Gerd Jakob

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.

Low-vibration high-cooling power 2-stage cryocoolers for ground-based astronomical instrumentation

This paper describes the outcome of a survey reviewing commercially available state-of-the-art high-cooling power 2- stage cryocooler systems for a potential use in powerful scientific instruments for ground-based astronomy. We present the development of a dedicated test-bed as well as vibration and performance measurements on different 2-stage refrigerator systems. As a result of this investigation program, one system was selected as ESOs new standard 20 K closed cycle cooler offering substantial advantages in flexibility and orientation insensitivity along with best compromise for a low vibration device with high cooling power. The new cryocooler type was integrated with VLT instrumentation. A concept for a comprehensive vibration test program at VLT is presented in order to define admissible vibration spectra for future instrumentation.

AQUARIUS: the next generation mid-IR detector for ground-based astronomy, an update.

ESO has already published data from a preliminary laboratory analysis on the new mid-IR detector, AQUARIUS, at the previous SPIE conference of 2012, held in Amsterdam2. This data analysis indicated that this new mid-IR Si:As IBC detector, from Raytheon Vision Systems, was an excellent astronomical detector when compared to previous generations of this detector type, specifically in terms of stability, read noise and cosmetic quality. Since that time, the detector has been deployed into the VISIR1 instrument at the VLT, with very mixed performance results, especially when used with the telescope secondary mirror, to chop between two areas of sky to do background subtraction and at the same time when many frames are co-added to improve the signal to noise performance. This is the typical mode of operation for a mid-IR instrument on a ground based telescope. Preliminary astronomical data analysis indicated that the new detector was a factor of two to three times less sensitive in terms of its signal to noise per unit time performance when directly compared to the old DRS detector that AQUARIUS was designed to replace. To determine the reason for this loss of sensitivity, the instrument was removed from the telescope and not offered to the ESO user community. A detector testing campaign was then initiated in our laboratory to determine the reasons for this loss of sensitivity, assuming that it was an issue with the new detector itself. This paper reports on our latest laboratory measurements to determine the reasons for this loss of sensitivity. We specifically report on indirect measurements made to measure the quantum efficiency of the detector, which can be difficult to measure directly. We also report on a little known source of noise, called Excess Low Frequency Noise (ELFN). Detailed analysis and testing has confirmed that this ELFN is the reason for the loss of instrument sensitivity. This has been proven by a re-commissioning phase at the telescope with the instrument and the detector. A new set of observing parameters and observational regime have been developed to help to mitigate the ELFN. We outline a possible explanation for the source of the EFLN, learnt from a literature search and discussion with the manufacturer.

Advanced high-cooling power 2-stage Gifford-McMahon refrigerator systems

This paper describes the development of high-cooling power systems by making use of multiple cold head operation with a minimized number of compressor units. These advanced cooling systems were investigated for optimization and their Carnot efficiencies were analyzed. Test series were performed to monitor and rank some of their critical operation parameters. Operating envelopes for different cold head / compressor configurations were defined for applications in various VLT instruments. This new concept of providing high pressure helium as a service point for a large number of detached cold heads is a first step towards a new cryogenic facility concept for the E-ELT.

AQUARIUS, the next generation mid-IR detector for ground-based astronomy

ESO has recently funded the development of the AQUARIUS detector at Raytheon Vision Systems, a new mega-pixel Si:As Impurity Band Conduction array for use in ground based astronomical applications at wavelengths between 3 – 28 μm. The array has been designed to have low noise, low dark current, switchable gain and be read out at very high frame rates. It has 64 individual outputs capable of pixel read rates of 3MHz, implying continuous data-rates in excess of 300 Mbytes/second. It is scheduled for deployment into the VISIR instrument at the VLT in 2012, for next generation VLTI instruments and base-lined for METIS, the mid-IR candidate instrument for the E-ELT. A new mid-IR test facility has been developed for AQUARIUS detector development which includes a low thermal background cryostat, high speed cryogenic pre-amplification and high speed data acquisition and detector operation at 5K. We report on all the major performance aspects of this new detector including conversion gain, read noise, dark generation rate, linearity, well capacity, pixel operability, low frequency noise, persistence and electrical cross-talk. We describe the many possible readout modes of this detector and their application. We also report on external issues with the operation of these detectors at such low temperatures. Finally we report on the electronic developments required to operate such a detector at the required high data rates and in a typical mid-IR instrument.

Upgrade of VISIR the mid-infrared instrument at the VLT

The European Southern Observatory (ESO) is preparing to upgrade VISIR, the mid-IR imager and spectrograph at the VLT. The project team is comprised of ESO staff and members of the original consortium that built VISIR: CEA Saclay and ASTRON. The goal is to enhance the scientific performance of VISIR and to facilitate its use by the ESO community. In order to capture the needs of the user community, we collected input from the users by means of a webbased questionnaire. In line with the results of the internal study and the input from the user community, the upgrade plan calls for a combination measures: installation of improved hardware, optimization of instrument operations and software support. The limitations of the current detector (sensitivity, cosmetics, artifacts) have been known for some time and a new 1k x 1k Si:As Aquarius array (Raytheon) will be the cornerstone of the VISIR upgrade project. A modified spectroscopic mode will allow covering the N-band in a single observation. Several new scientific modes (e.g., polarimetry, coronagraphy) will be implemented on a best effort basis. In addition, the VISIR operational scheme will be enhanced to ensure that optimal use of the observing conditions will be made. Specifically, we plan to provide a means to monitor precipitable water vapour (PWV) and enable the user to specify it as a constraint set for service mode observations. In some regions of the mid-IR domain, the amount of PWV has a fundamental effect on the quality of a given night for mid-IR astronomy. The plan also calls for full support by ESO pipelines that will deliver science-ready data products. Hence the resulting files will provide physical units and error information and all instrumental signatures will have been removed. An upgraded VISIR will be a powerful instrument providing diffraction-limited performance at an 8-m telescope. Its improved performance and efficiency as well as new science capabilities will serve the needs of the ESO community but will also offer synergy with various other facilities such as ALMA, JWST, VLTI and SOFIA. A wealth of targets for detailed study will be available from survey work done by VISTA and WISE. Finally, the upgraded VISIR will also serve as a pathfinder for potential mid-IR instrumentation at the European Extremely Large Telescope (E-ELT) in terms of technology as well as operations.

Different ways of reducing vibrations induced by cryogenic instruments

The infrared instruments and most of the detectors have to be operated at cryogenics temperatures. Today, this is generally achieved using mechanical coolers. Compared to traditional nitrogen systems, these coolers, which large implementation started 15 years ago, have the advantage of reducing considerably the operation effort at the observatories. Depending of the technology, these coolers are all generating a level of vibration which in most of the cases is not compatible with the extremely high stability requirement of the large size telescope. This paper described different ways which have been used at ESO to reduce the vibration caused by the large IR instruments. We show how we reached the goal to have the cryogenic instruments so quiet that they do not affect the operation of the interferometry mode of the VLT. The last section of the paper reports on a unique system based on a counter vibration principle.

CIAO: wavefront sensors for GRAVITY

GRAVITY is a second generation near-infrared VLTI instrument that will combine the light of the four unit or four auxiliary telescopes of the ESO Paranal observatory in Chile. The major science goals are the observation of objects in close orbit around, or spiraling into the black hole in the Galactic center with unrivaled sensitivity and angular resolution as well as studies of young stellar objects and evolved stars. In order to cancel out the effect of atmospheric turbulence and to be able to see beyond dusty layers, it needs infrared wave-front sensors when operating with the unit telescopes. Therefore GRAVITY consists of the Beam Combiner Instrument (BCI) located in the VLTI laboratory and a wave-front sensor in each unit telescope Coudé room, thus aptly named Coudé Infrared Adaptive Optics (CIAO). This paper describes the CIAO design, assembly, integration and verification at the Paranal observatory.

Perspective of imaging in the mid-infrared at the Very Large Telescope Interferometer

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 Konkoly Observatory.

VISIR upgrade overview: all's well that ends well

We present an overview of the VISIR instrument after its upgrade and return to science operations. VISIR is the midinfrared 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 was 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 manufactured by Raytheon. In addition, a new prism spectroscopic mode covers the whole N-band in a single observation. Finally, new scientific capabilities for high resolution and high-contrast imaging are offered by sub-aperture mask and coronagraphic 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. 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. It is inherent to the design chosen for this detector and cannot 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. After careful analysis, we have implemented fast (up to 4 Hz) chopping with field stabilization using the secondary mirror of the VLT. During commissioning, the upgraded VISIR has been confirmed to be more sensitive than the old instrument, and in particular for low-resolution spectroscopy in the N-band, a gain of a factor 6 is realized in observing efficiency. After overcoming several additional technical problems, VISIR is back in Science Operations since April 2015. In addition an upgrade of the IT infrastructure related to VISIR has been conducted in order to support burst-mode operations. Science Verification of the new modes was performed in Feb 2016. The upgraded VISIR is a powerful instrument providing close to background limited performance for diffraction-limited observations at an 8-m telescope. It offers synergies with facilities such as ALMA, JWST, VLTI and SOFIA, while a wealth of targets is available from survey works like WISE. In addition, it will bring confirmation of the technical readiness and scientific value of several aspects for future mid-IR instrumentation at Extremely Large Telescopes. We also present several lessons learned during the project.