Overview

Important

March, 2016: Most of the modes of the upgraded VISIR have now been re-commissioned. From P98 on, the offered modes of VISIR include regular and burst mode imaging, low-resolution spectroscopy, as well as long-slit and cross-dispersed high-resolution spectroscopy in both Service and Visitor Modes. Sparse Aperture Masking and coronagraphy using the Annular Groove Phase Mask (AGPM) or Four Quadrant Phase Mask (4QPM) are o ffered in Visitor Mode only. Robust sensitivities are currently being established through the accumulation of the respective measurements. The currently announced sensitivities for the new detectors are still based on low number statistics or have not yet been obtained at all. In the latter case, the pre-upgrade sensitivities should be assumed.

Large and Monitoring programmes will be accepted only for imaging, low-resolution spectroscopy, as well as long-slit and cross-dispersed high-resolution spectroscopy. Starting with P99, no Large and Monitoring programmes will be accepted because ESO is investigating the use of VISIR for a specific experiment which foresees its installation at UT4 during 2018 or 2019.

The PWV must be provided as an observing constraint during Phase 1 and Phase 2 preparation, otherwise programmes will be rejected.

Summary of instrument capabilities

VISIR provides imaging and long-slit spectroscopy in the thermal infrared between 5 and 24µm at several spatial and spectral resolutions. To remove the high thermal background from the atmosphere and telescope, differential measurements are performed by chopping (moving the secondary mirror of the telescope) and nodding (moving the telescope itself).

The imager and spectrograph are two sub-instruments. They have independent light paths, optics and detectors. The imager contains various broad and narrow band filters as well as three objectives for different spatial resolutions and fields of view. Additionally, coronagraphy (using an Annular Groove Phase Mask or a 4-Quadrant Phase Mask) and sparse aperture masking (SAM) are possible with the imager, both in pupil and in field tracking modes. In Burst Mode, all individual exposures of the detector are recorded instead of only the averages per nodding cycle. The spectrometer has two arms, one with a prism and low order gratings for the low and medium spectral resolution, the other with large echelle gratings providing high spectral resolution. Several slits with different widths are installed in the spectrometer.

The cryogenic optical bench is enclosed in a vacuum vessel. The vessel is a cylinder, 1.2m long and 1.5m in diameter. Standard Gifford-McMahon closed-cycle coolers are used to maintain the required temperature: 29K for most of the structure and optics and < 15K for the parts near the detector. The detectors are cooled down to ~10K. A quick overview of the imaging and spectroscopy modes of VISIR are given in the tables below.

Imaging

spectral range [µm]  pixel field of view [arcsec] total field of view [arcsec2] filter 
5, 8 – 13, 17 – 20 0.045 38.0 × 38.0, small field  (SF) several broad and narrow band filters; coronagraphy, sparse aperture masking
5, 8 – 13, 17 – 20 0.076 60.0 × 60.0, intermediate field (IF) several broad and narrow band filters

Spectroscopy

grating offered wavelengths [µm] resolving power
low resolution (LR) 8 – 13 µm R ~ 350 at 10 µm
medium resolution (MR) 7.5 – 9.3 µm, 10.2 – 13.0 µm, 17.1 – 19.0 µm, 20.12 µm R ~ 3200 at 10 µm
high resolution - long slit (HR) 8.02, 12.81 and 17.03 µm R ~ 25000 at 10 µm
HR - cross dispersed (HRX) 7.7 – 24.0 µm R ~ 25000 at 10 µm

The slits offered have selectable widths of 0.4", 0.75" and 1", and a length of 32.3".
In high-resolution, cross-dispersed mode, the slit length is 4.1".