Summary of instrument capabilities

VISIR provides imaging and long-slit spectroscopy in the thermal infrared between 5 and 24µm at two spatial and several 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. Currently, only the small field (SF) and the intermediate field (IF) modes are offered. Additionally, coronagraphy (using an Annular Groove 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.

VISIR is offered with regular and burst mode imaging, sparse aperture masking, low-resolution spectroscopy, as well as long-slit and cross-dispersed high-resolution spectroscopy in both Service and Visitor Modes. Coronagraphy is offered in Visitor Mode only. Large and monitoring programmes are accepted only for imaging, low-resolution spectroscopy, as well as long-slit and cross-dispersed high-resolution spectroscopy.

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 available 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.