A characterization of the optical turbulence vertical distribution and all the main integrated astroclimatic parameters
derived from the C2N and the wind speed profiles above Mt. Graham is presented. The statistic includes
measurements related to 43 nights done with a Generalized Scidar (GS) used in standard configuration with a
vertical resolution of ~1 km on the whole 20-22 km and with the new technique (HVR-GS) in the first kilometer.
The latter achieves a resolution of ~ 20-30 m in this region of the atmosphere. Measurements done in different
periods of the year permit us to provide a seasonal variation analysis of the C2N. A discretized distribution of the
profiles useful for the Ground Layer Adaptive Optics (GLAO) simulations is provided and a specific
analysis for the LBT Laser Guide Star system ARGOS case is done including the calculation of the 'gray zones'
for J, H and K bands. Mt. Graham confirms to be an excellent site with median values of the seeing without
dome contribution equal to 0.72", the isoplanatic angle equal to 2.5" and the wavefront coherence time equal to
4.8 msec. We provide a cumulative distribution of the percentage of turbulence developed below H* where H*
is included in the (0,1 km) range. We find that 50% of the whole turbulence develops in the first 80 m from the
ground. The turbulence decreasing rate is very similar to what has been observed above Mauna Kea.
Since November 2004 we measured the optical turbulence (C2N
profiles) with a Generalized Scidar (GS) placed at
the focus of the Vatican Advanced Technology Telescope at Mt.Graham, Arizona. The present statistic consists
in measurements related to 43 nights covering different periods of the solar year. In this paper we calculate the
statistics of the astroclimatic parameters
(C2N, seeing ε , isoplanatic angle θ0, wavefront coherence time τ0) and
we compare these values with those measured above other top level astronomic sites. All profiles are reduced
into a form suitable to be used as inputs for adaptive optics point spread function simulations for the conceptual
design of the Laser Guide Star Facility supported by a GLAO system of the Large Binocular Telescope. With
GS measurements done observing wide binaries (30-35 arcsec), the turbulence in the first kilometer above the
ground is characterized with the vertical resolution (200-250 m) required for the optimization of a 4 arcmin
field of view AO system. It is the first time that are published measurements of the optical turbulence vertical
distribution above a mid-latitude site with such a high vertical resolution and such a high statistical reliability.
On 8 of those nights, employing cross-correlation scintillation maps of wide binaries and the method described
in Ref. we characterize the distribution of the optical turbulence in the first kilometer at the extremely high
vertical resolution of 20-30 meters.
Mesoscale model such as Meso-Nh have proven to be highly reliable in reproducing 3D maps of optical turbulence
(see Refs. 1, 2, 3, 4) above mid-latitude astronomical sites. These last years ground-based astronomy has
been looking towards Antarctica. Especially its summits and the Internal Continental Plateau where the optical
turbulence appears to be confined in a shallow layer close to the icy surface. Preliminary measurements have
so far indicated pretty good value for the seeing above 30-35 m: 0.36" (see Ref. 5) and 0.27" (see Refs. 6, 7) at
Dome C. Site testing campaigns are however extremely expensive, instruments provide only local measurements
and atmospheric modelling might represent a step ahead towards the search and selection of astronomical sites
thanks to the possibility to reconstruct 3D C2N
maps over a surface of several kilometers. The Antarctic Plateau
represents therefore an important benchmark test to evaluate the possibility to discriminate sites on the same
plateau. Our group8 has proven that the analyses from the ECMWF global model do not describe with the required
accuracy the antarctic boundary and surface layer in the plateau. A better description could be obtained
with a mesoscale meteorological model. In this contribution we present the progress status report of numerical
simulations (including the optical
turbulence - C2N) obtained with Meso-Nh above the internal Antarctic Plateau.
Among the topic attacked: the influence of different configurations of the model (low and high horizontal resolution),
use of the grid-nesting interactive technique, forecasting of the optical turbulence during some winter
Dome C is considered a site particularly suited for wide-field imaging thanks to its shallow surface turbulent
layer and its weak turbulence in the free atmosphere. What is the quantitative gain one can hope to achieve
at Dome C with respect to a mid-latitude site? With the point spread function model defined analytically in
the spatial frequency domain we are better able to connect the morphological and statistical behaviour of the
turbulence profile to the trade-off between the adaptive telescope's field of view and a figure of merit for survey
rate. A familiar image quality figure of merit is the radius of 50% encircled energy, and for J-band images it
quickly identifies the requirement that will make a Dome C telescope, 8 meters above the ice, competitive with
a mid-latitude one. From the radius of 50% encircled energy we derive the wide-field survey rate equation to
estimate the impact of uncertainty in the vertical distribution of ground-layer turbulence on the trade-off between
field of view (in the domain 10-20 arcminutes) and their survey rate.
The atmospheric properties above three sites on the Internal Antarctic Plateau are investigated for astronomical
applications calculating the monthly median of the analysis-data from ECMWF (European Centre for Medium-Range Weather Forecasts) for an entire year (2005) thus covering all seasons. Radiosoundings extended on a
yearly time scale from Dome C and the South Pole are used to verify the reliability of the analyses in the
free atmosphere and to study the wind speed in the first 100 m as the analysis-data are not optimized for this
altitude-range. The wind speed in the free atmosphere is obtained from the ECMWF analyses from all three
sites. It appears that the strength of the wind speed in the upper atmosphere in winter is correlated to the
distance of the site from the centre of the polar high.
The Richardson number is employed to investigate the stability of the free atmosphere and, consequently,
the probability to trigger thermodynamic instabilities above the three sites. We find that, in a large majority of
the cases, the free atmosphere over the Internal Antarctic Plateau is more stable than at mid-latitude sites.
Given these data we can obtain a ranking of the three sites both with respect to wind speed, in the free
atmosphere as well as in the surface layer, and with respect to the stability of the atmosphere, using the
We arrive at a Ground Layer Adaptive Optics (GLAO) design that offers true seeing-improved performance and
operation for the red and infrared wavelengths. The design requires an adaptive secondary (AM2) and that the
sodium Laser Guide Star (LGS) launch telescope be able to steer four of the beams to 8.5 arcminutes off-axis.
When provided with this, the proposed design is potentially the simplest, lowest cost design that can take the
form of an upgrade. This is seen as a significant advantage over designs that would build an adaptive mirror
into each of the four arms of WFOS. We show that the performance penalty for using one mirror instead of four
to correct the entire 81 square arcminute WFOS field is minor.
We describe a simple and cost-effective concept for implementing a Ground Layer Adaptive Optics (GLAO) system on
Gemini that will feed all instruments mounted at the Cassegrain focus. The design concept can provide a GLAO
correction to any of the current or future seeing-limited optical or near-infrared Gemini instruments. The GLAO design
uses an adaptive secondary mirror and provides a significant upgrade to the current telescope acquisition-and-guide
system while reusing and building upon the existing telescope facilities and infrastructure.
This paper discusses the overall design of the GLAO system including optics, opto-mechanics, laser guide star facilities,
natural and laser guide stars wavefront sensors. Such a GLAO system will improve the efficiency of essentially all
observations with Gemini and also will help with scheduling since it virtually eliminates poor seeing.
Ground layer adaptive optics (GLAO) can significantly decrease the size of the point spread function (PSF) and
increase the energy concentration of PSFs over a large field of view at visible and near-infrared wavelengths. This
improvement can be realized using a single, relatively low-order deformable mirror (DM) to correct the wavefront
errors from low altitude turbulence. Here we present GLAO modeling results from a feasibility study performed
for the Gemini Observatory. Using five separate analytic and Monte Carlo models to provide simulations over the
large available parameter space, we have completed a number of trade studies exploring the impact of changing
field of view, number and geometry of laser guide stars, DM conjugate altitude and DM actuator density on the
GLAO performance measured over a range of scientific wavelengths and turbulence profiles.
In this paper, we provide an overview of the adaptive optics (AO) program for the Thirty Meter Telescope (TMT) project, including an update on requirements; the philosophical approach to developing an overall AO system architecture; the recently completed conceptual designs for facility and instrument AO systems; anticipated first light capabilities and upgrade options; and the hardware, software, and controls interfaces with the remainder of the observatory. Supporting work in AO component development, lab and field tests, and simulation and analysis is also discussed. Further detail on all of these subjects may be found in additional papers in this conference.
Nature provides reduced turbulence at intermediate altitudes and as a result the mean image quality of a ground
layer adaptive optics (GLAO) corrected field degrades slowly with increasing diameter. If this function has
a shallow slope all the way out to the maximum seeing limited field of view of the telescope then surveys at
that telescope may significantly benefit from GLAO. Using published optical turbulence profile models for Cerro
Pachon we show that the GLAO gains require that the full seeing limited field of view be corrected for two
example telescopes, each attempting two example science cases. We also show that using only four sodium laser
guide stars the very wide GLAO system with optimal actuator and subaperture pitch will not be affected by
servo lag and wavefront sensor noise.
Adaptive Optics (AO) will be essential for at least seven of the eight science instruments currently planned for the Thirty Meter Telescope (TMT). These instruments include three near infra-red (NIR) imagers and spectrometers with fields of view from 2 to 30 arc seconds, a mid-IR echelle spectrometer, a planet formation imager/spectrometer, a wide field optical spectrograph, and a NIR multi-object spectrometer with multiple integral field units deployable over a 5 arc minute field of regard. In this paper we describe the overall AO reference design that supports these instruments, which consists of a facility AO system feeding the first three instruments and dedicated AO systems for the remaining four. Key design challenges for these systems include very high-order, large-stroke wavefront correction, tip-tilt sensing with faint natural guide stars to maximize sky coverage, laser guidestar wavefront sensing on a very large aperture, and achieving extremely high contrast ratios for the detection of extra-solar planets and other faint companions of bright stars. We describe design concepts for meeting these challenges and summarize our supporting plans for AO component development.
Wide-Field Adaptive Optics (WFAO) is an AO mode in which one deformable mirror is used to achieve modest adaptive optics correction of the atmospheric turbulence, but in a much wider field of view than classical AO. At the heart of the concept is the desire to trade image quality at the center of the field of view for better image quality at the edge of a wide field (typically ~10') and is also called Improved Seeing AO (ISAO) or Ground Layer AO (GLAO) in the literature. An analytical (Fourier domain) model allows us to rapidly derive requirements on the number, brightness and distribution of guide stars for a WFAO system running on an 8-m or 30-m telescope, as well as basic AO system requirements such as loop rate and DM actuator density. In this paper we derive the Fourier domain filter that describes WFAO and present a method for evaluating WFAO performance and sky coverage. We test our performance evaluation on a pathological case, computing the scientifically relevant metric, radius of 50\% encircled energy for a typical Cn2 profile.
In this paper we evaluate the on-sky performance of Altair, the facility adaptive optics instrument at the Gemini North telescope.
We describe the method for doing this on-sky evaluation, which includes: 1) the choice of suitable stellar fields for PSF observations that must cover a range of guide star magnitudes and angular separations from the guide star; 2) the observation strategy and data reduction pipeline; and 3) the PSF database from which the performance results are queried. The database stores observatory system parameters and performance observations such as FWHM, Strehl, encircled energy, wave front sensor flux, as well as coherence length (ro) and outer scale (Lo) of the turbulence measured in closed loop and therefore coincident with the focal plane observations of the telescope. From the database we derive 20 to 24% Noll efficiency of the system and an estimated distribution of effective turbulence height above the summit to be 3.3 ± 0.6km. The performance evaluation strategy used on Altair is quite general and could be used for other adaptive optics systems.
Wide Field Adaptive Optics (WFAO) is a new proposed astronomical adaptive optics mode allowing a significant improvement of the seeing limited point spread function characteristics over large fields -- several arc minutes in diameter, using only one deformable mirror optically conjugated to an optimal altitude. In this paper, we present the WFAO upper limit performances, based on the assumption that the refractive index fluctuation field above the telescope is perfectly known. Our results are based on analytical developments for the residual phase power spectrum after WFAO correction, implemented in PAOLA, an analytical AO simulation tool, developed at the Herzberg Institute of Astrophysics. Results are presented for several sites: Mauna Kea, Cerro Tololo, Cerro Paranal. For each of these locations, we give the WFAO-PSF properties as a function of the field angle, the conjugation altitude of the deformable mirror, the imaging infrared wavelength, and the cone aperture angle over which the tomographic information is averaged to drive the deformable mirror actuators.