MATISSE is a mid-infrared spectro-interferometer combining the beams of up to four Unit Telescopes or Auxiliary
Telescopes of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory.
MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. New characteristics present in
MATISSE will give access to the mapping and the distribution of the material, the gas and essentially the dust, in the
circumstellar environments by using the mid-infrared band coverage extended to L, M and N spectral bands. The four
beam combination of MATISSE provides an efficient uv-coverage: 6 visibility points are measured in one set and 4
closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime.
We give an overview of the instrument including the expected performances and a view of the Science Case. We present
how the instrument would be operated. The project involves the collaborations of several agencies and institutes: the
Observatoire de la Côte d’Azur of Nice and the INSU-CNRS in Paris, the Max Planck Institut für Astronomie of
Heidelberg; the University of Leiden and the NOVA-ASTRON Institute of Dwingeloo, the Max Planck Institut für
Radioastronomie of Bonn, the Institut für Theoretische Physik und Astrophysik of Kiel, the Vienna University and the
ESO's Very Large Telescope (VLT) on Paranal mountain in northern Chile comprises four 8.2m diameter 'Unit
Telescopes' (also used for interferometry); four 1.8m movable outrigger telescopes dedicated to interferometry and two
survey telescopes - the 2.6m VST to be used in the visible and the 4m class, infrared VISTA telescope. Here I will give
an overview of the accompanying large instrument development programme which has so far delivered 11 operational
facility instruments for the UT's (leaving one visitor Nasmyth focus) and 2 major instruments for the interferometric
focus. In addition, a laser guide star facility has been added on UT4 to generate artificial (sodium) stars for the adaptive
optics assisted instruments NACO and SINFONI; the optical and infrared cameras for the survey telescopes are almost
ready; four major second generation instruments for the UTs (X-Shooter, KMOS, MUSE and SPHERE) are at various
stages of development throughout Europe; a fifth (high resolution spectrograph capable of 10cm/s radial velocity
stability at the incoherent combined focus of the four 8m telescopes) is the subject of a Call for Proposals; UT4 is being
converted to a fully adaptive telescope and three second generation interferometric instruments (MATISSE, GRAVITY
and VSI) have been approved following successful completion of their Phase A studies.
HAWK-I is the newly commissioned High Acuity Wide-field K-band Imager at the ESO Very Large Telescope. It is a
0.9-2.5 micron imager with a field of view of 7.5×7.5 arcmin sampled at 106 mas with four Hawaii2RG detectors. It has
a full reflective design that was optimised for image quality and throughput.We present an overview of its performance as
measured during the commissioning and first science runs. In particular, we describe a detector read-out mode that allows
us to increase the useful dynamic range of the detector, and a distortion calibration resulting in <5mas relative astrometry
across the field.
KMOS is a near-infrared multi-object integral field spectrometer which has been selected as one of a suite of second-generation instruments to be constructed for the ESO VLT in Chile. The instrument will be built by a consortium of UK and German institutes working in partnership with ESO and is currently at the end of its preliminary design phase. We present the design status of KMOS and discuss the most novel technical aspects and the compliance with the technical specification.
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.
The ESO Very Large Telescope (VLT) on Paranal mountain in northern Chile comprises four 8.2m diameter "Unit Telescopes"; three 1.8m movable outrigger telescopes (shortly to be 4) for interferometry and the enclosures for two survey telescopes - the 2.6m VST to be used in the visible and the 4m class, infrared VISTA telescope which will both start commissioning in 2006. Here I will give an overview of the accompanying large instrument development programme which has so far delivered 9 operational facility instruments for the UT's (plus a visitor focus) and 2 major instruments for the interferometric focus. In addition, the laser guide star facility to be used to generate artificial (sodium) stars for the adaptive optics assisted instruments NACO and SINFONI is being commissioned; the optical and infrared cameras for the survey telescopes are almost ready; the CRIRES high resolution infrared spectrograph is being commissioned on UT1; the infrared imager HAWK-I is being assembled at ESO Garching; four major second generation instruments for the UTs (X-Shooter, KMOS, MUSE and SPHERE) are at various stages of development throughout Europe and Phase A studies of three second generation interferometric instruments (MATISSE, VSI and GRAVITY) have just been launched.
Since adaptive optics on large telescopes provides almost diffraction limited resolution, Nyquist sampling of moderate
fields requires large format arrays. Because of limited substrate sizes there is a tendency to shrink the pixel size to extend
the array format beyond 2Kx2K. However, with smaller pixel sizes the coupling capacitance between neighboring pixels
becomes more important and its effect on performance and basic parameters of large format arrays has to be analyzed.
Therefore, techniques to measure the effect of the coupling capacitance on the conversion gain will be presented. The
capacitance comparison method and the autocorrelation technique will be discussed and compared quantitatively. It will
be shown that the "noise squared versus signal" method which is in common use to obtain the conversion gain, can only
be applied for negligible interpixel capacitance. The X-ray decay of Fe55 is a well established calibrator for silicon and
can be applied to Si-PIN diode arrays in order to verify the different methods. Finally, a new technique called single
pixel reset will be presented, which directly measures the impulse response or point spread function generated by the
inter-pixel capacitance. The measured PSF impulse response can be used for the deconvolution of images to compensate
the degradation of spatial resolution induced by the interpixel capacitance. The difference of interpixel capacitance
measured in infrared hybrid arrays and Si-PIN diode arrays hybridized to the same multiplexer will be discussed.
Keywords:, interpixel capacitance, conversion gain, point spread function, single pixel reset, CMOS hybrid, Hawaii-
2RG, HgCdTe, Si-PIN, HyViSI.
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.
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.
ESO celebrated the end of the first 5 years of very successful operation of its VLT in Chile on 1st April 2004 and is now set to expand this facility further with the addition of several 1.8 m, movable outrigger telescopes for interferometry, the VST (VLT survey telescope) and the VISTA (infrared) survey telescope. Here I will review briefly the status of the 8 operational 1st generation VLT/VLTI instruments plus the 3 in commissioning and the 3 still in development and will introduce the first 3 of the 2nd generation instruments which have recently been approved. I will also discuss briefly some of the 'Lessons Learned' from the development of the 1st generation instruments during an exercise conducted in 2003 with the participation of the external consortia of institutes to which ESO contracted out much of the instrument development.
The detector mounted in the VLTI fringe sensor FINITO is a 256x256 HgCdTe array with a cut-off wavelength of 1.9 micron. The same arrays having cut-off wavelengths of 2.5 micron will be used in the tip tilt sensor IRIS and the PRIMA instrument of the VLT interferometer. The arrays are part of an active control loop with integration times as short as a few hundred microseconds. The fringe tracker FINITO uses only 7 pixels of the array. To take advantage of the four parallel channels of the PICNIC multiplexer, the pixels illuminated in each quadrant are positioned at the same location within the quadrants. A noise analysis of the PICNIC array shows that the main sensitivity limitation of the array is contained in the low frequency part of the noise power spectrum. Similar behaviour has been observed with other infrared arrays. In an effort to optimize the unit cell pixel buffer to achieve high speed and low noise, a prototype multiplexer is being developed at Rockwell for adaptive optics. However, low frequency noise may still be the limiting factor dominating the noise performance of infrared arrays. To overcome this noise barrier, detector architectures have to be envisaged which should allow double correlated sampling on shorter time scales than a full exposure. This might be accomplished by some kind of gate in the IR material which allows charge to be shifted from an integrating well in the infrared pixel to a small sensing node capacitance of the multiplexer unit cell buffer.
VLT instruments increasingly require high sensitivity large format focal planes. Adaptive optics combined with multiple integral field units feeding high resolution spectrographs drive the pixel performance as well as the array format. Three VLT instruments, the wide field imager Hawk-I and the integral field spectrographs SINFONI and KMOS will be equipped with MBE-grown HgCdTe Hawaii-2RG arrays, which have a cut-off wavelength of 2.5 micron. The Hawaii-2RG array was originally developed for the near infrared camera of JWST having a cut-off wavelength of 5 micron. The Hawaii-2RG multiplexer is one of the most advanced readout architectures offering a large variety of operating modes. A special 32 channel package has been developed which allows reading out all 32 output channels of the detector in parallel. Symmetric cryogenic CMOS operational amplifiers are placed next to the focal plane instead of using ASIC’s which are not yet available. The internal bus of the detector is accessed directly, bypassing the on-chip buffer amplifier. Noise performance employing different techniques of using reference pixels is discussed. Basic performance characteristics of the Hawaii-2RG arrays will be presented. Unlike LPE arrays, which lose quantum efficiency at lower temperatures, MBE arrays with λc = 2.5 μm do not show this effect. However, the MBE arrays under test still suffer from persistence.
For the high-resolution IR Echelle Spectrometer CRIRES (1-5 μm range), to be installed at the VLT in 2005, ESO is developing a 512 x 4096 pixels focal plane array mosaic based on Raytheon Aladdin III InSb detectors with a cutoff wavelength of 5.2 microns. To fill the useful field of 135 mm in the dispersion direction and 21 mm in the spatial direction and to maximize simultaneous spectral coverage, a mosaic solution similar to CCD mosaics has been chosen. It allows a minimum spacing between the detectors of 264 pixels. ESO developed a 3-side buttable mosaic package for both the Aladdin II and Aladdin III detectors which are mounted on multilayer co-fired AlN ceramic chip carriers. This paper presents the design of the CRIRES 512 x 4096 pixel Aladdin InSb focal plane array and a new test facility for testing mosaic focal planes under low flux conditions.
CRIRES is a cryogenic, pre-dispersed, infrared echelle spectrograph designed to provide a resolving power of 105 between 1 and 5 μm at a Nasmyth focus of one of the 8m VLT telescopes. A curvature sensing adaptive optics sytem feed is used to minimize slit losses and a 4096x512 pixel mosaic of Aladdin arrays is being developed to maximixe the free spectral range covered in each order. Insertion of gas cells to measure high precision radial velocities is foreseen and the possibility of combining a Fresnel rhomb with a Wollaston prism for magnetic Doppler imaging is under study. Installation at the VLT is scheduled during the second half of 2004. Here we briefly recall the major design features of CRIRES and describe its current development status.
The Adaptive Optics NIR Instrument NAOS-CONICA has been commissioned at the VLT (UT4) between November 2001 and March 2002. After summarizing the observational capabilities of this multimode instrument in combination with the powerful AO-system, we will present first on sky results of the instrumental performance for several non-direct imaging modes: High spatial resolution slit-spectroscopy in the optical and thermal NIR region has been tested. For compact sources below 2 arcsec extension, Wollaston prism polarimetry is used. For larger objects the linear polarization pattern can be analyzed by wire grids down to the diffraction limit. Coronographic masks are applied to optimize imaging and polarimetric capabilities. The cryogenic Fabry-Perot Interferometer in combination with an 8m-telescope AO-system is shown to be a powerful tool for imaging spectroscopy (3D-scans).
We present and discuss the capabilities of the infrared polarimetric modes of the ESO-VLT adaptive optics system NAOS-CONICA. Commissioning results obtained both with wire-grids and Wollaston prisms are shown. In particular, NACO observations of the Calabash
reflection nebula are compared with earlier, seeing limited, results
obtained at ESO to illustrate the new potential offered by adaptive
optics assisted polarimetry on an 8m class telescope.
ESO is presently building an adaptive-optics fed Cryogenic
Infrared Echelle Spectrograph (CRIRES) for the VLT-observatory operating in the wavelength range from 1-5μm. Spectro-polarimetry with a focus on circular polarization in the infrared is particularly interesting as the ratio of Zeeman-splitting to intrinsic line widths improves linearly with wavelength. Also the contrast between absorption lines in starspots and the surrounding photosphere becomes more favourable when observing a longer wavelengths (i.e. closer to the Jeans-case). Moreover it is well known that even extremely red objects such as Brown Dwarf candidates show X-ray emission and hence must have magnetic activity. CRIRES shall be equipped with a reflective phase retarder and a Wollaston-prism allowing nearly
un-compromised measurements of circular polarization at a spectral resolution of 100000. Linear polarization measurements are also possible, but most likely with compromised performance. We show
preview spectra of Zeeman sensitive transitions in the infrared based
on Fourier-transform spectra of sunspots from literature.
High-z galaxies beyond redshift ~ 4 are essentially detected from ground based observations through their Lyα emission. The anticipated Lyα flux of galaxies at redshifts ~ 6 and beyond is a few times 0.1 10-17 ergs.s-1.cm-2 and its detection requires observations in low background conditions, when the observing wavelength is pushed into the near IR domain. We have carried out observations on 4-8 m telescopes to search for high z galaxies at 920 nm, 1060 and 1187 nm, resorting to various techniques: Narrow Band (NB) imaging and multi-slit windows. Observations, data reduction and preliminary results are described.
We have obtained ultradeep Js, H and Ks near-infrared imaging of the Hubble Deep Field South WFPC2 field with the ISAAC camera on the VLT. The total integration time of 100 hours resulted in the deepest ground-based infrared observations to date and the deepest Ks-band data ever taken. This depth allows us to determine the spectral energy distributions of the high-redshift galaxies with unprecendented accuracy. Together with existing optical observations, we use the multicolor data to select high-redshift galaxies by their rest-frame optical light, and study their statistical properties and morphologies. We find a wide variety of morphologies: some are large in the rest-frame optical and resemble normal spiral galaxies, others are barely detected in the observers optical and have red NIR colors. The latter belong to a new population of galaxies at redshifts z>2, that is notably absent in the HDF-North. The spectral energy distributions of many of such red galaxies show distinct breaks, which we identify as the balmer break/4000 Angstrom break, and their contribution to the stellar mass density is estimated to be substantial. At redshift z~3, we find a clear excess of superluminious galaxies (> 5 L*B(z=0)), which is consistent with 1 magnitude of luminosity evolution. Overall, the results show the necessity of deep near-infrared imaging to obtain a full census of the high redshift universe.
The VIRMOS consortium of French and Italian Institutes is manufacturing 2 wide field imaging multi-object spectrographs for the European Southern Observatory Very Large Telescope, with emphasis on the ability to carry over spectroscopic surveys of large numbers of sources. The Visible Multi-Object Spectrograph, VIMOS, is covering the 0.37 to 1 micron wavelength domain, with a full field of view of 4 by 7 by 8 arcmin2 in imaging and MOS mode. The Near IR Multi-Object Spectrograph, NIRMOS, is covering the 0.9 to 1.8 microns wavelength range, with afield of view 4 by 6 by 8 arcmin2 in MOS mode. The spectral resolution for both instrument scan reach up to R equals 5000 for a 0.5 arcsec wide slit. Multi-slit masks are produced by a dedicated Mask Manufacturing Machine cutting through thin Invar sheets and capable of producing 4 slit masks approximately 300 by 300 mm each with approximately slits 5.7 mm long in less than one hour. Integral field spectroscopy is made possible in VIMOS by switching in the beam specially build masks fed by 6400 fibers coming form a 54 by 54 arcsec2 integral field head with a 80 by 80 array of silica micro-lenses. NIRMOS has a similar IFS unit with a field of 30 by 30 arcmin2. These instruments are designed to offer very large multiplexing capabilities. In MOS mode, about 1000 objects can be observed simultaneously with VIMOS, with a S/N equals 10 obtained on galaxies with I equals 24 in one hour, and approximately 200 objects can be observed simultaneously with NIRMOS, with a S/N equals 10 obtained don galaxies with J equals 22, H equals 20.6 in 1h at Req equals 200. We present here the status of VIMOS, currently under final integration, with expected first light in the summer 2000, together with the final design of NIRMOS presented at the Final Design Review. The VLT-VIRMOS deep redshift survey of more with the final design of NIRMOS presented at the Final Design Review. The VLT-VIRMOS deep redshift survey of more than 150000 galaxies over the redshift range 0 < z < 5 will be undertaken based on 120 guaranteed nights awarded to the project.
The 105 resolution spectrograph 'CRIRES' for the 1-5 micrometers wavelength range is under construction for ESO's Very Large Telescope. The Nasmyth-mounted instrument sues an adaptive optics front-end for light concentration and 0.2 arcsec spatial resolution in the main long-slit spectroscopy mode. Three detector arrays will be used for large single- order wavelength coverage. Many components for this large cryogenic spectrograph are based on developments for earlier ESO IR instruments.
The first VLT IR instrument, ISAAC, was installed at the 8 meter Antu telescope in 1998. Experience and results with both InSb and HgCdTe large format arrays will be reported. Effects limiting the performance and strategies to partially overcome these limitations will be discussed.
The IR Spectrometer and Array Camera (ISAAC) at the VLT has ben used for follow-up spectroscopy of 11 candidate H(alpha) emitting galaxies at z approximately equals 2.2 detected in a preparatory, IR narrow band filter imaging survey. This survey was conducted with Son of ISAAC at the ESO NTT and covered an area of 100 sq. arcmin including the WFPC2 and STIS fields in the HDFS. Line emission within the wavelength range of the 2.1 micrometers narrow band filters is confirmed in 6 of the candidates and in all but one of the 6 detected at >= 4 (sigma) in the survey. Although only a single emission line is observed its identification with H(alpha) is relatively secure and the absence of (NII) (6548,6584 Angstrom) is consistent with high ionization and/or low metallicity systems. Velocity dispersions and one H(alpha) rotation curve imply masses of up to approximately 1010M. Star formation rates of the individual galaxies derived from the H(alpha) fluxes are 20-35 M yr-1 and the total star formation rate density is 0.12 M Mpc-3. This is the same as found at z approximately equals 1.3 from H(alpha) observations with NICMOS on HST$_1) and close to that at z equals 3-4.5 derived from the extinction corrected UV continuum fluxes of Lyman Break galaxies2.
ISAAC is an IR imager and array camera operating from 0.9 to 5 micrometers . This paper illustrates some preliminary scientific results which have been obtained during its first year of science operations. In Low Resolution Spectroscopy, we present results obtained on one Centaur, on TMR1-C and the detection of a methane brown dwarf. In Medium Resolution Spectroscopy, we present results obtained on kinematics studies in the central kpc of AGNs, and of emission line detections of distant galaxies at various redshift, from 0.6 to 3.
Instrument platforms like the VLT represent a new challenge to IR focal plane technology. Since the large telescope diameter and the improved image quality provided by adaptive optics reduce the pixel scale, larger array formats are needed. To meet this challenge ESO is participating in development programs for both InSb and HgCdTe large format arrays. To cover the spectral region of 1 to 5 micron ESO has funded a foundry run at SBRC to produce 1024 X 1024 InSb arrays, which will be installed in ISAAC, the IR Spectrometer and Array Camera built for the VLT. Since the delivery of the 1K X 1K InSb array is delayed, the test results obtained with a 256 X 256 InSb array and the application of off chip cryogenic amplifiers to InSb detectors will be discussed. Results obtained with a (lambda) c equals 2.5 micrometers Rockwell 1024 X 1024 HgCdTe array will be presented, where an off chip cryogenic operational amplifier was used yielding a rms read noise of 3 electrons. Sensitivity profiles of individual pixels have been measured with a single mode IR fiber. Limitations of PACE 1 technology, such as persistence, will be discussed. First results with the 1K X 1K array, which was installed in SOFI, an IR focal reducer providing 1-2.5 micron imaging and long slit grism spectroscopy at the NTT telescope, will be presented. Advanced techniques of real time image sharpening will also be included. An outlook to the development of (lambda) c equals 2048 X 2048 HgCdTe array formats will be given. The optical layout of NIRMOS, a multi-object spectrograph for the VLT telescope, is base don the availability of 2K X 2K HgCdTe arrays.
ISAAC is a 1 - 5 micrometer imager/spectrometer currently under construction at ESO and scheduled for installation at one of the Nasmyth foci of the first 8 m unit telescopes of the VLT in 1998. It comprises two cameras, optimized for the 1 - 2.5 micrometer and 2 - 5 micrometer wavelength ranges, which can be used to directly view the telescope focal plane or the intermediate spectrum formed by a grating spectrometer. The complete instrument is cryogenically cooled by means of a continuous flow liquid nitrogen circuit and two closed cycle coolers and is housed in an approximately 1.5 m diameter vacuum tank permanently attached to the Nasmyth adapter/rotator. The observing modes, instrument design, construction status and performance estimated using a recently developed software simulator of the instrument are described.
For the next generation of instruments which will be used at 8 m telescopes large format arrays are needed. Better image quality obtained by adaptive optics requires sampling to higher spatial frequencies. The large field of these instruments increases the demand for array formats as large as 1024 by 1024 and beyond. For this reason ESO is committed to the development of megapixel infrared detectors. In a multimode instrument covering the 1 to 5 micrometer spectral range a detector has to fulfill very different requirements. For high resolution spectroscopy low dark current and read noise are required. For broad band thermal imaging a high well capacity is needed to reduce the speed required to read out the array before it saturates. This paper gives a status report of ESO's activities related to large format arrays. An ultrafast data acquisition system has been developed to read out large format arrays. The performance goal is to achieve shot noise limited operation in the wavelength region of lambda equals 1 to 5 micrometer. The array controller is capable of handling the high data rates generated in the thermal infrared. The design of the controller was mainly driven by the requirement to read out the 32 parallel video channels of the SBRC 1024 by 1024 InSb detector in 50 msec. The array controller can also cope with the low read noise required for flux levels of less than 1 photon/sec. A new test camera for large format arrays has also been built. First test results obtained with the Rockwell 1024 by 1024 HgCdTe array are presented. The noise and dark current performance will be discussed with regard to OH line suppression. Read speed requirements will be defined for advanced readout techniques of image sharpening applying on chip tracking in the multiple nondestructive readout mode.
A cryogenic echelle spectrometer is planned for the ESO Very Large Telescope to perform high-resolution (R >= 100,000) observations in the 1 - 5 micrometers range. Primary design goals are optical quality to exploit the 8 m telescope advantage and large spectral field to exploit the sensitivity advantage of a dispersive instrument with a large detector array. The basic instrument uses established technology; some components potentially leading to enhanced performance or additional capabilities are under development.
Five infrared instruments are now under development or study for ESO's Very Large Telescope (VLT). ISAAC (infrared spectrometer and array camera) will be the first to be installed on the first of the 8 m unit telescopes where it will provide for imaging and long slit low and medium resolution spectroscopy in the 1 - 5 micrometers range. This instrument is being developed in-house at ESO and is now in the manufacturing and integration phase with installation scheduled for 1998. CONICA (high resolution near infrared camera) is intended primarily for 1 - 5 micrometers diffraction limited imaging and has been contracted to a consortium of institutes led by the Max Planck Institut fur Astronomie, Heidelberg, Germany with the Max Planck Institut fur Extraterrestrisches Physik, Garching, Germany as partner. Originally planed for coude, it has recently been redesigned for one of the Nasmyth foci of UT1 which will now be equipped with an adaptive optics system following a decision to delay installation of the coude foci. Following completion of a Phase A study, VISIR (mid-IR imager/spectrometer) has been selected as the next IR instrument and negotiation of a development contract is now in progress with the Service d'Astrophysique, Saclay, France who led the study consortium. This instrument is destined for the Cassegrain focus of UT2 and will provide both imaging and spectroscopic capabilities in the 10 micrometers and 20 micrometers windows. CRIRES (cryogenic IR echelle spectrometer), which aims for R approximately 100.000 in the 1 - 5 micrometers range, has been the subject of a concept definition study within ESO and has been rated highly enough scientifically to justify continuation of its associated immersion grating development program. NIRMOS (near IR multi-object spectrometer) has also been the subject of a concept definition study, led by the Observatoire de Meudon, and will soon enter a more detailed study phase of alternative concepts including the possibility of combining visible and near IR multi-object spectroscopy in a single instrument primarily for high redshift galaxy surveys.
Photovoltaic detectors for ground based astronomical applications have experienced dramatic improvements during the last decade. Both the array format has been increased and the pixel performance has improved and is approaching fundamental limits. In view of this development a detection limit for the photon flux of the ideal detector will be derived, depending only on the temperature and the impedance of the detector. It is shown, that this limit is approximated by state of the art infrared arrays for long on chip integrations. In a multimode instrument covering the 1 to 5 micrometers spectral range a detector has to fulfill very different requirements. For high resolution spectroscopy low darkcurrent and read noise are required. For broad band thermal imaging a high well capacity is needed to reduce the speed required to read out the array before it saturates. Different possibilities to increase the well depth of infrared arrays have been investigated. First, an extra capacity can be added to the gate of the source follower in the unit cell of the multiplexer. Alternatively, the pixel capacity can be increased by increasing the doping concentration of the detector diode. The third possibility is to apply a large reverse bias voltage. This requires exceptionally good low doped InSb junctions which can be operated at a reverse bias voltage of 1 volt.
A new non-destructive readout scheme for IR array detectors, which allows image sharpening by on chip tracking has been tested with the ESO IR array camera IRAC2. This camera is equipped with a large format NICMOS3 256*256 MCT array detector. The effect of the readout algorithm is equivalent to a first order wavefront correction of images degraded by atmospheric seeing. Correction is performed in real time for long on chip detector integrations. Results obtained at a 2.2 m telescope on the pre main sequence binary star S CrA, the Circinus galaxy and the cluster NGC3603 will be presented. Possible applications of this method at large 8 m telescopes will be discussed.
ESO has recently installed a new infrared array camera IRAC2 which is equipped with a large format NICMOS3 256*256 Hg1-xCdzTe array detector. The performance of the instrument and the detector array will be discussed briefly. A new nondestructive readout scheme of the array will be presented which allows first order wavefront corrections of images degraded by atmospheric seeing. During the stare time the integration ramp of the detector signal is sampled every 100 msec. A regressional fit of the sample data points yields the slope of the integration ramp which is proportional to the flux received by a detector pixel. To this readout mode which is commonly used for IR arrays a small software module can be added to compensate the image motion of the observed object by shifting the nondestructively sampled images. This has the same effect as a tip tilt correction by an active optical element--but without the extra complexity of such a device. Lab tests and first results obtained at the telescope are presented.
The ESO Very Large Telescope will be equipped with a complementary suite of instruments for infrared imaging and spectroscopy between 1 and 20 micrometers . ISAAC (Infrared Spectrometer and Array Camera) is being developed by ESO and is scheduled to be commissioned during 1997 on the first of the 8 m unit telescopes where it will provide for 1 - 5 micrometers imaging and long slit, low and medium resolution spectroscopy. Its design provides for separate 1 - 2.5 micrometers and 2.5 - 5 micrometers cameras equipped with large format array detectors which can be used either to re-image the telescope focal plane or the intermediate spectrum formed by a grating spectrometer. HgCdTe and InSb arrays of 256 X 256 pixels are currently baselined but the possibility of accommodating larger arrays of up to 1024 X 1024 pixels is foreseen if and when these become available. The complete instrument will be cryogenically cooled using two stage closed cycle coolers and housed in a approximately 1 m diameter vacuum vessel attached to the adapter/rotator at one of the Nasmyth foci. Subject to satisfactory results of prototype tests now in progress it is planned to use diamond turned metal mirrors for the reflecting optics and cryogenic stepper motor drive systems for all moving functions.
The design of a multimode IR spectrometer/imager for the ESO VLT is discussed. The system has two cameras optimized separately for the 1-2.5-micron and 2.5-5.0-micron spectral regions, used to either view the on-axis focal plane for direct imaging or to reimage the intermediate spectrum formed by a grating spectrometer whose input slit is located a few arcmin off axis. The characteristics of the imaging and the spectrometry are described, and a diagram of the opticaL layout of the system is presented together with the spot diagrams for the camera objectives and for the combined spectrometer/camera.