Starting with P98, 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 (SAM) 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 and are affected by the exchange of the entrance window.

Starting with P104, large and monitoring programmes will be accepted again but only for imaging, low-resolution spectroscopy, as well as long-slit and cross-dispersed high-resolution spectroscopy. Note carefully: as VISR is mounted on the Casssegrain focus of UT4 in its AO (NEAR) configuration (press release eso1702), the instrument's capabilities are limited in comparison to regular UT3 observations (see the "news" section, and P104 User manual). VISIR will be moved back and restored to its original self during P104.

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 (currently not offered) 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.


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


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".
Medium resolution spectroscopy is currently not offered. Please contact the instrument team if you are interested in using this mode.