Derek Ives

SAPHIRA detector for infrared wavefront sensing

The only way to overcome the CMOS noise barrier of near infrared sensors used for wavefront sensing and fringe tracking is the amplification of the photoelectron signal inside the infrared pixel by means of the avalanche gain. In 2007 ESO started a program at Selex to develop near infrared electron avalanche photodiode arrays (eAPD) for wavefront sensing and fringe tracking. In a first step the cutoff wavelength was reduced from 4.5 micron to 2.5 micron in order to verify that the dark current scales with the bandgap and can be reduced to less than one electron/ms, the value required for wavefront sensing. The growth technology was liquid phase epitaxy (LPE) with annular diodes based on the loophole interconnect technology. The arrays required deep cooling to 40K to achieve acceptable cosmetic performance at high APD gain. The second step was to develop a multiplexer tailored to the specific application of the GRAVITY instrument wavefront sensors and the fringe tracker. The pixel format is 320x256 pixels. The array has 32 parallel video outputs which are arranged in such a way that the full multiplex advantage is available also for small subwindows. Nondestructive readout schemes with subpixel sampling are possible. This reduces the readout noise at high APD gain well below the subelectron level at frame rates of 1 KHz. The third step was the change of the growth technology from liquid phase epitaxy to metal organic vapour phase epitaxy (MOVPE). This growth technology allows the band structure and doping to be controlled on a 0.1μm scale and provides more flexibility for the design of diode structures. The bandgap can be varied for different layers of Hg(1-x)CdxTe. It is possible to make heterojunctions and apply solid state engineering techniques. The change to MOVPE resulted in a dramatic improvement in the cosmetic quality with 99.97 % operable pixels at an operating temperature of 85K. Currently this sensor is deployed in the 4 wavefront sensors and in the fringe tracker of the VLT instrument GRAVITY. Initial results will be presented. An outlook will be given on the potential of APD technology to be employed in large format near infrared science detectors. Several of the results presented here have also been shown to a different audience at the Scientific Detector Workshop in October 2013 in Florence but this paper has been updated with new results [1].

Evaluation and optimization of NIR HgCdTe avalanche photodiode arrays for adaptive optics and interferometry

The performance of the current high speed near infrared HgCdTe sensors operating in fringe trackers, wavefront sensors and tip-tilt sensors is severely limited by the noise of the silicon readout interface circuit (ROIC), even if state-of-the- art CMOS designs are used. A major improvement can only be achieved by the amplification of the photoelectron signal directly at the point of absorption by means of avalanche gain inside the infrared pixel. Unlike silicon, HgCdTe offers noiseless avalanche gain. This has been verified with the LPE grown 320x256 pixel λc=2.5 μm HgCdTe eAPD arrays from SELEX both on a prototype ROIC called SWALLOW and on a newly developed ROIC, specifically designed for AO applications, called SAPHIRA. The novel features of the new SAPHIRA ROIC, which has 32 parallel video channels operating at 5 MHz, will be described, together with the new high speed NGC data acquisition system. Performance results will be discussed for both ROICs. The LPE material on the SWALLOW prototype was excellent and allowed operation at an APD gain as high as 33. Unfortunately, the LPE material of the first devices on the SAPHIRA ROIC suffers from problems which are now understood. However, due to the excellent performance of the SAPHIRA ROIC even with the limitations of present HgCdTe material, it is possible with simple double correlated sampling to detect test patterns with signal levels of 1 electron. An outlook will be given on further developments of heterojunctions grown by MOVPE, which eventually may replace eAPD arrays grown by LPE.

MICADO: first light imager for the E-ELT

MICADO will equip the E-ELT with a first light capability for diffraction limited imaging at near-infrared wavelengths. The instrument’s observing modes focus on various flavours of imaging, including astrometric, high contrast, and time resolved. There is also a single object spectroscopic mode optimised for wavelength coverage at moderately high resolution. This contribution provides an overview of the key functionality of the instrument, outlining the scientific rationale for its observing modes. The interface between MICADO and the adaptive optics system MAORY that feeds it is summarised. The design of the instrument is discussed, focusing on the optics and mechanisms inside the cryostat, together with a brief overview of the other key sub-systems.MICADO will be the first-light wide-field imager for the European Extremely Large Telescope (E-ELT) and will provide difiraction limited imaging (7mas at 1.2mm) over a ~53 arcsecond field of view. In order to support various consortium activities we have developed a first version of SimCADO: an instrument simulator for MICADO. SimCADO uses the results of the detailed simulation efforts conducted for each of the separate consortium-internal work packages in order to generate a model of the optical path from source to detector readout. SimCADO is thus a tool to provide scientific context to both the science and instrument development teams who are ultimately responsible for the final design and future capabilities of the MICADO instrument. Here we present an overview of the inner workings of SimCADO and outline our plan for its further development.

NAOMI adaptive optics system for the 4.2-m William Herschel telescope

NAOMI (Nasmyth Adaptive Optics for Multi-purpose Instrumentation) is a recently completed and commissioned astronomical facility on the 4.2m William Herschel Telescope. The system is designed to work initially with Natural Guide Stars and also to be upgradeable for use with a single laser guide star. It has been designed to work with both near infrared and optical instrumentation (both imagers and spectrographs). The system uses a linearised segmented adaptive mirror and dual-CCD Shack-Hartmann wavefront sensor together with a multiple-DSP real-time processing system. Control system parameters can be updated on-the-fly by monitoring processes and the system can self-optimize its base optical figure to compensate for the optical characteristics of attached scientific instrumentation. The scientific motivation, consequent specification and implementation of NAOMI are described, together with example performance data and information on future upgrades and instrumentation.

Development of high-speed, low-noise NIR HgCdTe avalanche photodiode arrays for adaptive optics and interferometry

The most promising way to overcome the CMOS noise barrier of infrared AO sensors is the amplification of the photoelectron signal directly at the point of absorption inside the infrared pixel by means of the avalanche gain. HgCdTe eAPD arrays with cut off wavelengths of λc ~2.64 μm produced by SELEX-Galileo have been evaluated at ESO. The arrays were hybridized to an existing non-optimized ROIC developed for laser gated imaging which has a format of 320×256 pixels and four parallel video outputs. The avalanche gain makes it possible to reduce the read noise to < 7 e rms. The dark current requirements of IR wavefront sensing are also met.

The E-ELT first light spectrograph HARMONI: capabilities and modes

HARMONI is the E-ELT’s first light visible and near-infrared integral field spectrograph. It will provide four different spatial scales, ranging from coarse spaxels of 60 × 30 mas best suited for seeing limited observations, to 4 mas spaxels that Nyquist sample the diffraction limited point spread function of the E-ELT at near-infrared wavelengths. Each spaxel scale may be combined with eleven spectral settings, that provide a range of spectral resolving powers (R ~3500, 7500 and 20000) and instantaneous wavelength coverage spanning the 0.5 – 2.4 μm wavelength range of the instrument. In autumn 2015, the HARMONI project started the Preliminary Design Phase, following signature of the contract to design, build, test and commission the instrument, signed between the European Southern Observatory and the UK Science and Technology Facilities Council. Crucially, the contract also includes the preliminary design of the HARMONI Laser Tomographic Adaptive Optics system. The instrument’s technical specifications were finalized in the period leading up to contract signature. In this paper, we report on the first activity carried out during preliminary design, defining the baseline architecture for the system, and the trade-off studies leading up to the choice of baseline.

VISIR upgrade overview and status

We present an overview of the VISIR upgrade project. VISIR is the mid-infrared imager and spectrograph at ESO’s VLT. The project team is comprised of ESO staff and members of the original VISIR consortium: CEA Saclay and ASTRON. The project plan is based on input from the ESO user community with the goal of enhancing the scientific performance and efficiency of VISIR by a combination of measures: installation of improved hardware, optimization of instrument operations and software support. The cornerstone of the upgrade is the 1k by 1k Si:As AQUARIUS detector array (Raytheon) which has been carefully characterized in ESO’s IR detector test facility (modified TIMMI 2 instrument). A prism spectroscopic mode will cover the N-band in a single observation. New scientific capabilities for high resolution and high-contrast imaging will be offered by sub-aperture mask (SAM) and phase-mask coronagraphic (4QPM/AGPM) modes. In order to make optimal use of favourable atmospheric conditions a water vapour monitor has been deployed on Paranal, allowing for real-time decisions and the introduction of a user-defined constraint on water vapour. During the commissioning in 2012 it was found that the on-sky sensitivity of the AQUARIUS detector was significantly below expectations and that VISIR was not ready to go back to science operations. Extensive testing of the detector arrays in the laboratory and on-sky enabled us to diagnose the cause for the shortcoming of the detector as excess low frequency noise (ELFN). It is inherent to the design chosen for this detector and can’t be remedied by changing the detector set-up. Since this is a form of correlated noise its impact can be limited by modulating the scene recorded by the detector. We have studied several mitigation options and found that faster chopping using the secondary mirror (M2) of the VLT offers the most promising way forward. Faster M2 chopping has been tested and is scheduled for implementation before the end of 2014 after which we plan to re-commission VISIR. In addition an upgrade of the IT infrastructure related to VISIR is planned in order to support burst-mode operations. The upgraded VISIR will be a powerful instrument providing close to background limited performance for diffraction-limited observations at an 8-m telescope. It will offer synergy with facilities such as ALMA, JWST, VLTI and SOFIA, while a wealth of targets is available from survey work (e.g. VISTA, WISE). In addition it will bring confirmation of the technical readiness and scientific value of several aspects of potential mid-IR instrumentation at Extremely Large Telescopes.

Design status of WFCAM: a wide-field camera for the UK Infrared Telescope

An update on the design status of the UKIRT Wide Field Camera (WFCAM) is presented. WFCAM is a wide field infrared camera for the UK Infrared Telescope, designed to produce large scale infrared surveys. The complete system consists of a new IR camera with integral autoguider and a new tip/tilt secondary mirror unit. WFCAM is being designed and built by a team at the UK Astronomy Technology Centre in Edinburgh, supported by the Joint Astronomy Centre in Hawaii. The camera uses a novel quasi-Schmidt camera type design, with the camera mounted above the UKIRT primary mirror. The optical system operates over 0.7 - 2.4 μm and has a large corrected field of view of 0.9° diameter. The focal plane is sparsely populated with 4 2K x 2K Rockwell HAWAII-2 MCT array detectors, giving a pixel scale of 0.4 arcsec/pixel. A separate autoguider CCD is integrated into the focal plane unit. Parallel detector controllers are used, one for each of the four IR arrays and a fifth for the autoguider CCD.

HARMONI: the first light integral field spectrograph for the E-ELT

HARMONI is a visible and near-infrared (0.47 to 2.45 μm) integral field spectrometer, providing the E-ELTs core spectroscopic capability, over a range of resolving powers from R (≡λ/Δλ)~500 to R~20000. The instrument provides simultaneous spectra of ~32000 spaxels at visible and near-IR wavelengths, arranged in a √2:1 aspect ratio contiguous field. HARMONI is conceived as a workhorse instrument, addressing many of the E-ELT’s key science cases, and will exploit the E-ELTs scientific potential in its early years, starting at first light. HARMONI provides a range of spatial pixel (spaxel) scales and spectral resolving powers, which permit the user to optimally configure the instrument for a wide range of science programs; from ultra-sensitive to diffraction limited, spatially resolved, physical (via morphology), chemical (via abundances and line ratios) and kinematic (via line-of-sight velocities) studies of astrophysical sources. Recently, the HARMONI design has undergone substantial changes due to significant modifications to the interface with the telescope and the architecture of the E-ELT Nasmyth platform. We present an overview of the capabilities of HARMONI, and of its design from a functional and performance viewpoint.

Characterization of VISTA IR detectors

VISTA awarded a contract to Raytheon Vision Systems in June 2002 to supply 16 SWIR HgCdTe 2Kx2K detectors. Raytheon has delivered VIRGO 2K x 2K readout multiplexers, engineering detectors and the first two science grade detectors. The UKATC has set up a low background test facility to test and characterize the VIRGO detectors and to confirm that the detector performance meets our specifications. The VIRGO 2Kx2K is a new detector being produced for VISTA by Raytheon. In this paper we present the first results and performance of the multiplexer and the science detector. The test facility includes a custom built low background close-cycle cooled cryostat, cryogenic pre-amplifier electronics and uses ESO’s Infrared array controller electronics and detector control and data acquisition software. The detector parameters being measured include trans-impedance conversion gain, quantum efficiency in J, H and K wave bands, read noise, dark generation rate, linearity, well capacity, pixel operability, drift with temperature, persistence and electrical cross-talk.