HOT

Full testbed name

High Order Testbench (HOT)

Managing institution

European Southern Observatory (ESO)

Main scientific focus

The high order testbench (HOT) was designed to implements an extreme adaptive optics on a test bench with realistic telescope conditions reproduced by star and turbulence generators. A 32×32 actuator micro deformable mirror (DM), one pyramid wave front sensor, one Shack-Hartmann wave front sensor (SHS), the ESO SPARTA real time computer and an essentially read-noise free L3 CCD60 provide an ideal cocoon to study the different behaviour of the two types of wave front sensors in terms of linearity, sensitivity to calibration errors, noise propagation, specific issues to pyramid or Shack-Hartmann wave front sensors, etc.

The main objectives can be summarized as:

• Study and characterize all the possible limitations which could reduce XAO correction.
• Characterize two kinds of WFS, Shack-Hartmann and Pyramid, for XAO. Study the different behaviour in terms of linearity, sensitivity to calibration errors and noise propagation. Spatial filtering on the SHS case or modulation for the pyramid, are taken into account.
• Test bench for new components. New deformable mirrors (MEMS) or EMCCDs are foreseen on next generation XAO systems, as well as new control techniques. It is necessary to check in laboratory their performance and limitations.
• XAO bench for High contrast techniques. It allows to test and compare the performance of several different types of coronagraphs and new post focal sensing techniques in order to achieve high contrast levels under realistic AO conditions.

Environment of the testbed

The optical platform is the central structural hub of HOT. The stainless steel table measures 1.5 x 3.5 m and is tapped with the standard M6 screw size on a 25 mm grid. All four "floating" legs of the optical platform have vibration damping servomechanisms that eliminate surrounding floor vibrations. the AO laboratory is also outfitted with two oxygen level sensors that will trigger a local alarm if the oxygen levels in the room drop below 19.0%.

Key hardware items

• Light Source: HOT possesses two independent light generating sources: a helium neon laser for alignment purposes and a halogen lamp for experiments. The light sources feed into the turbulence generator via optical fibers using either SMA or FC connectors.
• Turbulence Generator: An integral part of HOT is the Turbulence Generator. Turbulence is generated using two reflective phase screens with a diameter of 5.0 cm each. They are installed on rotating mounts, which turn in different directions and at adjustable speeds.
• Alpao DM: The ALPAO 52-actuator deformable mirror (DM52) is a discrete actuator deformable mirror, composed of a set of magnets attached to a thin mirror membrane used primarily for low -order wavefront aberrations.
• Boston Micromachines DM: The BMM has 1024 actuators in a 32 x 32 grid and is used for correcting high-order wavefront aberrations. It is installed on a customized board and connected to a electronics high voltage amplifier using 16 cables, with two cables per high voltage board. The high voltage electronics are driven through the real time computing SPARTA platform, developed to control the next generation of ESO adaptive optics systems.
• Shack-Hartmann WFS: The Shack-Hartmann wavefront sensor (SHS), developed by the university of Durham, is based on a 31x31 subaperture lenslet array and an ANDOR iXon L3CCD60 with 128x128 pixels. A anti-aliasing filter can be inserted in the front focal plane. The SHS can be equipped with two different lenslet arrays providing pixel scales of 0.25" or 0.5" for the 4x4 pixel subapertures. The SHS real- time computer (RTC) is an all-CPU architecture.
• Pyramid WFS: The Pyramid wavefront sensor (PWS), build by Arcetri, have two possible pupil sampling obtained changing the final camera lens namely a low sampling mode with 31x31 subapertures and a high sampling mode with 48x48 subapertures. Any other intermediate sampling between these two could be achieved using a proper camera lens. As the SHS, the PWS is based on the ANDOR iXon L3CCD60.
• Infrared Test Camera: The ITC is a Hawaii array (1K x 1K) with four contiguous quadrants (512 x 512). The borders of the quadrants are visible in camera images, so the centre of the Point Spread Function (PSF) is usually placed inside one of the quadrants. The camera’s internal optics are designed to enable a pixel-scale of 5.3 mas/pix. Pixel counts higher than 9000 ADU are considered over-exposed. The read- out noise is below 30 electrons. The ITC has a filter wheel with five different settings: J-band, H-band, K-band, blind, and no filter. The camera is cooled to 103 - 107 K with liquid nitrogen under vacuum conditions of 10-5 to 10-6 mbar for optimal performance. Liquid Nitrogen (LN2) is used as a coolant, and is provided by a standard nitrogen tank which maintains an internal pressure of about 0.6 bar.

Current status

Previously, HOT was equipped with both a SHS and a PWS. While SHSs and PWSs perform equally in regard to aberrations of spatial frequencies near to fc, the PWS performs much better in regard to low-order aberrations. Therefore, we have decided to focus our research only on PWS technologies, for their resolution is limited by the diameter of the telescope aperture and not by the diameter of the lenslets.

Also, for future demonstrations regarding the XAO capabilities of HOT, it is important that a faint secondary light source can be provided to simulate an faint stellar companion. Therefore, an adjustable stage to input a secondary light source was designed, manufactured in-house, and installed on the TG. Finally, it was decided to remove optical elements in the coronagraph path that were not being used. One focal point was left in the science path for introduction of a simple coronagraph. A vortex coronagraph from Thorlabs was acquired and it is planned to be integrated.