Reinhold J. Dorn

CRIRES: A High Resolution Infrared Spectrograph for ESO’s VLT

CRIRES is a cryogenic, pre-dispersed, infrared echelle spectrograph designed to provide a resolving power lambda/(Delta lambda) of 105 between 1 and 5mu m at the Nasmyth focus B of the 8m VLT unit telescope #1 (Antu). A curvature sensing adaptive optics system feed is used to minimize slit losses and to provide diffraction limited spatial resolution along the slit. A mosaic of 4 Aladdin~III InSb-arrays packaged on custom-fabricated ceramics boards has been developed. This provides for an effective 4096x512 pixel focal plane array, to maximize the free spectral range covered in each exposure. Insertion of gas cells to measure high precision radial velocities is foreseen. For measurement of circular polarization a Fresnel rhomb in combination with a Wollaston prism for magnetic Doppler imaging is foreseen. The implementation of full spectropolarimetry is under study. This is one result of a scientific workshop held at ESO in late 2003 to refine the science-case of CRIRES. Installation at the VLT is scheduled during the first half of 2005. Here we briefly recall the major design features of CRIRES and describe its current development status including a report of laboratory testing.

HAWK-I: the high-acuity wide-field K-band imager for the ESO Very Large Telescope

We describe the design, development, and performance of HAWK-I, the new High-Acuity Wide-field K-band Imager for ESO’s Very Large Telescope, which is equipped with a mosaic of four 2 k × 2 k arrays and operates from 0.9−2.4 μm over 7.5 � × 7.5 � with 0.1 �� pixels. A novel feature is the use of all reflective optics that, together with filters of excellent throughput and detectors of high quantum efficiency, has yielded an extremely high throughput. Commissioning and science verification observations have already delivered a variety of excellent and deep images that demonstrate its high scientific potential for addressing important astrophysical questions of current interest.

HAWK-I: A new wide-field 1- to 2.5-μm imager for the VLT

HAWK-I (High Acuity, Wide field K-band Imaging) is a 0.9 μm - 2.5 μm wide field near infrared imager designed to sample the best images delivered over a large field of 7.5 arcmin x 7.5 arcmin. HAWK-I is a cryogenic instrument to be installed on one of the Very Large Telescope Nasmyth foci. It employs a catadioptric design and the focal plane is equipped with a mosaic of four HAWAII 2 RG arrays. Two filter wheels allow to insert broad band and narrow band filters. The instrument is designed to remain compatible with an adaptive secondary system under study for the VLT.

HAWK-I: the new wide-field IR imager for the VLT

HAWK-I is a new wide-field infrared camera under development at ESO. With four Hawaii-2RG detectors, a 7.5 arcminute square field of view and 0.1 arcsecond pixels, it will be an optimum imager for the VLT, and a major enhancement to existing and future infrared capabilities at ESO. HAWK-I will eventually make use of ground-layer AO achieved through a deformable secondary mirror/laser guide star facility planned for the VLT.

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.

A novel CCD design for curvature wavefront sensing

At the European Southern Observatory (ESO) in Garching, Germany, several adaptive optics systems using curvature wavefront sensors are being developed for the Very Large Telescope (VLT) and the VLT interferometer (VLTI). Curvature AO-systems have traditionally used avalanche photodiodes (APDs) as detectors due to strict requirements of very short integration times (200 microsec) and very low readout noise. Advances in CCD technology motivated an investigation of the use of a specially designed CCD as the wavefront sensor detector in a 60-element curvature AO system. A CCD has never been used before as the wavefront sensor in a low light level curvature adaptive optics system. This CCD can achieve nearly the same performance as APDs at a fraction of the cost and with reduced complexity for high order wavefront correction. Moreover the CCD has higher quantum efficiency and a greater dynamic range than APDs. A readout noise of less than 1.5 electrons at 4000 frames per second was achieved. Back-illuminated thinned versions of this CCD can replace APDs as a new detector for high order curvature wavefront sensing.

The Differential Tip-Tilt Sensor of SPHERE

The SPHERE instrument aims at detecting giant extrasolar planets in the vicinity of bright stars. Such a challenging goal requires the use of a high performance Adaptive Optics (AO) system, a coronagraphic device to cancel out the flux coming from the star itself, and smart focal plane techniques to calibrate residual uncorrected turbulent and/or static wavefronts. Inside the adaptive optic system, a specific tool is developed in SPHERE to ensure that the star is always well centered on the coronagraph. This tool called Differential Tip-Tilt Sensor (DTTS) measures the position of the star at the same wavelength than the science instruments. It is located very close to the focal plane to minimize drifts between DTTS and the coronagraph. After describing the DTTS, we will describe the tests and laboratory results on stability measurement of the DTTS; stability which is crucial for SPHERE performance.

A CCD-based Curvature Wavefront Sensor for Adaptive Optics in Astronomy

Advances in Charge-Coupled Device (CCD) technology motivated an investigation of the use of a specially designed CCD as the wavefront sensor detector in a 60 element curvature AO system. A CCD has never been used before as the wavefront sensor in a low light level curvature adaptive optics system. A CCD can achieve nearly the same performance as APDs at a fraction of the cost and with reduced complexity for high order wavefront correction. Moreover the CCD has higher quantum efficiency and a greater dynamic range an APD. A readout noise of less than 1.5 ē at 4000 frames/sec was achieved. A back-illuminated thinned version of this CCD can replace APDs as the best detector for high order curvature wavefront sensing.

High contrast polarimetry in the infrared with SPHERE on the VLT

The instrument SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch), recently installed on the VLT-UT3, aims to detected and characterize giant extra-solar planets and the circumstellar environments in the very close vicinity of bright stars. The extreme brightness contrast and small angular separation between the planets or disks and their parent stars have so far proven very challenging. SPHERE will meet this challenge by using an extreme AO, stellar coronagraphs, an infrared dual band and polarimetric imager called IRDIS, an integral field spectrograph, and a visible polarimetric differential imager called ZIMPOL. Polarimetry allows a separation of the light coming from an unpolarized source such as a star and the polarized source such as a planet or protoplanetary disks. In this paper we present the performance of the infrared polarimetric imager based on experimental validations performed within SPHERE before the preliminary acceptance in Europe. We report on the level of instrumental polarization in the infrared and its calibration limit. Using differential polarimetry technique, we quantify the level of speckle suppression, and hence improved sensitivity in the context of imaging extended stellar environments.

A CMOS Visible Silicon Imager hybridized to a Rockwell 2RG multiplexer as a new detector for ground based astronomy

For the past 25 years Charge Coupled Devices (CCDs) have been used as the preferred detector for ground based astronomy to detect visible photons. As an alternative to CCDs, silicon-based hybrid CMOS focal plane array technology is evolving rapidly. Visible hybrid detectors have a close synergy with IR detectors and are operated in a similar way. This paper presents recent test results for a Rockwell 2K x 2K silicon PIN diode array hybridized to a Hawaii-2RG multiplexer, the Hybrid Visible Silicon Imager (HyViSI). Since the capacitance of the integrating node of Si-PIN diodes is at least a factor of two smaller than the capacitance of the Hawaii-2RG IR detector pixel, lower noise was expected. However, those detectors suffer from interpixel capacitance which introduces an error to the value of the conversion factor measured with the photon transfer method. Therefore QE values have been overestimated by almost a factor of two in the past. Detailed test results on QE, noise, dark current, and other basic performance values as well as a discussion how to interpret the measured values will be presented. Two alternative methods, direct measurement of the nodal capacity and the use of Iron-55 X-rays to determine the actual nodal capacitance and hence the conversion factor will be briefly presented. PSF performance of this detector was analyzed in detail with an optical spot and single pixel reset measurement.