235 research outputs found
The opto-mechanical alignment procedure of the VLT Survey Telescope
The VLT Survey Telescope is a f/5.5 modified Ritchey-Chretien imaging
telescope, which is being installed at the ESO-Paranal Observatory. It will
provide a one square degree corrected field of view to perform survey-projects
in the wavelength range from UV to I band. In this paper we describe the
opto-mechanical alignment procedure of the 2.61m primary mirror, the secondary
and correctors lenses onto the mechanical structure of the telescope. The
alignment procedure does not rely on the mechanical precision of the mirrors.
It will be achieved using ad-hoc alignment tools, described in the paper, which
allows the spatial determination of optical axes (and focuses where necessary)
of the optical components with respect to the axis defined by the rotation of a
laser beam mounted on the instrument bearing.Comment: 10 pages, 10 figures, Proceeding 773357 of the SPIE Conference
"Ground-based and Airborne Telescopes III", Sunday 27 June 2010, San Diego,
California, US
Characterization of a Double Mesospheric Bore Over Europe
Observations of a pair of mesospheric bore disturbances that propagated through the nighttime mesosphere over Europe are presented. The observations were made at the Padua Observatory, Asiago (45.9\ub0N, 11.5\ub0E), by the Boston University all-sky imager on 11 March 2013. The bores appeared over the northwest horizon, approximately 30 min apart, and propagated toward the southeast. Using additional satellite and radar data, we present evidence indicating the bores originated in the mesosphere from a single, larger-scale mesospheric disturbance propagating through the mesopause region. Furthermore, the large-scale mesospheric disturbance appeared to be associated with an intense weather disturbance that moved southeastward over the United Kingdom and western Europe during 10 and 11 March
Recovering pyramid WS gain in non-common path aberration correction mode via deformable lens
It is by now well known that pyramid based wavefront sensors, once in closed
loop, have the capability to improve more and more the gain as the reference
natural star image size is getting smaller on the pyramid pin. Especially in
extreme adaptive optics applications, in order to correct the non-common path
aberrations between the scientific and sensing channel, it is common use to
inject a certain amount of offset wavefront deformation into the DM(s),
departing at the same time the pyramid from the optimal working condition. In
this paper we elaborate on the possibility to correct the low order non-common
path aberrations at the pyramid wavefront sensor level by means of an adaptive
refractive lens placed on the optical path before the pyramid itself, allowing
the mitigation of the gain loss
FrogEye, the Quantum Coronagraphic mask. The Photon Orbital Angular Momentum and its applications to Astronomy
We propose to realize an optical device based on the properties of photon
orbital angular momentum (POAM) to detect the presence of closeby faint
companions in double systems using Laguerre-Gaussian (L-G) modes of the light.
We test also the possibility of using L-G modes to build coronagraph mask. We
realized in the laboratory a prototype using a blazed l=1 hologram to simulate
the separation between two stars, as observed with a telescope, in
Laguerre-Gaussian modes.Comment: 2 pages, XLIX meeting of the Italian Astronomical Society (SAIT
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
Optical vortices with starlight: Implications for ground-based stellar coronagraphy
Using an l = 1 blazed fork-hologram at the focal plane of the Asiago 122 cm
telescope, we obtained optical vortices from the stellar system Rasalgethi
(alpha Herculis) and from the single star Arcturus (alpha Bootis). We have
analyzed the structure of the optical vortices obtained from non-monochromatic
starlight under very poor seeing conditions using a fast CCD camera to obtain
speckle patterns and carry out the lucky imaging technique, alternative to
adaptive optics. With the insertion of a red filter and of a Lyot stop we
performed l = 1 optical vortex coronography the double star HD74010. The
results are in agreement with theory and numerical simulations. Our results
open the way to applications of optical vortices to ground based astronomical
observations, in particular for coronagraphy with l > 1 masks. No intrinsic
orbital angular momentum was detected in the starlight.Comment: 4 pages, 5 figures. Revised data analysi
Overcoming the Rayleigh Criterion Limit with Optical Vortices
We experimentally and numerically tested the separability of two independent
equally-luminous monochromatic and white light sources at the diffraction
limit, using Optical Vortices (OV), related to the Orbital Angular Momentum
(OAM) of light. The diffraction pattern of one of the two sources crosses a
phase modifying device (fork-hologram) on its center generating the
Laguerre-Gaussian (L-G) transform of an Airy disk. The second source, crossing
the fork-hologram in positions different from the optical center, acquires
different OAM values and generates non-symmetric L-G patterns. We formulated a
criterion, based on the asymmetric intensity distribution of the superposed L-G
patterns so created, to resolve the two sources at angular distances much below
the Rayleigh criterion. Analogous experiments carried out in white light allow
angular resolutions which are still one order of magnitude below the Rayleigh
criterion. The use OVs might offer new applications for stellar separation in
future space experiments.Comment: 4 pages, 5 figure
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