keck AO system would allow the first true images of
quasar host galaxies, with which one could look for smaller scale features such as dust
lanes and star forming knots to which previous studies were not sensitive. By resolving
such structures we can also separate the older, more distributed, stellar population. This is
crucial since even a relatively small number of new stars will wipe out the signature of
the presence of a large older population in the optical and UV, where HST work has been
done.
2.2 Initial steps
At a minimum, precision adaptive optics will require reduction of the final delivered
wavefront error to a suitably small value to increase the Strehl ratio. For example, a goal
of a minimum Strehl ratio of 90% for K-band observations would require a final
wavefront error of 100 nm or less. Understanding how to achieve wavefront errors this
low -- or indeed if it is even a possibility -- will require a significantly increased
knowledge of the error budget of the telescope, the structure of turbulence in the
atmosphere above the observatory, and the performance of the current AO system.
Acquisition of this knowledge is a necessary first step before any serious design
considerations should begin.
2.2.1 Telescope Error Budget
Precision adaptive optics requires a precise telescope. The individual factors affecting the
wavefront delivered by the telescope are currently not well characterized. The predicted
aberrations used for the design of the current AO system are in many cases no more than
educated guesses; some of these guesses are now suspected to be quite far from reality.
A high priority therefore needs to be placed on developing the tools to measure telescope
aberration sources such as warping harness residuals, segment phasing and stacking
errors, ACS noise, and segment vibration. We strongly recommend the immediate start to
the PCS upgrade proposed by Chanan as one part of this effort. Although the new PCS
system will not provide a new capability, it will set the stage for future upgrades and
ensures that we do not lose a key capability. Once the sources of aberration are known, it
is entirely possible that a significant amount of the spending for the development of a
precision AO instrument will actually be in developing methods of determining and
correcting telescope aberrations.
2.2.2 Atmospheric characterization
One limitation to the wavefront error possible for an advanced laser guide star based AO
system will be the inherent focal anisoplanatism (FA) imposed by the atmosphere. The
magnitude of this effect depends directly on the height of typical turbulent layers in the
atmosphere and can only be made smaller with multiple laser guide stars. The number of
lasers required will depend directly on the magnitude of the FA at Mauna Kea, which is
currently unknown. Current estimates suggest that a best value might be ~170 nm,
implying that as many as 9 lasers would be required to bring the FA down to levels
necessary for a 100 nm system. It is critical to know the true value and range of values,
however, as the required number of lasers goes as approximately the square of the FA
and it is possible that this number could be incorrect by a factor of 2. The AOWG
therefore urges that a high priority be placed on working with the ongoing campaigns to
characterize the Mauna Kea atmosphere and measure typical Cn2 profiles. Ideally, data
would be collected over several years before final decisions would be made on critical
parts of the design like the total numbers of lasers.
2.2.3 Current AO system characterization
Before design of any next generation AO system commences, it is critical to fully
understand the reasons for the underperformance of the current system. As discussed at
the beginning of this document, continued characterization and optimization of the
current AO system remains a top priority of the AOWG.
3. Extreme Adaptive Optics Planet Imager (XAOPI)
For the first time in history, technology has permitted astronomers to image the
environments of nearby stars on scales comparable to the size of our solar system.
Exploitation of these capabilities has led to the discovery of new classes of objects
including dusty circumstellar debris disks and brown dwarfs. These discoveries have
galvanized intense public interest in science and technology and have led to profound
new insights into the formation and evolution of planetary systems. It is now within our
grasp to develop an AO system with the potential to make the first direct detection of the
light from planets in orbit around solar-type stars. XAOPI, the eXtreme Adaptive Optics
Planet Imager, will be a 3000-actuator AO system optimized for ultra-high-contrast
imaging around bright (mR<7-10) stars
Figure 1: Simulated XAOPI
15 minute integration on a
solar-type star (centered
behind the occulting spot)
showing a 8 Jupiter-mass
planet. The characteristic
square null, 1.6” on a side, in
scattered light from the
primary star is caused by
XAOPI’s spatially-filtered
wavefront sensor.
Knowledge of brown dwarfs and extra-solar planets is growing rapidly. Since 1995, the
first sub-stellar objects have been discovered using low order AO coronagraph (Nakajima
et al. 1995), precision Doppler measurements (Mayor & Queloz 1995) and field surveys
(Kirkpatrick et al. 1997; Strauss et al. 1999). Doppler searches have succeeded
spectacularly; the technique is mature with more than seventy known extra-solar planets.
The 2MASS, DENIS and Sloan field surveys have been fruitful for finding faint brown
dwarfs; many examples are now known. The latter two techniques are handicapped by
strong biases: Doppler searches will continue to find planets in close orbits with short
periods (<10 Yr), while field surveys only find massive, isolated objects.
None of the planetary companions detected so far resembles the giants of the solar
system. This fact is, however, consistent with the notion that our solar system is a typical
planetary system. Selection effects can easily account for the lack of detections of
Jupiter-like planets - exoplanets detected to date cannot resemble the planets of our solar
system because the Doppler technique used to detect them is not sufficiently sensitive
and has not been in use long enough to detect a Jupiter-mass planet in a 5 AU orbit. If
the sun were a target star no definitive Doppler signature would yet have been detected.
The detected exoplanets may well be the observable 5% tail of the main concentration of
massive planets of which Jupiter is typical. Moreover, Doppler searches are indirect: they
can neither image nor study the physics, chemistry or thermodynamics of the objects they
discover. XAOPI is a proposed AO system which will be the next step toward the
detailed study of extra-solar planets, brown dwarfs and circumstellar disks. In this highcontrast regime, sensitivity scales roughly with Strehl ratio S and telescope diameter D as
SD2/(1-S), giving an enormous advantage to the S=0.95 XAOPI system over
conventional AO or even KPAO. In addition, high-contrast imaging is often limited by
small residual static phase errors. XAOPI will be designed from the start to minimize
these static errors and to use a specially-designed wavefront sensor to precisely control
phase errors, even those caused by discontinuities in the primary mirror, out to its spatial
frequency cutoff. Simulations indicate that XAOPI will achieve contrast levels of 107-108
and be able to detect a significant population of extrasolar planets through their near-IR
emission. Once a planet has been detected it can be characterized through IFU
spectroscopy, allowing measurements of temperature and composition and inferences
about formation history. Such an instrument will allow Keck to dominate the field of
direct detection and characterization of extrasolar planets as it has so far dominated
radial-velocity detection. XAOPI will also be capable of near-IR imaging of circumstellar
dust around a large population of stars, imaging of the environments of evolved stars, and
ultra-high-resolution imaging at high Strehl ratios of bright solar-system targets.
Figure 2: Monte Carlo simulation (Graham et al 2003) of extrasolar planet detection.
Stars indicate actual planets discovered by radial-velocity techniques; small dots indicate
planets discovered by a simulated astrometric survey using the Keck outriggers; and
filled circles indicate planets that would be found by a XAOPI survey of field stars.