VSTPOL is a project to provide a new polarimetric capability to the VST. With its 2.6m primary mirror and 1 degree × 1 degree field of view, the upgrade will make the VST the first large wide field survey telescope with optical polarimetry, filling a specific niche in the astronomical instrumentation landscape. The polarimetric mode will replace the electro-mechanical system that hosts the ADC, which currently sits unused, so that the filter can be accommodated without compromising the ordinary optical configuration. The upgrade requires the design of the mechanical interface to the telescope structure and optics, and the integration of the instrument electronic and software systems. In this paper we present an overview of the approach adopted for the project management and system engineering towards the design of the polarimetric mode addition. In particular, this includes the activities related to the definition of schedule, product and work breakdown structure, deliverables, technical requirements analysis and interfaces.
VSTPOL enhances VST’s capabilities by adding optical polarimetry via a linear polarized filter. This will make the VST the first large wide-field survey telescope with optical polarimetry. The project addresses the need for optical follow-up observations of Cherenkov Telescope Array (CTA) sources and transients. This paper describes software upgrades required for the new polarimetric mode. The current instrument control software, based on ESO VLT software 2011, manages pointing, acquisition, and active optics. The polarimetric mode necessitates two additional motorized movements: inserting the filter and selecting polarization while tracking the object. Traditionally, VLT systems use a Local Control Unit (LCU) on VxWorks for motor control, but this system is outdated. Since compatibility with modern hardware is crucial, we resorted to a PLC-based system, which are unsupported by the installed VLTSW. Fortunately, the ICS Fieldbus Extension allows for a dedicated Device Control Environment (DCE). This DCE, using an updated VTLSW release, acts as a gateway to control electronics, minimizing system-wide impact and reducing update-related risks.
Since the start of operations in 2011, the VLT Survey Telescope (VST) has been one of the most efficient wide-field imagers in the optical bands. However, in the next years the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will be a game-changer in this field. Hence, the timing is appropriate for specializing the VST with additions that can make it unique in well-defined scientific cases. VSTPOL is a project that aims to provide the addition of wide-field polarimetric capabilities to the VST telescope, making it the first large survey telescope for linear optical polarimetry. Actually, while there are quite a number of optical telescopes, the telescopes providing polarimetric instrumentation are just a few. The number of relatively large mirror polarimetric telescopes is small, although they would be specifically needed e.g. to support many science cases of the Cherenkov Telescope Array (CTA) that, in the southern hemisphere, is co-located with the VST. The VST telescope is equipped with a single instrument, the OmegaCAM wide-field imaging camera operating in the visible bands with a field of view of 1° × 1°. The polarimetric mode will be implemented through the insertion of a large rotatable polarizer installed on the field-corrector optics, which will be exchangeable with the non-polarimetric corrector optics. The limiting polarimetric systematic errors due to variable atmospheric conditions and instrumental polarization can be corrected down to a level of ∼ 10−3 by leveraging the large amount of unpolarized stars within each field-of-view. By the user point of view, VSTPOL will be an additional mode for the VST wide-field imaging camera.
The VST (VLT Survey Telescope) is a 2.6m telescope installed in the ESO Observatory of Cerro Paranal, equipped with a wide-field imaging camera operating in the visible band (OmegaCAM). One of the goals of the Cherenkov Telescope Array Plus (CTA+) program, included in the EU Recovery Plan (PNRR), is to upgrade this ground-based optical facility adding a new polarimetric mode to allow the follow-up and monitoring of the CTA transient sources. The VSTPOL design aims to replace the actual electro-opto-mechanical system connected to the back side of the primary mirror cell of the telescope with a new system, consisting of two motorized functions: a linear exchanger mechanism to switch between the traditional imaging mode and the new polarimetric mode; a rotating device equipped with a polarimetric filter, replacing the unused ADC functionality, that enables tracking to compensate for the field rotation, following the movement of the OmegaCAM. Here we present the VSTPOL control electronics architecture, based on the new ESO electronics standards. All the control electronics are hosted in a wall-mountable and properly cooled enclosure installed on-board of the telescope: Commercial Off-The-Shelf (COTS) industrial components (e.g. Beckhoff PLC and EtherCAT fieldbus modules) represent the core of the system to increase the overall reliability and maintainability.
An accurate alignment of the optical surfaces of a telescope is essential to guarantee an optimal image quality since even small displacements introduce aberrations increasing towards the edges of the field. This effect is especially detrimental in wide-field imagers. This work proposes the derivation of a fully analytical model of the wavefront error as a function of the most likely system misalignments. An accurate response of the telescope under a predefined set of misaligned conditions is obtained through simulations in Zemax OpticStudio. The resulting data is combined through an integrated modeling approach, obtaining a map of the aberrations as a function of a vector of perturbations applied to the optical system. The analytical wavefront error allows for a quick and accurate assessment of the theoretical PSF across the entire image field. As a case study, the example of the Rubin Observatory is adopted, featuring an 8.4m primary mirror and a large field of view.
In wide-field telescopes, relatively small misalignments in the optical system can cause large aberrations. The nominal system is normally designed to show a good optical performance over the whole field of view but, in presence of misalignments, the symmetry is broken and the aberrations increase towards the edge of the field. No new aberrations arise, but the known aberrations behave differently and originate multiple nodes, according to the Nodal Aberration Theory. The effects, in terms of image quality degradation, can be especially deleterious for wide-field imagers. This issue can be studied in detail by the ray-tracing programs that are normally adopted for the optical design. Nevertheless, these codes are not optimal for applications where a high execution speed is needed. Here, an application of PSF reconstruction for a wide-field telescope by using an integrated modeling approach is presented. Ray-tracing data are adopted as input to build a fully analytical model. The example of the VST telescope (1x1 deg field of view) is discussed as a case study.
The axes servo control of optical telescopes and antennas acts in two typical phases: the slew to a new target and the subsequent accurate tracking of the source. Although the tracking error minimization is paramount, a good design of the slewing phase is needed as well. In fact, saturations of velocity and acceleration can easily occur during telescope slew, introducing non-linearities in the control system which may lead to undesired behaviors. Also, sudden accelerations may trigger vibrations of the telescope structure, which may increase the slew time or even prevent a stable target acquisition. In this paper, a command pre-processor is adopted to provide recursively a valid path to reach the assigned target, never exceeding the specified rate and acceleration limits. Different generation methods are considered, with different degrees of smoothness and slewing time. Numerical simulations show their main features in different test cases, for both radio and optical telescopes.
Every night the VST Telescope Control Software logs large text files including information on the telescope and instrument operations, executed commands, failures, weather conditions and anything is relevant for the instrument maintenance and the identification of problem sources. These log files are a precious tool, daily used by the observatory personnel for the analysis of any issue raised by the telescope operators during the night. One of the most frequent use of these data is then to trace back telescope, instrument or enclosure problem sources and analyze them. Consequently, these _les are often analyzed looking only for specific issues and for solving well identified problems, in the framework of dedicated and focused efforts. Thus, a minimal part of the information is useful for this kind of daily maintenance. Nevertheless, the log files contain a gold mine of other data, which often make sense only when analyzed on a long time span. This paper describes a 5-year effort, started in 2015, for the systematic collection and analysis of log files, aiming at the identification of useful long-term trends and statistics which are normally overlooked in the daily telescope life. The specific case of the active optics open-loop corrections is discussed as case study.
The servo control algorithms of the TNG, developed in the nineties, have been working for more than 20 years with no major updates. The original hardware was based on a VME-bus based platform running a real time operating system, a rather popular choice for similar applications at the time. Recently, the obsolescence of the hardware and the lack of spares pushed the observatory towards a complete replacement of the electronics that is now being implemented in steps, respecting the basic requirement of never stopping the observatory night operations. Within the framework of this major hardware work, we are taking the opportunity to review and update the existing control schemes. This servo control update, crucial for the telescope performance, envisages a new study from scratch of the controlled plant, including a re-identification of the main axes transfer functions and a re-design of the control filters in the two nested position and speed loops. The ongoing work is described, including preliminary results in the case study of the azimuth axis and our plans for possible further improvements.
The VST telescope is a wide field survey telescope being installed at Cerro Paranal (Chile). Due to the geological nature
of the area, telescopes in Chile are always submitted to unpredictable and sometimes severe earthquake conditions. In
order to clarify some aspects of VST telescope seismic behavior not well represented by linear procedures like Response
Spectrum Analysis, a transient nonlinear analysis of the whole telescope has been foreseen. A mixed approach Finite
Element - Matlab-Simulink has been introduced and a linear FE model of the telescope has been developed, with all
nonlinear devices sources modelled as linear elements. The FE model has been exported to Simulink, using a space state
representation. In Simulink all nonlinearities are appropriately modeled and a base excitement corresponding to
accelerograms compliant with Paranal MLE response spectrum is applied. Resulting force-time histories are then applied
to a detailed finite element model of mirror, to compute stress field. The paper describes both Simulink and mirror FE
analysis, giving also an overview of the actual safety system mechanical implementation, based on analysis results.
The VST primary mirror is a 2.6-m meniscus made of Astro-Sitall. An active optics system is implemented to correct
surface errors due to manufacturing or induced by gravity and temperature changes. The primary mirror is axially
supported by 84 supports disposed in four concentric rings. Three of the supports, symmetrically placed and much stiffer
than the other ones, define the axial plane of the primary mirror acting as fixed points. The remaining 81 supports are
force controlled actuators, used to change the shape of the mirror according to wavefront measurements in closed loop
operation, or to a look-up table in open loop. This paper describes the solutions adopted for the axial actuator, as well as
the test campaign to assess their performance and degree of reliability.
The VST telescope is equipped with an active optics system based on a wavefront sensor, a set of axial actuators to
change the primary mirror shape and a secondary mirror positioner stage. The secondary mirror positioning capability
allows the correction of defocus and coma, caused by incorrect relative positions of the two mirrors arising from the
deformation of the telescope tube and of the optical train under the effect of gravity and thermal espansion. Periodically
the image analyser calculates the deviation of the image from the best quality and the telescope control software
decomposes the deviation into the single optical contributions. The new position and orientation of the secondary mirror
is computed by the telescope control software and transmitted to the secondary mirror support system for execution. The
secondary mirror positioner is a hexapod, i.e. a parallel robot with a mobile platform moved by six linear actuators acting
simultaneously. This paper describes the secondary mirror support system and the qualification test campaign performed
both in laboratory and at the telescope.
The 2.6-m primary mirror of the VST telescope is equipped with an active optics system in order to correct low-order
aberrations, constantly monitoring the optical quality of the image and controlling the relative position and the shape of
the optical elements. Periodically an image analyser calculates the deviation of the image from the best quality. VST is
equipped with both a Shack-Hartmann in the probe system and a curvature sensor embedded in the OmegaCAM
instrument. The telescope control software decomposes the deviation into single optical contributions and calculates the
force correction that each active element has to perform to achieve the optimal quality. The set of correction forces, one
for each axial actuator, is computed by the telescope central computer and transmitted to the local control unit of the
primary mirror system for execution. The most important element of the VST active optics is the primary mirror, with its
active support system located within the primary mirror cell structure. The primary mirror support system is composed
by an axial and a lateral independent systems and includes an earthquake safety system. The system is described and the
results of the qualification test campaign are discussed.
The VST telescope is equipped with an Atmospheric Dispersion Corrector to counterbalance the spectral dispersion
introduced by the atmosphere. The well known effect of atmospheric refraction is the bending of incoming light due to
variable atmospheric density along the light path. This effect depends on the tangent of the zenith angle and also varies
with altitude, humidity and wavelength. Since the magnitude of refraction depends on the wavelength, the resulting
effect is not only a deviation of the light beam from its original direction but also a spectral dispersion of the beam. This
effect can be corrected by introducing a dispersing element in the instrument. In the VST case the device that
compensates for this effect is based on a set of four prisms in two cemented doublet pairs. The system provides an
adjustable counter dispersion by counter-rotating the two pairs of prisms. The counter-rotating angle depends on the
atmospheric dispersion, which is computed with an atmospheric model using both environmental data (temperature,
pressure, humidity) and the telescope position. Two different approaches have been compared for the computations to
cross-check the results. The electromechanical system has been assembled, tested and debugged prior to the shipping to
Chile. This paper describes the atmospheric models used in the VST case and the most recent phases of work.
The most important element of the VST active optics is the primary mirror, with its active support system located within
the primary mirror cell structure. The primary mirror support system is composed by an axial and a lateral independent
systems and includes an earthquake safety system. The primary mirror system software has been designed with a system
engineering approach. The software has to change the mirror shape during observations, but also shall allow the user to
perform a number of other activities. It has to support: periodic maintenance operations like the alignment, the mirror
removal and installation for recoating; the functional tests; the engineering operations; the recalibration of several
parameters. This paper describes how the primary mirror system software has been developed to support both the
observations and engineering activities.
The VST telescope is equipped with an active optics system based on a wavefront sensor, a set of axial actuators to
change the primary mirror shape and a secondary mirror positioner stage. The secondary mirror positioning capability
allows the correction of defocus and coma optical aberrations, mainly caused by incorrect relative positions of the optics.
The secondary mirror positioner is a 6-6 Stewart platform (also called "hexapod"). It is a parallel robot with a mobile
platform moved by 6 linear actuators acting simultaneously. This paper describes the secondary mirror support system
and the current status of the work.
The VST telescope is a wide field survey telescope partially installed at Cerro Paranal (Chile). Due to the geological
nature of the area, telescopes in Chile are always submitted to unpredictable and sometimes severe earthquake
conditions. The proper design of an "ad hoc" safety system for the main mirror inside the cell is therefore mandatory for
a safe operation of the telescope, in order to avoid unpleasant consequences in terms of critical loss of the main optical
element. A multi-step detailed dynamic analysis has been performed in order to asses the characteristic of safety devices,
in term of stiffness, damping and positions with respect to the mirror. The paper describes the procedure followed while
giving an overview of dynamic analysis capabilities of MSC-Nastran code. The procedure proposed follows the
requirements of Eurocode 8 regulations for the design of earthquake safe structures.
In order to perform detailed dynamic simulation the FEM model must be reduced in order to decrease simulation
calculation time while maintaining the essential mechanical characteristics. A mathematical approach has been exploited
instead of a "trial and error" procedure, with springs and masses: a more accurate fitting to the FEM model is therefore
available. This paper describes the integration of Nastran and Ansys finite element software capabilities within the
Matlab - Simulink computational environment.
The integrated modelling approach is fundamental in telescopes design where it is necessary to merge different
disciplines together. This paper describes the integration of optical ray-tracing capabilities within the Matlab
computational environment. This approach allows to write automatic procedures to implement a huge number of
computations, that are very unpractical to perform in interactive mode by ray tracing software packages. Data produced
by computations are stored and automatically analyzed. One of the main benefits from this approach comes from the
traceability of the work, that is intrinsically impossible when the optical designer works in interactive mode. The right
procedure is built and tuned just the first time and the computation software is available for inspection and check.
Furthermore computations and results are easily reproducible simply re-running Matlab scripts. An automatic approach
is especially helpful in wide-field telescope projects where the optical quality has to be studied over a wide field of view.
This leads to repeat the same computations many times in a number of fields. In interactive mode this would cause a
significant waste of optical designer time to repeat many times the same manual procedures. The solution proposed here
allows to save time and prevent occasional mistakes.
VST is a survey telescope with a wide field of view (1°x1°). This makes necessary to investigate the image quality over
the whole field of view and not just in the centre. The image quality is studied in terms of diameter enclosing the 80% of
the PSF energy. Several error sources are analyzed and optical sensitivity analysis are carried out to know the effect of
each individual error source on the image quality. The individual contributions are added quadratically in the assumption
they are uncorrelated. The worst case is generally found in the border of the field. The average of the results obtained in
a number of fields along the whole field of view is considered as a metric for the telescope optical quality. The overall
budget is a continuous work in progress because the requirements for the different subsystems, starting from the initial
predictions, are then fed back by test data and further analysis.
In active optics systems obviously a fundamental role is played be the choice of polynomials to describe the primary
mirror deformations. The well known Zernike polynomials are widely used because of their immediate interpretation
in terms of optical aberrations. Nevertheless in an active optics correction system context, the choice of the so called
"minimum energy modes" as the polynomials to represent the mechanical deformations is best justified by their
derivation from mechanical properties. This is the approach followed for the 2.6m primary mirror of the VST
telescope, to be hosted on top of the Cerro Paranal ESO observatory. The calibration forces to compensate a given
amount of each aberration mode are computed and discussed.
Active optics systems have been designed for most of the recently built medium and large telescopes. Anyway the number of working active optics systems is still relatively small and there are some differences in the various technologies and solutions. This paper describes a toolbox for active optics simulation developed in the framework of a specific project, but general enough to be useful in most of the cases. The toolbox deals with both off-line and online activities: it generates fictitious Shack-Hartmann fits images, supports different kinds of polynomials (Zernike, quasi-Zernike, normalized Zernike, minimum energy modes) to fit the wavefront data, and performs calculation of primary mirror actuator forces and secondary mirror position corrections, simulating also the usage of hexapod systems.
KEYWORDS: Mirrors, 3D modeling, Finite element methods, Integrated modeling, Active optics, Systems modeling, Actuators, MATLAB, Differential equations, Device simulation
In all the new technology telescopes (medium, large, extremely large) the active optics is one of the most delicate
systems. Sometimes only a perfect control of the mirrors can justify the whole telescope project. No assessed
simulation tool exists, one of the reason is that the approach can be different for segmented or monolithic, medium or
large mirrors, and so each telescope design staff usually develops its own simulation system. In this paper an
integrated modeling approach is proposed, combining finite element analysis, dynamical systems simulation methods
and optical performance analysis.
This paper is about VST active optics system design, specifications and status. The VST is a modified Ritchey-Chretien wide field Alt-Az telescope with a corrector camera (1 square degree field of view), so when all optical components are correctly aligned, only residual aberrations in whole field are present. The major amounts of these aberrations can be introduced by gravitational and thermo opto-mechanical deformations and mirror misalignments. For these reasons active control of the primary mirror shape and secondary mirror position are required to lessen optical aberrations. The aim of active optics is to correct all optical telescope errors in order to make them small compared with external seeing. The VST is essentially compensated for static or slow frequency deformations and misalignments with a band pass from dc to 1/30 Hz, since the corresponding integration time is sufficient to integrate out the external seeing, giving a round image corresponding to the integrated external seeing quality. VST decentering, coma and defocus are corrected by mean of a secondary mirror position control system (a two-stage hexapode system) and spherical, astigmatism, quad-astigmatism and tri-coma are corrected by mean of M1 mirror shape deformation (axial and radial support system). For optical aberrations and guiding measurement an optical sensing arm has been designed.
The effects of atmospheric differential refraction on astrophysical measurements are well known. In particular, as a ray of light passes through the atmosphere, its direction is altered by the effects of atmospheric refraction. The amount of this effect depends basically on the variation of the refractive index along the path of the ray. The real accuracy needed in the atmosphere model and in the calculation of the correction to be applied is of course, considerably worse, especially at large zenith angles. On the VLT Survey Telescope (VST) the use of an Atmospheric Dispersion Corrector (ADC) is foreseen at a wide zenith distance range. This paper describes the software design and implementation aspects regarding the analytical correction law discovered to correct the refraction effect during observations with VST.
Dario Mancini, Guido Mancini, Francesco Perrotta, Luigi Ferragina, D. Fierro, Valentina Fiume Garelli, Lorenzo Pellone, Oreste Caputi, Giovanni Sciarretta, Michele Valentino
The VLT Survey Telescope (VST) is a cooperative program between the European Southern Observatory (ESO) and the INAF Capodimonte Astronomical Observatory (OAC), Naples, for the study, design, and realization of a 2.6-m wide-field optical imaging telescope to be operated at the Paranal Observatory, Chile. The VST has been specifically designed to carry out stand-alone observations in the UV to I spectral range and to supply target databases for the ESO Very Large Telescope (VLT). The telescope design, manufacturing and integration are responsibility of TWG and have been carried out on the base of a model of optimized design not only for mechanics but for all telescope subsystems. The paper is an overview on the telescope mechanical design and optimization.
The VLT Survey Telescope (VST) is a cooperative program between the European Southern Observatory (ESO) and the INAF Capodimonte Astronomical Observatory (OAC), Naples, for the study, design, and realization of a 2.6-m wide-field optical imaging telescope to be operated at the Paranal Observatory, Chile. The VST has been specifically designed to carry out stand-alone observations in the UV to I spectral range and to supply target databases for the ESO Very Large Telescope (VLT). The telescope design, manufacturing and integration are responsibility of OAC. The telescope is in Cassegrain configuration and for this reason the primary mirror cell represents one of the most complex telescope subsystems, designed to host a large amount of auxiliary sub-systems and to support a wide field camera. The paper describes the solutions adopted as a result of an integrated optimized optical and mechanical design.
Dario Mancini, Oliver LeFevre, Oreste Caputi, Luigi Ferragina, Valentina Fiume Garelli, Guido Mancini, P. Tommasi Mavar, C. Parrella, Francesco Perrotta, M. Russo, R. Russo, M. Schibeci, Pietro Schipani
The VIRMOS consortium of French and Italian institutes is building the Visible Multi-Object Spectrograph (VIMOS) and the NIRMOS for the European Southern Observatory Very Large Telescope. Inside the VIRMOS consortium the Technology Working Group of the Osservatorio Astronomico di Capodimonte is in charge of the design, realization and integration of mechanics, opto-mechanical interfaces, control electronics and low level control software. This paper is an overview on the VIMOS technical project managed by the TWG.
The VST (Very Large Telescope Survey Telescope) is an 2.6 m class Alt-Az telescope which will be installed in the European Southern Observatory (ESO) Paranal site, Chile. It has been designed by the Technology Working Group of the Astronomical Observatory of Capodimonte, Italy. The VST is an 1 degree(s) X 1 degree(s) wide-field imaging facility planned to supply databases for the ESO VLT science and carry out stand-alone observations in the UV to I spectral range starting in the year 2001. All the solutions adopted in the VST design comply to the ESO VLT standards. This paper reports a technical overview of the telescope design.
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