FORS 1 and FORS 2

FOcal Reducer and low dispersion Spectrograph

“Of all instruments at Paranal, this one is the Swiss Army knife”. This is the way Henri Boffin, the instrument scientist behind the FOcal Reducer and low dispersion Spectrograph 2 or FORS2, describes the instrument that is most in demand at ESO's Paranal Observatory. The key to success is that FORS2, installed on UT1 (Antu) of the Very Large Telescope (VLT), is able to study many different astronomical objects in many different ways.

For example, FORS2 can take images of relatively large areas of the sky with very high sensitivity. No wonder that some of the most iconic photos taken with the VLT used this instrument (see eso9845d, eso9948f, eso0202a, eso0338a, eso0338c, eso0617a, and more recently eso1244a and eso1348a).

But FORS2 can also take spectra of one (eso9920r), two or even several tens of objects in the sky simultaneously (eso0223b). “When used as a spectrograph, FORS2 disperses the light into very sophisticated rainbows that help astronomers study chemical composition or estimate the distances of remote objects,” says Boffin.

And this is not all. FORS2 can also measure the polarisation of light and is therefore used at the VLT to determine whether some astronomical objects have strong magnetic fields.

Observations with FORS2 and its twin brother FORS1 (decommissioned in 2009) have together led to almost 1800 papers in scientific journals as of 2014, with an average of about 100 scientific papers per year. “Basically, whatever you can think of, you can do it with FORS2. Apart from making the coffee the astronomers need at night!”

Science highlights with FORS

  • Cosmological gamma-ray bursts and hypernovae linked by FORS1 and FORS2 observations (eso0318)
  • VLT “rediscovered” life on Earth (eso1210)
  • FORS was used to spot “Dark Galaxies”, an early phase of galaxy formation, which are essentially gas-rich galaxies without stars (eso1228)
  • Comet Halley in the cold – the most distant view of a regular visitor (eso0328)
  • FORS1 and FORS2 broke several distance records: the most distant gamma-ray burst (eso0034), the most distant group of galaxies (eso0212), the most distant galaxy (eso0314)

IC 2944 with FORS2

This raw image, straight from the instrument, was used, together with many others, to produce the main photo at the top of this page. The images taken with astronomical instruments are always monochromatic: the information on the colours is obtained by taking exposures through different glass filters. The thick black line cutting the field is the gap between the two detectors in the camera. Long “bleeds” caused by bright stars saturating the detector are also visible.

A raw spectrum obtained with FORS

A long, narrow slit isolates a small strip of sky. On this image, the slit is vertical. The spectrograph then splits the light from the slit into its individual colours, each point of the slit forming a horizontal rainbow. In this spectrum, the light from a distant galaxy appears as a faint horizontal line, peppered with bright dots corresponding to the colours emitted by the gas in the galaxy.


The authoritative technical specifications as offered for astronomical observations are available from the Science Operation page.

Name: FOcal Reducer and low dispersion Spectrograph 2
Site: Cerro Paranal
Telescope: VLT UT1
Focus: Cassegrain
Type: Spectro-imager, with multiple object spectroscopy, and polarimetric capabilities
Wavelength coverage: Visible, 330–1100 nm
Spatial resolution: Seeing limited; smallest pixel scale is 0.125"
Spectral resolution: Low to medium resolution (260 to 1600)
First light date: FORS1: 1998 (eso9845)
FORS2: 1999 (eso9948)
Images taken with the instrument: Link
Images of the instrument: Link

Videos of the instrument:


Press Releases with the instrument: Link
Data papers: FORS1, FORS2
Science goals: multi-purpose, versatile instrument


Landessternwarte Heildelberg

Universitaets-Sternwarte Muenchen

Universitaets-Sternwarte Goettingen