132 research outputs found
An upper limit to the sodium layer longitudinal and transversal altitude structure function from MCAO data
The Gemini Multi-Conjugate Adaptive Optics System is the first sodium-based Multi-Laser Guide Star (LGS) Adaptive Optics system used for astronomy. It uses five LGSs distributed on a 1 arcmin2 constellation to measure and compensate for atmospheric disto
Characterising latency for AO optical sensors: An implementation
The latency of electro-optical components is of high importance in the design of Adaptive Optics systems, as it limits the performance of the control loop. There exists a need for a latency measurement method that can be constructed with simple components found in most Adaptive Optics labs that still provides a measurement accurate to sub-microseconds. Through a combination of research and experimentation, potential methodologies were investigated with the aim of producing reliable latency measurements. This document will discuss one such method, involving coupling a LED pulse output and detected pulse input signals to the same clock for easy comparison. For this method, a proof-of-concept was developed using MATLAB and small analogue electronics, and the performance characterised. This characterisation showed that although there is some merit to the method, improvements are necessary to increase the precision of the measurement to a level usable in Adaptive Optics systems
Multiconjugate Adaptive Optics for Astronomy
Since the year 2000, adaptive optics (AO) has seen the emergence of a variety
of new concepts addressing particular science needs; multiconjugate adaptive
optics (MCAO) is one of them. By correcting the atmospheric turbulence in 3D
using several wavefront sensors and a tomographic phase reconstruction
approach, MCAO aims to provide uniform diffraction limited images in the
near-infrared over fields of view larger than 1 arcmin square, i.e., 10 to 20
times larger in area than classical single conjugated AO. In this review, we
give a brief reminder of the AO principles and limitations, and then focus on
aspects particular to MCAO, such as tomography and specific MCAO error sources.
We present examples and results from past or current systems: MAD
(Multiconjugate Adaptive Optics Demonstrator) and GeMS (Gemini MCAO System) for
nighttime astronomy and the AO system, at Big Bear for solar astronomy. We
examine MCAO performance (Strehl ratio up to 40percent in H band and full width
at half maximum down to 52 mas in the case of MCAO), with a particular focus on
photometric and astrometric accuracy, and conclude with considerations on the
future of MCAO in the Extremely Large Telescope and post-HST era.Comment: 38 pages, published in Annual Review of Astronomy and Astrophysic
Astrometric performance of the Gemini multi-conjugate adaptive optics system in crowded fields
The Gemini Multi-conjugate adaptive optics System (GeMS) is a facility
instrument for the Gemini-South telescope. It delivers uniform,
near-diffraction-limited image quality at near-infrared wavelengths over a 2
arcminute field of view. Together with the Gemini South Adaptive Optics Imager
(GSAOI), a near-infrared wide field camera, GeMS/GSAOI's combination of high
spatial resolution and a large field of view will make it a premier facility
for precision astrometry. Potential astrometric science cases cover a broad
range of topics including exo-planets, star formation, stellar evolution, star
clusters, nearby galaxies, black holes and neutron stars, and the Galactic
center. In this paper, we assess the astrometric performance and limitations of
GeMS/GSAOI. In particular, we analyze deep, mono-epoch images, multi-epoch data
and distortion calibration. We find that for single-epoch, un-dithered data, an
astrometric error below 0.2 mas can be achieved for exposure times exceeding
one minute, provided enough stars are available to remove high-order
distortions. We show however that such performance is not reproducible for
multi-epoch observations, and an additional systematic error of ~0.4 mas is
evidenced. This systematic multi-epoch error is the dominant error term in the
GeMS/GSAOI astrometric error budget, and it is thought to be due to
time-variable distortion induced by gravity flexure.Comment: 16 pages, 22 figures, accepted for publication in MNRA
Single detector stereo-SCIDAR for Mount Stromlo
Satellite tracking and imaging is conducted by the ANU Research School of Astronomy and Astrophysics and Electro-Optic Systems (EOS) at Mount Stromlo Observatory, Canberra, Australia, as part of the Space Environment Management Cooperative Research Centre (SERC) to support the development in space situational awareness. Atmospheric turbulence leads to distortions in the measured data. Adaptive optics (AO) systems counteract those distortions and improve the resolution of the tracking and imaging systems. To assist in the design of the AO systems, we need to gather information on the atmosphere at Mount Stromlo: r0, τ 0, and the turbulence Cn2 profile. With the SCIntillation Detection And Ranging (SCIDAR) Technique the scintillation of two stars is measured and their autocorrelation function is computed, providing a measurement of the turbulence profile. This technique usually uses one detector recording the two images of the stars simultaneously. However, the images overlap leading to an underestimation of the Cn2 values. The introduction of stereo-SCIDAR1 over- comes this issue by separating the two stars and imaging them on two separate image sensors. To reduce costs, we introduce a new stereo-SCIDAR system separating the beams from the two stars, but using only one single detector. This has been shown for a Low Layer SCIDAR (LOLAS) system with wide double stars (200 arcsec). We investigate this technique by detecting the scintillation patterns of double stars with separation from 10 to 25 arcsec, allowing some flexibility in the altitude span and resolution, while retaining a simple optical setup. We selected a low noise sCMOS camera as the imager. We show the current design of this system and investigate its feasibility for further development
First performance of the gems + gmos system. Part1. Imaging
During the commissioning of the Gemini MCAO System (GeMS), we had the
opportunity to obtain data with the Gemini Multi-Object Spectrograph (GMOS),
the most utilised instrument at Gemini South Observatory, in March and May
2012. Several globular clusters were observed in imaging mode that allowed us
to study the performance of this new and untested combination. GMOS is a
visible instrument, hence pushing MCAO toward the visible.We report here on the
results with the GMOS instruments, derive photometric performance in term of
Full Width Half Maximum (FWHM) and throughput. In most of the cases, we
obtained an improvement factor of at least 2 against the natural seeing. This
result also depends on the Natural Guide Star constellation selected for the
observations and we then study the impact of the guide star selection on the
FWHM performance.We also derive a first astrometric analysis showing that the
GeMS+GMOS system provide an absolute astrometric precision better than 8mas and
a relative astrometric precision lower than 50 mas.Comment: 13 pages, 11 figures, accepted for publication in MNRAS on March 23rd
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Lens mounting techniques for precise radial location of fragile lenses in the NGS2 and Veloce instruments
We present novel methods for mounting lenses in a pair of instruments that presented challenging optical and
mechanical requirements. The first instrument is the replacement Natural Guide Star Sensor (NGS2) for CANOPUS
at Gemini South, which incorporates an objective consisting of a stack of six lenses mounted in a common
bore. A compliant radial spacer was used to eliminate lens decentre resulting from the additional radial clearance
required to accommodate differential thermal strains between the low thermal expansion lenses and a common
bore. In the same instrument, tangent contact toroidal spacers were deployed in place of traditional conical spacers
to further reduce contact stresses in fragile calcium fluoride lens elements. The toroidal faces were specified
with a 10µm profile tolerance to avoid possible edge contact between the spacers and lenses. We investigated
milling and turning machining processes for the production of the spacers by comparing their results via Coordinate
Measuring Machine (CMM) measurements. In the second instrument, Veloce, built for the Anglo-Australian
Telescope, a lens decentre requirement of 40µm led us to develop a simple means of in-situ centring adjustment
of the cell mounted lens. Physical testing of the finished instruments verified the performance of each of these
methods. NGS2 produced images at the factory acceptance test in which 94% of encircled energy was captured
by a single 16um detector pixel, surpassing the specification of 80%. Bench testing of Veloce during assembly
showed that the adjustment mechanism allowed centring of the lens over a range of +/- 0.1mm with a precision
of 5µm
AO corrected satellite imaging from Mount Stromlo
The Research School of Astronomy and Astrophysics have been developing adaptive optics systems for space situational awareness. As part of this program we have developed satellite imaging using compact adaptive optics systems for small (1-2 m) telescopes such as those operated by Electro Optic Systems (EOS) from the Mount Stromlo Observatory. We have focused on making compact, simple, and high performance AO systems using modern high stroke high speed deformable mirrors and EMCCD cameras. We are able to track satellites down to magnitude 10 with a Strehl in excess of 20% in median seeing
Characterization of the sodium layer at Cerro Pachon, and impact on laser guide star performance
Detailed knowledge of the mesospheric sodium layer characteristics is crucial to estimate and optimize the performance of laser guide star (LGS) assisted adaptive optics (AO) systems. In this paper, we present an analysis of two sets of data on the mesospheric sodium layer. The first set comes from a laser experiment that was carried out at Cerro Tololo to monitor the abundance and altitude of the mesospheric sodium in 2001, during six runs covering a period of one year. These data are used to derive the mesospheric sodium column density, the sodium layer thickness and the temporal behaviour of the sodium layer mean altitude. The second set of data was gathered during the first year of the Gemini Multi-Conjugate Adaptive Optics (MCAO) System (GeMS) commissioning and operations. GeMS uses five LGSs to measure and compensate for atmospheric distortions. Analysis of the LGS wavefront sensor (WFS) data provides information about the sodium photon return and the spot elongation seen by the WFS. All these parameters show large variations on a yearly, nightly and hourly basis, affecting the LGS brightness, shape and mean altitude. The sodium photon return varies by a factor of 3-4 over a year, and can change by a factor of 2 over a night. In addition, the comparison of the photon returns obtained in 2001 with those measured a decade later using GeMS shows a significant difference in laser format efficiencies. We find that the temporal power spectrum of the sodium mean altitude follows a linear trend, in good agreement with the results reported by Pfrommer & Hickson
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