February 03, 2015: As the re-commissioning after the VISIR upgrade is still on-going, VISIR is offered only for a limited set of observing modes for Period 95: regular imaging (0.045" pixel scale), low-resolution spectroscopy, and high-resolution long-slit and cross-dispersed spectroscopy. Chop throws are limited to 20" for the time being. Currently the final sensitivities are still being established. By consequence, 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.
Imaging and long-slit spectroscopy with VISIR can be achieved at several spatial and spectral resolutions. The imager and spectrograph are two sub-instruments. They have independent light paths, optics and detectors. 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. 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 ~9K. 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|
8 –13, 17 – 24
38.0 × 38.0
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".