The VISIR Upgrade Project
VISIR, the mid-infrared imager and spectrograph on the Very Large Telescope has now been successfully in operation for more than five years. Since the introduction of VISIR, mid-infrared detector technology has advanced significantly, and ESO, in collaboration with members of the consortium that built VISIR (CEA Saclay, France and ASTRON, the Netherlands) is currently launching a project to upgrade VISIR and its capabilities. A new detector will be the most prominent upgrade to the hardware providing better sensitivity, stability and overcoming the electronic artifacts. Based on feedback from the user community, it was decided that several other measures to optimize hardware, software and operations will also be implemented as part of the VISIR Upgrade Project.
The main part of the VISIR upgrade project will be the upgrade of its current DRS (former Boeing) 256x256 BIB detector to a Raytheon 1024x1024 pixel Aquarius array. The new detector will offer a larger field-of-view (for imaging), and extended recorded spectral range (for spectroscopy), better cosmetics, and -- especially for spectroscopy -- improved sensitivity. In addition to this main part of the upgrade, several other new features will be implemented:
During the upgrade project, a prism will be installed in VISIR that will enable the entire N-band (8-12 μm) spectrum to be recorded in a single exposure in low-resolution (R = 200) spectroscopy mode. This new low-resolution prism mode is envisioned to largely replace VISIR's current low-resolution grism spectroscopy mode, which needs four different exposures to cover an equivalent wavelength range. This new observing mode will be offered in both visitor- and service-mode.
The stability of the images delivered by VISIR will be improved. We are working toward offering diffraction-limited image quality in the N- and Q-bands under median seeing conditions on Paranal with the post-upgrade VISIR.
- New coronographic and sparse aperture masking (SAM) modes will be implemented in the course of the VISIR upgrade project. It is expected that initially these modes would be offered for visitor-mode observations only.
- Precipitable Water Vapour (PWV) is the main source of opacity at mid-infrared wavelengths. Furthermore, the PWV contents of the atmosphere is highly variable. To be able to better match the scientific needs of each programme to the ambient conditions, ESO is installing a PWV monitor on Paranal as part of the VISIR upgrade project. PWV will be a user-defined constraint for VISIR service-mode observations starting with ESO period 89.
The VISIR data-reduction pipeline will be upgraded to support the new detector and the new observing modes. In addition, several existing shortcomings will be remedied.
The VISIR Upgrade Project has succesfully passed it Preliminary Design Review (PDR) and Final Design Review (FDR) in September 2010 and July 2011, respectively. VISIR will be unavailable for science observations during the implementation phase of the upgrade project, foreseen for May-August 2012. The post-upgrade VISIR will be offered to the user community for regular science observations at the end of ESO period 89 (from September 2012 onwards). Further news about the upgrade project will be listed in the News section of this web-page
The initial (engineering-grade) Aquarius array delivered by Raytheon is currently being tested and characterized in Garching using TIMMI-2 as a test facility. Initial results show this detector to have much improved sensitivity and cosmetic quality as compared to the current VISIR detector.
A 183 GHz Precipitable Water Vapour (PWV) monitor has been delivered to ESO by its manufacterer, Radiometer Physics GMBH of Meckenheim, Germany, and has successfully passed Preliminary Acceptance Europe (PAE) after being tested at the UFS Schneefernerhaus (2650 m, below Zugspitze) in the Alps.
View of the UFS Schneefernerhaus.
The new Water Vapour Radiometer on the terrace of the UFS Schneefernerhaus.
The Precipitable Water Vapour (PWV) monitor has been installed and successfully commissioned on Paranal! Initial estimates of the performance of the PWV monitor show that it meets all specifications:
- PWV range 0.5-9 mm validated
- PWV precision: ca 30 μm
- PWV accuracy: ca 0.1 mm
- High time resolution (sec)
- All sky pointing, 2D capability
- Autonomous operation
The new Water Vapour Radiometer with the commissioning team in front of VLT UT4 (Yepun).
Comparison of Water Vapour Radiometer measurements (blue dots) with in-situ water-vapour measurements from radiosonde launches attached to weather balloons (black diamonds) and values derived from various other instruments.
The hardware upgrade of VISIR has started! After passing the shipment readiness review, the new detectors and other components of the hardware upgrade have been shipped to Paranal! The last set of observations on-sky obtained with VISIR in its old state was a set of close-out calibrations (a special set of measurements designed to get a quantitative estimate of the instrument performance before the upgrade) on the standard star HD 178345 on May 9, 2012. Subsequently, VISIR was taken off the telescope and taken to the integration hall where the new hardware will be integrated into the instrument.
The empty VISIR cryostat (instrument disassembled)
First light has been achieved (in the integration hall) with the new Aquarius detectors assembled within VISIR!
VISIR has been successfully re-assembled, and has been mounted back on VLT UT3 (Melipal). Re-commissioning of the upgraded VISIR will start on July 28th.
August 1: First light on-sky has been achieved with the new low-resolution prism mode!
August 12: The first VISIR re-commissioning run has ended. The optical quality of the new VISIR is superb, and all modifications including the new prism based low resolution spectroscopy work well. Offline and on-sky tests of the new modes (various flavours of coronagraphy and SAM-imaging) are very encouraging.
However, VISIR is not yet ready for science operations. During the first re-commissioning run there were performance problems of the new Aquarius arrays read-out by the new NGC detector controller. There is no reason to believe that there is a fundamental problem here, but in its present state the instrument does not meet the sensitivity expectations.
VISIR will therefore be dis-mounted from VLT UT3 (Melipal) before the end of August 2012, for further testing. If deemed necessary, the imaging detector may be exchanged before re-mounting VISIR back on the telescope in early September. At present, we expect further re-commissioning activities of VISIR to take place in the bright time period in October. Regular science operations with VISIR are expected to resume at the end of October 2012.
Further testing has confirmed that VISIR's sensitivity after the upgrade is out of specification. Therefore the instrument has been taken off the telescope and various issues are still being worked on in order to properly diagnose the problem and quantitatively characterize the performance parameters. The situation is being analyzed and a short term as well as a long term recovery plan are being drafted. Until the instrument behaviour is fully understood and characterized, VISIR will not be available for operations. This situation is expected to last for the remainder of ESO period 90 (until the end of March 2013). Therefore all VISIR observing runs for ESO period 90 have been canceled.
Work on the VISIR recovery plan is on-going. In collaboration with the manufacturer, ways to mitigate the effects on the final sensitivity of the behaviour of the new detectors in the high-background regime (for imaging) are being studied. In addition, the detector for the spectroscopic arm will be exchanged. No observing time on VISIR will be allocated until the instrument has successfully been re-commissioned. Therefore no VISIR proposals are accepted for ESO period 91 and 92.
The work on the VISIR upgrade is on-going. Modes for single-setting LR spectroscopy from 7 to 14 microns, new science modes for Sparse Aperture Masking and Coronography, and the improvemed observatory operations using a dedicated Particulate Water Vapout (PWV) monitor have all been succesfully implemented. However, on-sky tests performed in August of this year show that the performance with the new Aquarius detectors is still lacking in sensitivity (factor of 2-3 lower than with the old DRS detector). Extensive laboratory testing of the new Aquarius detectors over the course of the last 12 months has shown that the quantum efficiency of the new detectors is fine (around 40%), but that the on-sky sensitivity is severely compromised by the presence of additional correlated noise. The presence of this extremely low frequency noise (ELFN) is intrinsic to Si:As detectors developed by Raytheon and other manufacturers and is not easily avoided in the detector design. Several mitigation efforts to bring VISIR back with improved sensitivity, including faster chopping, drift-scanning, post-processing of the data, and the procurement of new detectors, are being investigated at the moment. As these efforts are on-going, we expect VISIR to be back at the telescope no earlier than mid-2014. Hence VISIR will not be offered for science observations in ESO periods 93 and 94.