147 research outputs found
Laser Guide Star for 3.6m and 8m telescopes: Performances and astrophysical implications
We have constructed an analytical model to simulate the behavior of an
adaptive optics system coupled with a sodium laser guide star. The code is
applied to a 3.6-m and 8m class telescopes. The results are given in terms of
Strehl ratio and full width at half maximum of the point spread function. Two
atmospheric models are used, one representing good atmospheric conditions (20
per cent of the time), the other median conditions. Sky coverage is computed
for natural guide star and laser guide star systems, with two different
methods. The first one is a statistical approach, using stellar densities, to
compute the probability to find a nearby reference. The second is a
cross-correlation of a science object catalogue and the USNO catalogue. Results
are given in terms of percentage of the sky that can be accessed with given
performances, and in terms of number of science object that can be observed,
with Strehls greater than 0.2 and 0.1 in K and J bands.Comment: 14 pages, 17 figures, accepted for publication in MNRAS. Also
available at: http://www-obs.univ-lyon1.fr/~lelouarn
New challenges for Adaptive Optics: Extremely Large Telescopes
The performance of an adaptive optics (AO) system on a 100m diameter ground
based telescope working in the visible range of the spectrum is computed using
an analytical approach. The target Strehl ratio of 60% is achieved at 0.5um
with a limiting magnitude of the AO guide source near R~10, at the cost of an
extremely low sky coverage. To alleviate this problem, the concept of
tomographic wavefront sensing in a wider field of view using either natural
guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use
3 or 4 reference sources and up to 3 deformable mirrors, which increase up to
8-fold the corrected field size (up to 60\arcsec at 0.5 um). Operation with
multiple NGS is limited to the infrared (in the J band this approach yields a
sky coverage of 50% with a Strehl ratio of 0.2). The option of open-loop
wavefront correction in the visible using several bright NGS is discussed. The
LGS approach involves the use of a faint (R ~22) NGS for low-order correction,
which results in a sky coverage of 40% at the Galactic poles in the visible.Comment: 11 pages, 9 figures, 4 tables. Accepted for publication in MNRA
Validation Through Simulations of a Cn2 Profiler for the ESO/VLT Adaptive Optics Facility
The Adaptive Optics Facility (AOF) project envisages transforming one of the
VLT units into an adaptive telescope and providing its ESO (European Southern
Observatory) second generation instruments with turbulence corrected
wavefronts. For MUSE and HAWK-I this correction will be achieved through the
GALACSI and GRAAL AO modules working in conjunction with a 1170 actuators
Deformable Secondary Mirror (DSM) and the new Laser Guide Star Facility
(4LGSF). Multiple wavefront sensors will enable GLAO and LTAO capabilities,
whose performance can greatly benefit from a knowledge about the stratification
of the turbulence in the atmosphere. This work, totally based on end-to-end
simulations, describes the validation tests conducted on a Cn2 profiler adapted
for the AOF specifications. Because an absolute profile calibration is strongly
dependent on a reliable knowledge of turbulence parameters r0 and L0, the tests
presented here refer only to normalized output profiles. Uncertainties in the
input parameters inherent to the code are tested as well as the profiler
response to different turbulence distributions. It adopts a correction for the
unseen turbulence, critical for the GRAAL mode, and highlights the effects of
masking out parts of the corrected wavefront on the results. Simulations of
data with typical turbulence profiles from Paranal were input to the profiler,
showing that it is possible to identify reliably the input features for all the
AOF modes.Comment: 15 pages, 12 figures, accepted for publication in the MNRAS Accepted
2015 January 22. Received 2015 January 21; in original form 2014 December
The High-Superior-Tension Technique: Evolution of Lipoabdominoplasty
Because abdominoplasty is associated with complications such as seroma and necrosis as well as epigastric bulging and a suprapubic scar located too high, the demand for this procedure is not as high as it otherwise might be. However, although these negative effects were common many years ago, their incidence has decreased dramatically with modern abdominoplastic techniques. One approach using a combination of abdominoplasty and liposuction or lipoabdominoplasty has resolved many of the problems faced with earlier techniques, offering aesthetically pleasing results and excellent reliability. The keys to successful lipoabdominoplasty, first developed as the high-superior-tension technique, are extensive liposuction, preservation of lymphatic trunks, preaponeurotic epigastric dissection, major muscle fascia plication, two high-tension paraumbilical sutures, hypogastric tension sutures, and closure of the dead spaces. The most recent updates to this technique are described in this article
Simulations of Laser Guide Star Adaptive Optics Systems for the European Extremely Large Telescope
Abstract. End-to-end simulations of ground layer adaptive optics (GLAO) and laser tomography adaptive optics (LTAO) systems for the European extremely large telescope (EELT) are presented. This paper focuses on wavefront sensing issues; reconstruction is addressed in the paper by Bechet in these proceedings. For laser guide star based systems with Shack-Hartmann wavefront sensors without noise-optimal reconstruction, the simulations show that approximately 1.5 times as many photons are required for side launch of the laser as central launch to achieve the same performance in both LTAO and GLAO modes. The matched filter wavefont sensing algorithm provides better performance than correlation or center-of-gravity over the expected range of photon flux
Simulations of Adaptive Optics Systems for the E-ELT
ABSTRACT In this paper, we present simulation work done on AO systems for the E-ELT. We study the influence of the number of Laser Guide Stars (LGS) on system performance. Then, we investigate the impact of the conjugation height of the M4 adaptive mirror on GL/LT/MC-AO. Finally, we compare the results of a Fourier code and end-to-end models on the position of the LGS in the field of view
A comparison of next-generation turbulence profiling instruments at Paranal
A six-night optical turbulence monitoring campaign has been carried at Cerro Paranal observatory in February and March, 2023 to facilitate the development and characterisation of two novel atmospheric site monitoring instruments - the ring-image next generation scintillation sensor (RINGSS) and 24-hour Shack Hartmann image motion monitor (24hSHIMM) in the context of providing optical turbulence monitoring support for upcoming 20-40m telescopes. Alongside these two instruments, the well-characterised Stereo-SCIDAR and 2016-MASS-DIMM were operated throughout the campaign to provide data for comparison. All instruments obtain estimates of optical turbulence profiles through statistical analysis of intensity and wavefront angle-of-arrival fluctuations from observations of stars. Contemporaneous measurements of the integrated turbulence parameters are compared and the ratios, bias, unbiased root mean square error and correlation of results from each instrument assessed. Strong agreement was observed in measurements of seeing, free atmosphere seeing and coherence time. Less correlation is seen for isoplanatic angle, although the median values agree well. Median turbulence parameters are further compared against long-term monitoring data from Paranal instruments. Profiles from the three small-telescope instruments are compared with the 100-layer profile from the stereo-SCIDAR. It is found that the RINGSS and SHIMM offer improved accuracy in characterisation of the vertical optical turbulence profile over the MASS-DIMM. Finally, the first results of continuous optical turbulence monitoring at Paranal are presented which show a strong diurnal variation and predictable trend in the seeing. A value of 2.65″ is found for the median daytime seeing
Ground-layer wavefront reconstruction from multiple natural guide stars
Observational tests of ground layer wavefront recovery have been made in open
loop using a constellation of four natural guide stars at the 1.55 m Kuiper
telescope in Arizona. Such tests explore the effectiveness of wide-field seeing
improvement by correction of low-lying atmospheric turbulence with ground-layer
adaptive optics (GLAO). The wavefronts from the four stars were measured
simultaneously on a Shack-Hartmann wavefront sensor (WFS). The WFS placed a 5 x
5 array of square subapertures across the pupil of the telescope, allowing for
wavefront reconstruction up to the fifth radial Zernike order. We find that the
wavefront aberration in each star can be roughly halved by subtracting the
average of the wavefronts from the other three stars. Wavefront correction on
this basis leads to a reduction in width of the seeing-limited stellar image by
up to a factor of 3, with image sharpening effective from the visible to near
infrared wavelengths over a field of at least 2 arc minutes. We conclude that
GLAO correction will be a valuable tool that can increase resolution and
spectrographic throughput across a broad range of seeing-limited observations.Comment: 25 pages, 8 figures, to be published in Astrophys.
Prism matching for piston segmentation correction with adaptive optics systems on extremely large telescopes
Images observed at ground-based telescopes are
blurred by Earth’s atmosphere. Adaptive optics systems can correct for this blurring by using a wavefront sensor to measure the
instantaneous wavefront aberration created by the atmosphere,
and a deformable mirror to apply correction to the aberrated
wavefront. The European Extremely Large Telescope, one of the
next generation of telescopes currently under construction, will
have large supporting struts or arms (spiders) for the secondary
mirror that obscure whole rows and columns of subapertures in
the wavefront sensor. This phase discontinuity can allow large
segment piston errors to arise between neighbouring segments,
because the deformable mirror can produce the segment modes
but the wavefront sensor senses them poorly. The spider for
the EELT will have six arms, and we propose in this paper
employing a six-sided prism for the wavefront sensor instead
of the traditional four sided pyramid. We show that when the
diffraction spikes from the spider arms are aligned in the middle
of the prism faces, the sensitivty of the sensor, as measured by
the sum of the singular values of the interaction matrix for the
six segment piston modes, is 15% larger than if the diffraction
spikes are aligned with the prism edges
Modeling of pulsed laser guide stars for the Thirty Meter Telescope project
The Thirty Meter Telescope (TMT) has been designed to include an adaptive
optics system and associated laser guide star (LGS) facility to correct for the
image distortion due to Earth's atmospheric turbulence and achieve
diffraction-limited imaging. We have calculated the response of mesospheric
sodium atoms to a pulsed laser that has been proposed for use in the LGS
facility, including modeling of the atomic physics, the light-atom
interactions, and the effect of the geomagnetic field and atomic collisions.
This particular pulsed laser format is shown to provide comparable photon
return to a continuous-wave (cw) laser of the same average power; both the cw
and pulsed lasers have the potential to satisfy the TMT design requirements for
photon return flux.Comment: 16 pages, 20 figure
- …