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What is SHARK-NIR?
What is SHARK-NIR?
SHARK-NIR is a near-infrared (0.96 μm to 1.7 μm) instrument proposed for the Large Binocular Telescope (LBT) in the framework of the “2014 Call for Proposals for Instrument Upgrades and New Instrument" (Farinato et al. 2014).
Once installed on the left side of the central Gregorian focal station of the LBT, coupled with its visible counterpart SHARK-VIS (on the right side), it will enable to exploit unique challenging science, including exoplanet and extragalactic science cases. SHARK will provide simultaneous spectral coverage from B to H band, taking advantage of the outstanding performance of LBT binocular Extreme Adaptive Optics (XAO) capability. The XAO features two Adaptive Secondary Mirrors (ASM) and pyramid wavefront sensors. Moreover, the SOUL upgrade will allow to reach unprecedent sensitivity in the faint-end regime.
INAF-Padua is the lead institution of a consortium formed by other INAF Observatories, such as Rome, Arcetri, Milan, Trieste, Brera, Catania, Palermo and Turin. The collaboration also includes the Steward Observatory (SO) of Tucson, University of Arizona (USA), The Max Planck Institut fur Astronomie (MPIA) of Heidelberg (Germany) and the Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), in France.
The project is now in the AIV phase at the clean room of the Padova Observatory, where the optical and mechanical components are being integrated on the SHARK-NIR optical bench and the final performance of the instrument's coronagraphy subsystem is being tested. Its first light is foreseen in early 2023.
SHARK-NIR is an extreme-AO equipped system (nominal Strehl SR > 98% in all bands) with imaging and spectroscopic capabilities that will be observing the NIR regime, covering the wavelength range from 0.96 to 1.7 microns.
The field-of-view is 18” x 18” with a pixel scale of 14.5 mas/pixel. Observing modes include classical imaging w/o coronagraphic mode (with Gaussian, Shaped pupil, and FQPM coronagraphic masks), dual-band imaging (thanks to a Wollaston prism), and long slit coronagraphic spectroscopy (with resolutions of R=100 and R=700 over the whole wavelength range). The performances are expected to reach contrasts down to 10^-5/^-6 at 300-500 mas, in order to accomplish the main scientific requirements.
The main science cases will cover exoplanet detection and characterisation (extrasolar giant planets on wide orbits), circumstellar discs and jets around young stars, solar system objects, evolved stars, AGN and QSOs.
The main SHARK-NIR Specification:
Recap of main SHARK-NIR Specifications
Coverage: 960-1700 nm
FoV: 18"x18"
Pixel Scale: 14.5 mas/pixel
Spectral Resolution: R=100/700
Filters: Y,J,H bands + narrow band e.g., FeII, HeI, PaB, and others
Coronagraphs: Gaussian Lyot, Shaped pupil, FQPM
Optical layout of SHARK-NIR (top view of the optical bench). The light beam enters the instrument on the left side of the figure.
The opto-mechanical concept of the SHARK-NIR optical bench.
The opto-mechanical concept of the SHARK-NIR optical bench + the cryostat.
FROM CAD TO REALITY: The optical bench fully populated and aligned.
SHARK-NIR CORONAGRAPHIC COMPONENTS
SHARK-NIR CORONAGRAPHIC COMPONENTS
Coronagraphs on SHARK-NIR
Coronagraphs on SHARK-NIR
SHARK-NIR is equipped for the coronagraphic subsystem with 3 Shaped Pupil masks, a Gaussian Lyot coronagraph and a Four-Quadrant Phase Mask.
As suggested by its name, the Shaped Pupil (SP) technique consists in re-shaping the telescope pupil so to generate dark areas in the focal plane via destructive interference. Strictly speaking, “shaping'' here means constraining transmission to be either 0 or 1 across the pupil.
In principle, the only binary mask is sufficient to create high contrast. However, there are two main drawbacks: first, the shaped PSF features a bright central core, which may easily saturate the detector. The second important drawback is stray light.
In order to mitigate these effects, we can exploit SHARK-NIR intermediate focal plane to put a hard edge occulter that masks the PSF core. A Lyot stop in the second intermediate pupil plane then blocks the residual light diffracted by the occulter, as in the classical Lyot configuration. The three-plane configuration also allows to go deeper in contrast with respect to the apodizer-only configuration.
In a Gaussian Lyot coronagraph, the electric field amplitude in the focal plane is spatially modulated according to a Gaussian transmission profile. Continuous (greyscale) transmission profiles cannot be accurately reproduced with standard metal deposition methods in the NIR. For this reason, we decided to approximate the Gaussian profile using microdots, a technique named halftoning. Halftoning requires the theoretical Gaussian grayscale profile to be first converted to binary (only 0 and 1 transmission). To do so, we used a standard error-diffusion algorithm.
The Four-Quadrant Phase-Mask (FQPM) coronagraph suppresses on-axis starlight by means of a phase mask in the focal plane. The mask divides the FP into four quadrants and induces a π phase shift on two of them on one diagonal. Provided that the image of the star is perfectly centered on the common vertex of the quadrants, then the four outcoming beams combine destructively at infinity and the stellar light in the downstream pupil plane is totally rejected outside of the pupil area. This light is then easily blocked by means of a Lyot stop.
MIRRORS GLUING (3 Flat Mirror and 4 Off-Axis Parabola)
MIRRORS GLUING (3 Flat Mirror and 4 Off-Axis Parabola)
It's a dirty job, but someone's gotta do it!
It's a dirty job, but someone's gotta do it!
The gluing procedure is in 5 steps:
Prepare the parts to be glued. Clean the back surface of the mirror with acetone. Slightly abrade the plateau of the mount with ScotchBrite 7447 pads. Lightly abraded surfaces give a better profiled surface for adhesive bonding than do highly polished surfaces. Clean the pads + plateau with acetone. The mount has also been cleaned with UltraSound cleaner before applying acetone.
Mix the two components of the 2216 B/A glue accordingly to the ratios recommended in the data sheet. Mixing by weight it is 5 parts of Base (white) and 7 parts of Accelerator (Gray). We weighed the two components using a precision balance and mixed them using two metal spatulas.
Pour all the glue into a glue dispenser syringe and then apply the glue on the 3 ring plateau of the mount.
After injection of the glue into the designed areas, the mirrors have been manually inserted into their respective seats and covered with a piece of optical paper. For each mirror, we inserted four 50 μm thick brass shims, spaced by 90°, in the circular gap between the mirror and the mount. To ensure that the mirrors were actually lying on the pads and not floating on the glue, we applied a small pressure on the mirror with the fingers. Of course gloves + optical paper has been used to prevent mirror surface from contamination.
Let the glue curing for 72 hours (a full cure at 24 °C would require 7 days, but after 12 hours the glue reaches its handling strength).
SHARK-NIR BENCH CHARACTERIZATION AND OFF-AXIS PARABOLA ALIGNMENT
SHARK-NIR BENCH CHARACTERIZATION AND OFF-AXIS PARABOLA ALIGNMENT
Inside the Clean Room
Inside the Clean Room
The whole AIV phase of SHARK-NIR will take place in a controlled environment, inside a clean room class ISO5 to prevent the contamination of the optics from dust and particles.
This because being SHARK-NIR fundamentally a coronagraphic camera, it is severely impacted by the dust-scattered light.
The SHARK-NIR optical bench, initially empty, is hold by a dedicated handling in order to have the surface of the bench positioned perpendicular to the gravity vector and with the rotation axis at ~ 1 m above the ground.
Two alignment beams are foreseen. These are:
An on-axis, narrow and collimated laser beam, used for the materialization of the bearing rotation axis, the alignment of the refractive opitics and for visual aid in the alignment of all the reflective optics.
A F/15 beam produced by a Zygo interferometer, simulating an on axis beam coming from LBT. Used for the fine alignment of the OAPs.
These two beams are of course co-aligned, following the procedure described in RD2, and it is possible to switch from one beam to the other by means of a motorized linear stage, which inserts a flat mirror in the beam. The insertion of the mirror in the optical path inject the light from the laser into SHARK-NIR, while removing the mirror we have the interferometer light entering into SHARK-NIR.
The whole alignment procedure is based on a mechanical/optical approach, meaning that as a first step we will mechanically pre-align all the optics to a mechanical reference representing the on-axis entrance focal plane of SHARK-NIR. This mechanical reference is a sphere, positioned and aligned by the SHARK-NIR mechanics manufacturer, whose center lies on the bearing rotation axis.
The mechanical alignment is based on the use of a Coordinate Measuring Machine (CMM) Space Plus 1800 from Tomelleri Engineering and rely on adjusting the position of the optics with respect to the sphere till reaching the nominal configuration.
Other optical and mechanical components
Other optical and mechanical components
SHARK-NIR .. travel preparations!
SHARK-NIR .. travel preparations!
Box #1: SHARK-NIR family portrait
Box #1: SHARK-NIR family portrait
From bottom left: Jacopo Farinato, Gabriele Umbriaco, Daniele Vassallo, Davide Ricci, Elena Carolo, Fulvio Laudisio, Valentina Viotto, Maria Bergomi, Luca Marafatto, Simone Di Filippo, Davide Greggio and Marco Dima.
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