The Mid-High Wavefront Sensors (MHWS) are components of the adaptive optics system of LINC-NIRVANA, the Fizeau interferometer that will be mounted at the LBT. These sensors, one for each telescope arm, will measure the atmospheric turbulence in the high altitude layers, using up to 8 reference stars in a 2 arcmin Field of View, and they will be coupled with two Ground Layer WFSs that will measure the lower part of the atmospheric turbulence using up to 12 stars over an annular Field of View from 2 to 6 arcmin in diameter. We will describe the opto-mechanical layout of the MHWS and the Assembly, Integration and Test (AIT) phase of the first sensor in the laboratory of the Bologna Observatory.
LINC-NIRVANA is the IR Fizeau interferometric imager of the Large Binocular Telescope (LBT) in Arizona.
Here we describe in particular the design, realization and preliminary tests of the so-called Patrol Camera. It
can image (in the range 600-900 nm) the same 2 arcmin FoV seen by the Medium- High-Wavefront Sensor
(MHWS), adequately sampled to provide the MHWS star enlargers with the positions of the FoV stars with
an accuracy of 0.1 arcsec. To this aim a diffraction-limited performance is not required, while a distortion free
focal plane is needed to provide a suitable astrometric output. Two identical systems will be realized, one for
each single arm, which corresponds to each single telescope. We give here the details concerning the optical
and mechanical design, as well as the CCD and the control system. The interfaces with LINC-NIRVANA are
also presented both in terms of matching the carbon fiber optical bench and developing of suitable software
procedures. Since the major components have been already gathered, the laboratory tests and the integration
are currently in progress.
LINC-NIRVANA is an infrared camera that will work in Fizeau interferometric way at the Large Binocular Telescope (LBT). The two beams that will be combined in the camera are corrected by an MCAO system, aiming to cancel the turbulence in a scientific field of view of 2 arcminutes. The MCAO wavefront sensors will be two for each arm, with the task to sense the atmosphere at two different altitudes (the ground one and a second height variable between a few kilometers and a maximum of 15 kilometers). The first wavefront sensor, namely the Ground layer Wavefront sensor (GWS), will drive the secondary adaptive mirror of LBT, while the second wavefront sensor, namely the Mid High layer Wavefront Sensor (MHWS) will drive a commercial deformable mirror which will also have the possibility to be conjugated to the same altitude of the correspondent wavefront sensor. The entire system is of course duplicated for the two telescopes, and is based on the Multiple Field of View (MFoV) Layer Oriented (LO) technique, having thus different FoV to select the suitable references for the two wavefront sensor: the GWS will use the light of an annular field of view from 2 to 6 arcminutes, while the MHWS will use the central 2 arcminutes part of the FoV. After LINC-NIRVANA has accomplished the final design review, we describe the MFoV wavefront sensing system together with its current status.
It is shown that one can build a 100m-class wavefront sensing with today existing components and that 100m-scale wavefront sensing of layers in the atmosphere is also possible with today existing technology
LINC-NIRVANA is an imaging interferometer for the Large Binocular Telescope (LBT) and will make use of multi-conjugated adaptive optics (MCAO) with two 349 actuators deformable mirrors (DM), two 672 actuator deformable secondary mirrors and a total of 4 wavefront sensors (WFS) by using 8 or 12 natural guide stars each. The goal of the MCAO is to increase sky coverage and achieve a medium Strehl-ratio over the 2 arcmin field of view. To test the concepts and prototypes, a laboratory setup of one MCAO arm is being built. We present the layout of the MCAO prototype, planned and accomplished tests, especially for the used Xinetics DMs, and a possible setup for a test on sky with an existing 8m class telescope.
On the way to the Extremely Large Telescopes (ELT) the Large Binocular
Telescope (LBT) is an intermediate step. The two 8.4m mirrors create a masked aperture of 23m. LINC-NIRVANA is an instrument taking advantage of this opportunity. It will get, by means of Multi-Conjugated Adaptive Optics (MCAO), a moderate Strehl Ratio over a 2 arcmin field of view, which is used for Fizeau (imaging) interferometry in J,H and K. Several MCAO concepts, which are
proposed for ELTs, will be proven with this instrument. Studies of sub-systems are done in the laboratory and the option to test them on sky are kept open. We will show the implementation of the MCAO concepts and control aspects of the instrument and present the road map to the final installation at LBT. Major milestones of LINC-NIRVANA, like preliminary design review or final design review are already done or in preparation. LINC-NIRVANA is one of the
few MCAO instruments in the world which will see first light and go into operation within the next years.
Several multi-conjugate adaptive optics (MCAO) systems using the layer-oriented approach are under construction and will soon be tested at different facilities in several instruments. One of these instruments is LINC-NIRVANA, a Fizeau interferometer for the Large Binocular Telescope (LBT). This instrument uses a ground layer wavefront sensor (GWS) and a combined mid-high layer wavefront sensor (MHWS) with different fields of view (concept of multiple field of view), a 2-6 arcmin annular ring for the GWS and a 2 arcmin diameter central field of view for the MHWS. Both sensors are Pyramid wavefront sensors which optically co-add light from multiple natural guide stars.
The opto-mechanical problems concerning these sensors are related to the fast focal ratio of the beam on the pyramids coupled with the available pixelscale of detectors. This leads to very tight requirements on the moving systems (linear stages) for the star enlargers (SE) used to pick off the light of individual stars.
As there are 40 star enlargers in the overall system, additional efforts were put into the alignment system of the optics of the star enlargers and the reduction in size of the star enlargers to minimize the distance between available guide stars.
It is generally believed that very fast cameras imaging large Fields of View translate into huge optomechanics and mosaics of very large contiguous CCDs. It has already been suggested that seeing limited imaging cameras for telescopes whose diameters are larger than 20m are considered virtually impossible for a reasonable cost. We show here that, using existing technology and at a moderate price, one can build a Smart Fast Camera, a device that placed on aberrated Field of View, including those of slow focal ratios, is able to provide imaging at an equivalent focal ratio as low as F/1, with a size that is identical to the large focal ratio focal plane size.
The design allows for easy correction of aberrations over the Field of View. It has low weight and size with respect to any focal reducer or prime focus station of the same performance. It can be applied to existing 8m-class telescopes to provide a wide field fast focal plane or to achieve seeing-limited imaging on Extremely Large Telescopes. As it offers inherently fast read-out in a massive parallel mode, the SFC can be used as a pupil or focal plane camera for pupil-plane or Shack-Hartmann wavefront sensing for 30-100m class telescopes.
Wide-Field imaging at visible wavelengths with seeing of the order of 0.1-0.2arcsec is believed to be possible from the high Antarctic plateau site of Dome-C by the removal of ground layer effects only. We present a proposal for a 2m-class telescope specifically designed for the science case of short duration (~10s or greater) microlensing events in the crowded central regions of Galactic Globular Clusters and nearby galaxies where the achievement of a spatial resolution of the order of a fraction of arcsec is essential.
The philosophy behind the telescope proposal is discussed in detail. It is emphasized that this is a project with a specific unique science goal in mind and not a large scale facility instrument. A preliminary design for the optics, ground layer removal using a deformable secondary and "static" wavefront sensor and telescope structure is presented. In particular, it is shown that substantial simplification in the design can be achieved by having a specific science goal in mind, so reducing the complexity and increasing reliability. Transport and logistics for the successful deployment and operation of the telescope at the Dome-C site are discussed.
Layer Oriented represented in the last few years a new and promising aproach to solve the problems related to the limited field of view achieved by classical Adaptive Optics systems. It is basically a different approach to multi conjugate adaptive optics, in which pupil plane wavefront sensors (like the pyramid one) are conjugated to the same altitudes as the deformable mirrors. Each wavefront sensor is independently driving its conjugated deformable mirror thus simplifying strongly the complexity of the wavefront computers used to reconstruct the deformations and drive the mirror themselves, fact that can become very important in the case of extremely large telescopes where the complexity is a serious issue. The fact of using pupil plane wavefront sensors allow for optical co-addition of the light at the level of the detector thus increasing the SNR of the system and permitting the usage of faint stars, improving the efficiency of the wavefront sensor. Furthermore if coupled to the Pyramid wavefront sensor (because of its high sensitivity), this technique is actually peforming a very efficient usage of the light leading to the expectation that, even by using only natural guide stars, a good sky coverage can be achieved, above all in the case of giant telescopes. These are the main reasons for which in the last two years several projects decided to make MCAO systems based on the Layer Oriented technique. This is the case of MAD (an MCAO demonstrator that ESO is building with one wavefront sensing channel based on the Layer Oriented concept) and NIRVANA (an instrument for LBT). Few months ago we built and successfully tested a first prototype of a layer oriented wavefront sensor and experiments and demonstrations on the sky are foreseen even before the effective first light of the above mentioned instruments. The current situation of all these projects is presented, including the extensive laboratory testing and the on-going experiments on the sky.
LINC-NIRVANA is a Fizeau interferometer which will be built for the Large Binocular Telescope (LBT). The LBT exists of two 8.4m mirrors on one mounting with a distance of 22.8m between the outer edges of the two mirrors. The interferometric technique used in LINC-NIRVANA provides direct imaging with the resolution of a 23m telescope in one direction and 8.4m in the other. The instrument uses multi-conjugated adaptive optics (MCAO) to increase the sky coverage and achieve the diffraction limit in J, H, K over a moderate Field of View (2 arcmin in diameter). During the preliminary design phase the team faced several problems similar to those for an instrument at a 23m telescope. We will give an overview of the current design, explain problems related to 20m class telescopes and present solutions.
We are currently working on four projects employing Multi Conjugate Adaptive Optics in a Layer-Oriented fashion. These ranges from experimental validations, to demonstration facility or full instrument to be offered to an astronomical community and involves telescopes in the range of 4m to 24m equivalent telescope aperture. The current status of these projects along with their brief description is here given.
In order to achieve moderate Field of View (2 arcmin in diameter) and nearly diffraction limited capabilities, at the reddest portion of the visible spectrum in the interferometric mode of LBT, two sophisticated MCAO channels are required. These are being designed to perform a detailed correction of the atmospheric turbulence through three deformable mirrors per telescope arm: the secondary adaptive mirror and two commercial piezostack mirrors, leading to an overall number of degree of freedom totaling ~ 3000. A combination of numerical and optical coaddition of light collected from natural reference stars located inside the scientific Field of View and in an annular region, partially vignetted, and extending up to ≈ 6 arcmin in diameter, allows for such a performance with individual loops characterized by a much smaller number of degree of freedom, making the real-time computation, although still challenging, to more reasonable levels. We implement in the MCAO channel the dual Field of View layer-oriented approach using natural guide stars, only allowing for limited, but significant, sky coverage.