185 research outputs found
Simulations of ELT-GMCAO performance for deep field observations
The Global-Multi Conjugated Adaptive Optics (GMCAO) approach offers an
alternative way to correct an adequate scientific Field of View (FoV) using
only natural guide stars (NGSs) to extremely large ground-based telescopes.
Thus, even in the absence of laser guide stars, a GMCAO-equipped ELT-like
telescope can achieve optimal performance in terms of Strehl Ratio (SR),
retrieving impressive results in studying star-poor fields, as in the cases of
the deep field observations. The benefits and usability of GMCAO have been
demonstrated by studying 6000 mock high redshift galaxies in the Chandra Deep
Field South region. However, a systematic study simulating observations in
several portions of the sky is mandatory to have a robust statistic of the
GMCAO performance. Technical, tomographic and astrophysical parameters,
discussed here, are given as inputs to GIUSTO, an IDL-based code that estimates
the SR over the considered field, and the results are analyzed with statistical
considerations. The best performance is obtained using stars that are
relatively close to the Scientific FoV; therefore, the SR correlates with the
mean off-axis position of NGSs, as expected, while their magnitude plays a
secondary role. This study concludes that the SRs correlate linearly with the
galactic latitude, as also expected. Because of the lack of natural guide stars
needed for low-order aberration sensing, the GMCAO confirms as a promising
technique to observe regions that can not be studied without the use of laser
beacons. It represents a robust alternative way or a risk mitigation strategy
for laser approaches on the ELTs.Comment: 18 pages, 10 figures, accepted for publication on PAS
Hierarchichal-segmented AO in order to attain wide field compensation in the visible on an 8m class telescope
We describe the preliminary optimized layout for a partially optimized
concept of an optical-8m class VLT-like 2x2 segmented camera where each channel
is assisted by an equivalent of an MCAO system where the ground layer
correction is commonly employed while the high altitude ones is performed in an
open-loop fashion. While we derive the basic relationships among the Field of
View and attainable correction with a pre-defined choice for the hardware, we
discuss sky coverage and wavefront sensing issues employing natural and
artificial references, involving the latest stateof-the-art in the development
of wavefront sensing. We show that a flexible approach allow for a compensated
Field of View that is variable and can be properly tuned matching the current
turbulence situation and the requirement in term of quality of the
compensation. A preliminary description of the overall optomechanical package
is given as well along with a rough estimates of the efforts required to
translates such a concept into reality.Comment: 6 pages, 4 figures, in AO4ELT5 Proceeding
Multiple Spatial Frequencies Pyramid WaveFront Sensing
A modification of the pyramid wavefront sensor is described. In this conceptually new class of devices, the perturbations are split at the level of the focal plane depending upon their spatial frequencies, and then measured separately. The aim of this approach is to increase the accuracy in the determination of some range of spatial frequency perturbations, or a certain classes of modes, disentangling them from the noise associated to the Poissonian fluctuations of the light coming from the perturbations outside of the range of interest or from the background in the pupil planes; the latter case specifically when the pyramid wavefront sensor is used with a large modulation. While the limits and the effectiveness of this approach should be further investigated, a number of variations on the concept are shown, including a generalization of the spatial filtering in the point-diffraction wavefront sensor. The simplest application, a generalization to the pyramid of the well-known spatially filtering in wavefront sensing, is showing promise as a significant limiting magnitude advance. Applications are further speculated in the area of extreme adaptive optics and when serving spectroscopic instrumentation where “light in the bucket” rather than Strehl performance is required
MAVIS: system modelling and performance prediction
The MCAO Assisted Visible Imager and Spectrograph (MAVIS) Adaptive Optics
Module has very demanding goals to support science in the optical: providing
15% SR in V band on a large FoV of 30arcsec diameter in standard atmospheric
conditions at Paranal. It will be able to work in closed loop on up to three
natural guide stars down to H=19, providing a sky coverage larger than 50% in
the south galactic pole. Such goals and the exploration of a large MCAO system
parameters space have required a combination of analytical and end- to-end
simulations to assess performance, sky coverage and drive the design. In this
work we report baseline performance, statistical sky coverage and parameters
sensitivity analysis done in the phase-A instrument study.Comment: 12 pages, 9 figures, 7 tables. SPIE conference Astronomical
Telescopes and Instrumentation, 14 - 18 December 2020, digital foru
Ingot Laser Guide Stars Wavefront Sensing
We revisit one class of z-invariant WaveFront sensor where the LGS is fired
aside of the telescope aperture. In this way there is a spatial dependence on
the focal plane with respect to the height where the resonant scattering
occurs. We revise the basic parameters involving the geometry and we propose
various merit functions to define how much improvement can be attained by a
z-invariant approach. We show that refractive approaches are not viable and we
discuss several solutions involving reflective ones in what has been nicknamed
"ingot wavefront sensor" discussing the degrees of freedom required to keep
tracking and the basic recipe for the optical design.Comment: 6 pages, 4 figures, AO4ELT5 Conference Proceeding, 201
A Holographic Diffuser Generalised Optical Differentiation Wavefront Sensor
The wavefront sensors used today at the biggest World's telescopes have
either a high dynamic range or a high sensitivity, and they are subject to a
linear trade off between these two parameters. A new class of wavefront
sensors, the Generalised Optical Differentiation Wavefront Sensors, has been
devised, in a way not to undergo this linear trade off and to decouple the
dynamic range from the sensitivity. This new class of WFSs is based on the
light filtering in the focal plane from a dedicated amplitude filter, which is
a hybrid between a linear filter, whose physical dimension is related to the
dynamic range, and a step in the amplitude, whose size is related to the
sensitivity. We propose here a possible technical implementation of this kind
of WFS, making use of a simple holographic diffuser to diffract part of the
light in a ring shape around the pin of a pyramid wavefront sensor. In this
way, the undiffracted light reaches the pin of the pyramid, contributing to the
high sensitivity regime of the WFS, while the diffused light is giving a sort
of static modulation of the pyramid, allowing to have some signal even in high
turbulence conditions. The holographic diffuser zeroth order efficiency is
strictly related to the sensitivity of the WFS, while the diffusing angle of
the diffracted light gives the amount of modulation and thus the dynamic range.
By properly choosing these two parameters it is possible to build a WFS with
high sensitivity and high dynamic range in a static fashion. Introducing
dynamic parts in the setup allows to have a set of different diffuser that can
be alternated in front of the pyramid, if the change in the seeing conditions
requires it.Comment: 11 pages, 5 figure
Pathfinder first light: alignment, calibration, and commissioning of the LINC-NIRVANA ground-layer adaptive optics subsystem
We present descriptions of the alignment and calibration tests of the
Pathfinder, which achieved first light during our 2013 commissioning campaign
at the LBT. The full LINC-NIRVANA instrument is a Fizeau interferometric imager
with fringe tracking and 2-layer natural guide star multi-conjugate adaptive
optics (MCAO) systems on each eye of the LBT. The MCAO correction for each side
is achieved using a ground layer wavefront sensor that drives the LBT adaptive
secondary mirror and a mid-high layer wavefront sensor that drives a Xinetics
349 actuator DM conjugated to an altitude of 7.1 km. When the LINC-NIRVANA MCAO
system is commissioned, it will be one of only two such systems on an 8-meter
telescope and the only such system in the northern hemisphere. In order to
mitigate risk, we take a modular approach to commissioning by decoupling and
testing the LINC-NIRVANA subsystems individually. The Pathfinder is the
ground-layer wavefront sensor for the DX eye of the LBT. It uses 12 pyramid
wavefront sensors to optically co-add light from natural guide stars in order
to make four pupil images that sense ground layer turbulence. Pathfinder is now
the first LINC-NIRVANA subsystem to be fully integrated with the telescope and
commissioned on sky. Our 2013 commissioning campaign consisted of 7 runs at the
LBT with the tasks of assembly, integration and communication with the LBT
telescope control system, alignment to the telescope optical axis, off-sky
closed loop AO calibration, and finally closed loop on-sky AO. We present the
programmatics of this campaign, along with the novel designs of our alignment
scheme and our off-sky calibration test, which lead to the Pathfinder's first
on-sky closed loop images
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