February, 2015: As the re-commissioning after the VISIR upgrade is still on-going, VISIR is offered only for a limited set of observing modes. Currently, these are regular imaging (0.045" pixel scale), coronography with the AGPM, low-resolution spectroscopy, high-resolution long-slit and cross-dispersed spectroscopy. The other observing modes will be successively released in the coming periods. Robust sensitivities are currently being established through the accumulation of the respective measurements. The currently announced sensitivities for the new detectors are only based on a few individual measurements or have not yet been obtained at all. In the latter case, the pre-upgrade sensitivities should be assumed.
VISIR provides imaging and long-slit spectroscopy in the thermal infrared between 7 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, coronography (using an Annular Groove Phase Mask or a 4-Quadrant Phase Mask) and sparce 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.
|spectral range [µm]||pixel field of view [arcsec]||total field of view [arcsec2]||filter|
|7 – 13, 17 – 20||0.045||38.0 × 38.0, small field (SF)||several broad and narrow band filters; coronography, sparce aperture masking|
|7 – 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".