An increasing number of astronomical spectrographs employ optical fibres to collect and deliver light. For integral-field and high multiplex multi-object survey instruments, fibres offer unique flexibility in instrument design by enabling spectrographs to be located remotely from the telescope focal plane where the fibre inputs are deployed. Photon-starved astronomical observations demand optimum efficiency from the fibre system. In addition to intrinsic absorption loss in optical fibres, another loss mechanism, so-called focal ratio degradation (FRD) must be considered. A fundamental cause of FRD is stress, therefore low stress fibre cables that impart minimum FRD are essential. The FMOS fibre instrument for Subaru Telescope employed a highly effective cable solution developed at Durham University. The method has been applied again for the PFS project, this time in collaboration with a company, PPC Broadband Ltd. The process, planetary stranding, is adapted from the manufacture of large fibre-count, large diameter marine telecommunications cables. Fibre bundles describe helical paths through the cable, incorporating additional fibre per unit length. As a consequence fibre stress from tension and bend-induced ‘race-tracking’ is minimised. In this paper stranding principles are explained, covering the fundamentals of stranded cable design. The authors describe the evolution of the stranding production line and the numerous steps in the manufacture of the PFS prototype cable. The results of optical verification tests are presented for each stage of cable production, confirming that the PFS prototype performs exceptionally well. The paper concludes with an outline of future on-telescope test plans.
To approach optimal performance advanced Adaptive Optics (AO) systems deployed on ground-based telescopes must have accurate knowledge of atmospheric turbulence as a function of altitude. Stereo-SCIDAR is a high-resolution stereoscopic instrument dedicated to this measure. Here, its profiles are directly compared to internal AO telemetry atmospheric profiling techniques for CANARY (Vidal et al. 20141), a Multi-Object AO (MOAO) pathfinder on the William Herschel Telescope (WHT), La Palma. In total twenty datasets are analysed across July and October of 2014. Levenberg-Marquardt fitting algorithms dubbed Direct Fitting and Learn 2 Step (L2S; Martin 20142) are used in the recovery of profile information via covariance matrices - respectively attaining average Pearson product-moment correlation coefficients with stereo-SCIDAR of 0.2 and 0.74. By excluding the measure of covariance between orthogonal Wavefront Sensor (WFS) slopes these results have revised values of 0.65 and 0.2. A data analysis technique that combines L2S and SLODAR is subsequently introduced that achieves a correlation coefficient of 0.76.
Optical turbulence characterisation is crucial to understanding astronomical site and observational limitations. The Differential Image Motion Monitor (DIMM) is a widely used, low cost and portable instrument for measuring the total integrated seeing. We have designed and tested a variation on the DIMM design that utilises a low order Shack-Hartmann (SH) lenslet array instead of the standard two hole aperture mask. This instrument, which is comprised of readily available components, is known as SHIMM. This alternative design utilises more of the telescope aperture, in comparison to the DIMM, and therefore increases the signal to noise ratio, as well as providing a more accurate method of noise estimation. In future the instrument will be developed to provide estimation of the coherence timescale, limited turbulence altitude information, and to correct for scintillation effects on the seeing measurements. We describe the instrument and present measurements from two identical SHIMM seeing monitors, as well as a comparison with simultaneous optical turbulence profiles recorded with Stereo-SCIDAR on the 2.5m Isaac Newton Telescope, La Palma.
Estimating the outer scale profile, L0(h) in the context of current very large and future extremely large telescopes is crucial, as it impacts the on-line estimation of turbulence parameters (Cn2(h), r0, θ0 and τ0) and the performance of Wide Field Adaptive Optics (WFAO) systems. We describe an on-line technique that estimates L0(h) using AO loop data available at the facility instruments. It constructs the cross-correlation functions of the slopes of two or more wavefront sensors, which are fitted to linear combinations of theoretical responses for individual layers with different altitudes and outer scale values.
We analyze some restrictions found in the estimation process, which are general to any measurement technique. The insensitivity of the instrument to large values of outer scale is one of them, as the telescope becomes blind to outer scales larger than its diameter. Another problem is the contradiction between the length of data and the stationarity assumption of the turbulence (turbulence parameters may change during the data acquisition time).
Our method effectively deals with problems such as noise estimation, asymmetric correlation functions and wavefront propagation effects. It is shown that the latter cannot be neglected in high resolution AO systems or strong turbulence at high altitudes. The method is applied to the Gemini South MCAO system (GeMS) that comprises five wavefront sensors and two DMs. Statistical values of L0(h) at Cerro Pachón from data acquired with GeMS during three years are shown, where some interesting resemblance to other independent results in the literature are shown.
Vertical profiles of the atmospheric optical turbulence strength and velocity is of critical importance for simulating, designing, and operating the next generation of instruments for the European Extremely Large Telescope. Many of these instruments are already well into the design phase meaning these profies are required immediately to ensure they are optimised for the unique conditions likely to be observed.
Stereo-SCIDAR is a generalised SCIDAR instrument which is used to characterise the profile of the atmospheric optical turbulence strength and wind velocity using triangulation between two optical binary stars. Stereo-SCIDAR has demonstrated the capability to resolve turbulent layers with the required vertical resolution to support wide-field ELT instrument designs. These high resolution atmospheric parameters are critical for design studies and statistical evaluation of on-sky performance under real conditions. Here we report on the new Stereo-SCIDAR instrument installed on one of the Auxillary Telescope ports of the Very Large Telescope array at Cerro Paranal. Paranal is located approximately 20 km from Cerro Armazones, the site of the E-ELT. Although the surface layer of the turbulence will be different for the two sites due to local geography, the high-altitude resolution profiles of the free atmosphere from this instrument will be the most accurate available for the E-ELT site.
In addition, these unbiased and independent profiles are also used to further characterise the site of the VLT. This enables instrument performance calibration, optimisation and data analysis of, for example, the ESO Adaptive Optics facility and the Next Generation Transit Survey. It will also be used to validate atmospheric models for turbulence forecasting. We show early results from the commissioning and address future implications of the results.
We present high resolution optical turbulence profiles of the dome and ground-layers measured using a set of Shack-
Hartmann wavefront sensors deployed over a field of view of between 0.5 and 1.0 degrees at the focal planes of the
University of Hawaii 2.2-m telescope and the Canada-France-Hawaii Telescope on Maunakea, Hawaii. Observations with the experiment were made over the course of several nights on each telescope. We obtain estimates of the strength, distribution, and velocities of optical turbulence from the covariance matrices and maps of the measured wavefront gradients and a decomposition of the measured wavefronts into Zernike polynomials. We find agreement with previous measurements on Maunakea that the ground layer is largely confined within the first tens of meters above the ground and moves at the ground wind velocity. In addition, we spatially resolve the optical turbulence that arises from within the dome. For both facilities we find that the dome seeing is a major component of the overall turbulence strength accounting for more than half of the turbulence within the ground layer and that the dome seeing changes very slowly with a characteristic frequency of less than 1 Hz. While the variety of observing conditions sampled is low, we find that the characteristics of the dome seeing with observation elevation angle and the azimuth angle with respect to the ground wind are quite different on the two telescopes suggesting a different origin to the seeing within the two enclosures.
CANARY is an on-sky Laser Guide Star (LGS) tomographic AO demonstrator that has been in operation at the 4.2m William Herschel Telescope (WHT) in La Palma since 2010. In 2013, CANARY was upgraded from its initial configuration that used three off-axis Natural Guide Stars (NGS) through the inclusion of four off-axis Rayleigh LGS and associated wavefront sensing system. Here we present the system and analysis of the on-sky results obtained at the WHT between May and September 2014. Finally we present results from the final ‘Phase C’ CANARY system that aims to recreate the tomographic configuration to emulate the expected tomographic AO configuration of both the AOF at the VLT and E-ELT.
We present on-sky results obtained with Carmen, an artificial neural network tomographic reconstructor. It was tested during two nights in July 2013 on Canary, an AO demonstrator on the William Hershel Telescope. Carmen is trained during the day on the Canary calibration bench. This training regime ensures that Carmen is entirely flexible in terms of atmospheric turbulence profile, negating any need to re-optimise the reconstructor in changing atmospheric conditions. Carmen was run in short bursts, interlaced with an optimised Learn and Apply reconstructor. We found the performance of Carmen to be approximately 5% lower than that of Learn and Apply.
This paper presents preliminary daytime profiles taken using a Wide-Field Shack-Hartmann Sensor at the Swedish
Solar Telescope (SST), La Palma. These are contrasted against Stereo-SCIDAR data from corresponding nights to
assess the validity of the assumptions currently used for simulating the performances of possible Multi-Conjugate
Adaptive Optics (MCAO) systems for future solar telescopes, especially the assumption that the structure of the high
altitude turbulence is mostly similar between the day and the night. We find that for our data both the altitude and the
strength of the turbulence differ between the day and the night, although more data is required to draw any conclusions
about typical behaviour and conditions.
An MOAO corrected multi-IFU instrument, such as the EAGLE instrument proposed for the E-ELT has a
deformable mirror correcting each IFU sub-field. Additionally, EAGLE will also use the E-ELT deformable M4
mirror to apply a global (closed-loop) MOAO correction. Here, we investigate the impact on MOAO performance
if a global GLAO correction is applied across the whole field, rather than the optimised global MOAO correction
(which may or may not be identical). The differences in M4 correction (between GLAO and optimal MOAO)
will depend on the position of IFU pick-offs in the science field, and also on the turbulence, for example, MOAO
DM stroke may be minimised if more than the ground layer is corrected by the M4 DM, depending on how
fast turbulence decorrelates across the field of view. We consider the impact on MOAO DM stroke, the effect
on performance, and study both tomographic and non-tomographic GLAO corrections. Such a situation may
arise if for example a combined multi-object spectrograph and multi-IFU instrument is designed, such as would
result from the integration of EAGLE with another proposed E-ELT instrument. We demonstrate here that
performance of EAGLE will not be significantly affected by being placed behind another such instrument. The
results presented will be obtained using full end-to-endMonte-Carlo simulations using the Durham AO Simulation
Platform. We also present a number of algorithms which can be used to improve AO performance, both in pixel
processing and multi-mirror control.
Multi-object adaptive optics requires a tomographic reconstructor to compute the AO correction for scientific targets
within the field, using measurements of incoming turbulence from guide stars angularly separated from the science
targets. We have developed a reconstructor using an artificial neural network, which is trained in simulation only.
We obtained similar or better results than current reconstructors, such as least-squares and Learn and Apply, in
simulation and also tested the new technique in the laboratory. The method is robust and can cope well with
variations in the atmospheric conditions. We present the technique, our latest results and plans for a full MOAO experiment.
In this paper the Paranal Surface Layer characterization is presented. Causes, physics and behavior of the SL above
Paranal surface are discussed. The analysis is developed using data from different turbulence profilers operated during
several campaigns between 2007 and 2009. Instruments used are SL-SLODAR, DIMM, Elevated DIMM, MASS, Lunar
Scintillometer and Ultrasonic Anemometers with temperature sensors positioned at different strategic heights.
The use of software based simulation packages is essential for the design of adaptive optics systems on next
generation ELT scale telescopes. We present Monte-Carlo AO simulation results for an E-ELT multi-IFU spectrograph
instrument comprising multiple laser and natural guide stars with wavefront correction along multiple
lines of sight. We discuss the techniques used to perform these simulations. Considerations are also given to
compressed reconstructor representations which can greatly simplify the design of real-time control systems. We
also discuss work on the use of GPUs for AO simulation.
The CANARY on-sky MOAO demonstrator is being integrated in the laboratory and a status update about its
various components is presented here. We also discuss the alignment and calibration procedures used to improve
system performance and overall stability. CANARY will be commissioned at the William Herschel Telescope at
the end of September 2010.
The 'Imaka project is a high-resolution wide-field imager proposed for the Canada-France-Hawaii telescope
(CFHT) on Mauna Kea. 'Imaka takes advantage of two features of the optical turbulence above Mauna Kea:
weak optical turbulence in the free-atmosphere and boundary layer turbulence which is highly confined within a
surface layer tens of meters thick and or the telescope enclosures. The combination of the two allows a groundlayer
adaptive optics system (GLAO) to routinely deliver an extremely-wide corrected field of view of one-degree
at an excellent free-atmosphere seeing limit at visible wavelengths. In addition, populating the focal-plane with
orthogonal-transfer CCDs provides a second level of image improvement on the free-atmosphere seeing and the
residual GLAO correction. The impact of such an instrument covers a broad range of science and is a natural
progression of CFHT's wide-field expertise.
We have made high order (32x32 subaperture) Shack-Hartmann wavefront sensor observations of binary stars
with separations of approximately 20 arcseconds using the University of Hawaii 2.2 m telescope. We present
preliminary results of a Slope Detection and Ranging (SLODAR) analysis of the data yielding measurements of
turbulence strength, wind velocity and velocity dispersion as a function of altitude, with approximately 500 m
vertical resolution. The aim of the investigation is to explore the validity of the Taylor frozen flow approximation
and the implications for layer-oriented predictive AO reconstruction algorithms.
A method that has been followed to produce performance estimates for the adaptive optics (AO) aspect of the
EAGLE instrument proposed for the European Extremely Large Telescope (ELT) (E-ELT) using Durham Monte-Carlo simulation code is presented. These simulations encompass a wide range of possible configurations for EAGLE, including multi-object adaptive optics (MOAO), segmented multi-conjugate adaptive optics (MCAO)
and other more novel techniques. Particular emphasis is placed on the techniques used to enable a good simulation
turn-around rate, allowing the large parameter space associated with optimising high performance AO systems
to be explored. Performance estimate results for some AO system configurations are also provided.
We present observations of the high-speed variations of the altitude of the telluric sodium layer. In this experiment we observed the Gemini-North sodium laser guide star from approximately 80 meters
off-axis using the UH-2.2m telescope on Mauna Kea, Hawaii. Observations were made using an electron-multiplying camera at a rate of about 100Hz. The temporal power spectrum of the layer centroid follows a power law between 0.001 and 1Hz and we find that the exponent of the power law (α=-1.8) is similar to that found at lower temporal frequencies from lidar experiments. This data set taken with the lidar results shows that the power spectrum of the sodium layer mean altitude follows a simple power law over 5 orders of magnitude from 10-4.5 Hz to 1Hz. The approach taken in this experiment is difficult due to telescope jitter in any of the three telescopes (Gemini-N, Gemini-N LGS launch telescope, or from the observing UH2.2m) and atmospheric tip/tilt wave front aberrations. We circumvented these problems by analyzing the differential motion between two distinct features that appeared in the sodium layer during that night.
We describe the current status of the SLODAR optical turbulence monitors, developed at Durham University, for support
of adaptive optics for astronomy. SLODAR systems have been installed and operated at the Cerro Paranal and Mauna
Kea observatories, and a third will be deployed at the South African Astronomical Observatory in 2008. The instruments
provide real-time measurements of the atmospheric turbulence strength, altitude and velocity. We summarize the
capabilities of the systems and describe recent enhancements. Comparisons of contemporaneous data obtained with
SLODAR, MASS and DIMM monitors at the ESO Paranal site are presented.
Results of numerical simulations of the performance of GLAS (Ground-layer Laser Adaptive optics System) are
presented. GLAS uses a Rayleigh laser guide star (LGS) created at a nominal distance of 20km from the 4.2m William
Herschel Telescope primary aperture and a semi-analytical model has been used to determine the observed LGS
properties. GLAS is primarily intended for use with the OASIS spectrograph working at visible wavelengths although a
wider-field IR imaging camera can also use the AO corrected output. Image quality metrics relating to scientific
performance for each instrument are used showing that the energy inside every OASIS lenslet across the 10" instrument
FOV is approximately doubled, irrespective of atmospheric conditions or wavelength of observation.
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.
Numerical Simulation is an essential part of the design and optimisation of astronomical adaptive optics systems. Simulations of adaptive optics are computationally expensive and the problem scales rapidly with telescope aperture size, as the required spatial order of the correcting system increases. Practical realistic simulations of AO systems for extremely large telescopes are beyond the capabilities of all but the largest of modern parallel supercomputers. Here we describe a more cost effective approach through the use of hardware acceleration using field programmable gate arrays. By transferring key parts of the simulation into programmable logic, large increases in computational bandwidth can be expected. We show that the calculation of wavefront sensor images (involving a 2D FFT, photon shot noise addition, background and readout noise), and centroid calculation can be accelerated by factor of 400 times when the algorithms are transferred into hardware. We also provide details about the simulation platform and framework that we have developed at Durham.
We describe SPLASH (Sky Projected Laser Array Shack-Hartmann) which is a method of laser guide star (LGS) wavefront sensing with reduced focal anisoplanatism (FA). We present the results of a semi-theoretical analysis and a semi-geometrical simulation of SPLASH, allowing a direct comparison between SPLASH and a conventional laser guide star system. We show that SPLASH is significantly less susceptible to focal anisoplanatism than a conventional LGS.
An experimental Ground Layer Adaptive Optics system utilizing a low-altitude Rayleigh Laser Guide Star is presented. This demonstrator is designed for the GHRIL Nasmyth platform of the 4.2m William Herschel Telescope, where it uses a low-altitude (~4km) focused-spot 523nm Rayleigh-scatter beacon, launched from behind the secondary mirror using an independent beam launch telescope. A novel range-gate is used to select the LGS return altitude for wavefront sensing, whilst wavefront correction uses a 97-actuator continuous phase sheet deformable mirror and separate tip-tilt mirror. The performance can be monitored on-axis and off-axis. These and other aspects of the demonstration system are described in detail, including optical design, laser launch technique, laboratory performance, and a preliminary assessment of potential on-sky performance.