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

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

Modeling pyramidal sensors in ray-tracing software by a suitable user-defined surface

Following the unprecedented results in terms of performances delivered by the first light adaptive optics system at the Large Binocular Telescope, there has been a wide-spread and increasing interest on the pyramid wavefront sensor (PWFS), which is the key component, together with the adaptive secondary mirror, of the adaptive optics (AO) module. Currently, there is no straightforward way to model a PWFS in standard sequential ray-tracing software. Common modeling strategies tend to be user-specific and, in general, are unsatisfactory for general applications. To address this problem, we have developed an approach to PWFS modeling based on user-defined surface (UDS), whose properties reside in a specific code written in C language, for the ray-tracing software ZEMAX™. With our approach, the pyramid optical component is implemented as a standard surface in ZEMAX™, exploiting its dynamic link library (DLL) conversion then greatly simplifying ray tracing and analysis. We have utilized the pyramid UDS DLL surface-referred to as pyramidal acronyms may be too risky (PAM2R)-in order to design the current PWFS-based AO system for the Giant Magellan Telescope, evaluating tolerances, with particular attention to the angular sensitivities, by means of sequential ray-tracing tools only, thus verifying PAM2R reliability and robustness. This work indicates that PAM2R makes the design of PWFS as simple as that of other optical standard components. This is particularly suitable with the advent of the extremely large telescopes era for which complexity is definitely one of the main challenges

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

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

Autonomous Observations in Antarctica with AMICA

The Antarctic Multiband Infrared Camera (AMICA) is a double channel camera operating in the 2-28 micron infrared domain (KLMNQ bands) that will allow to characterize and exploit the exceptional advantages for Astronomy, expected from Dome C in Antarctica. The development of the camera control system is at its final stage. After the investigation of appropriate solutions against the critical environment, a reliable instrumentation has been developed. It is currently being integrated and tested to ensure the correct execution of automatic operations. Once it will be mounted on the International Robotic Antarctic Infrared Telescope (IRAIT), AMICA and its equipment will contribute to the accomplishment of a fully autonomous observatory.Comment: 12 pages, 4 figures, Advances in Astronomy Journal, Special Issue "Robotic Astronomy", Accepted 11 February 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