VISIR  contains, within a single cryostat and under unified control, both an imager and a spectrometer. A French/Dutch consortium is responsible for VISIR as an integrated instrumental system.

The VISIR layout, click on the picture to see the details

The VISIR reality. The detector cover is removed to look inside

The ESO VISIR web pages contains prime information about this great instrument. It can be found here. 

This "System Specifications" (for a pure-astronomer audience) illustrates the topmost layer of  modern instrument design. It reflects the situation in April 2001. Jan Willem Pel's full version is available here in pdf.

Many of the most important astrophysical processes in the universe take place inside dense clouds of gas and dust: bursts of star formation in young galaxies, the energetic phenomena in active galactic cores, the birth of stars and planetary systems, the final phases of stellar evolution.

The infrared wavelength range, in particular the 'mid-infrared' with wavelengths from about 5 to 30 micro-meters (mm), is ideally suited for the study of these processes, for the following reasons:

  • This wavelength range is very rich in atomic and molecular spectral lines.
  • Many absorption bands of interstellar dust are situated in the mid-infrared. These solid-state features provide unique information about the chemistry and physics of the dust particles.
  • The radiation of typical 'warm' interstellar dust peaks in the mid-infrared.
  • At mid-infrared wavelengths the obscuration by dust is very much smaller than in the optical. This allows probing of the deep interiors of dust and gas clouds that are totally opaque at shorter wavelengths.

In comparison to the optical wavelength domain, the mid-infrared is still poorly explored. The main reason for this is the Earth's atmosphere. The atmospheric gases, water vapor in particular, absorb large parts of the infrared spectrum. At the same time the atmosphere's thermal radiation causes a high sky background that makes it difficult to observe faint astronomical sources. Ground-based mid-infrared observations can only be made in two atmospheric transparency 'windows', the N-band (wavelengths between 7.5 and 14 mm) and the Q-band (16-28 mm), but even in these windows the atmospheric absorption and radiation are a major disturbance.

Since observations from space do not have these problems, big international efforts have been made over the past few decades to launch infrared satellite experiments such as IRAS and ISO. Whereas these space observatories have collected a wealth of new mid-infrared data and demonstrated the great potential of this wavelength range, they were restricted by poor spatial resolving power and naturally had a very limited lifetime.

In order to make sensitive mid-infrared measurements from the ground, one needs the combination of three factors: a large telescope, an excellent observing site in a very dry climate and sensitive infrared detector arrays.

Design: Kuenst.    Development: Dripl.    © 2022 ASTRON