FORS2, FOcal Reducer/low dispersion Spectrograph 2

FORS2 is a multi mode (imaging, polarimetry, long slit and multi-object spectroscopy) optical instrument mounted on the UT1 Cassegrain focus. FORS2 works in the wavelength range 330-1100 nm. Two different magnifications can be used with pixel scales of 0.25''/pixel (with the Standard Resolution collimator) and 0.125''/pixel (with the High Resolution collimator). The corresponding field sizes are 7.1' x 7.1' and 4.25' x 4.25' respectively. The two different magnifications are chosen by selecting one of two different collimators, hence each magnification has to be calibrated independently. An unbinned CCD readout mode is only offered for applications that specifically require it and must be explicitly requested in the proposal.


FORS2 offers imaging with a wide range of broad- and narrow-band filters. The narrow-band filters are exchangeable and chosen from a large range available filters depending on the user-request. It is also possible to use the jaws of the MOS unit as occulting bars to avoid saturation by unwanted bright objects.


FORS2 has a number of grisms available with different resolutions, including a number of high-throughput Volume-Phased Holographic (VPH) grisms.

Long-Slit (LSS) mode

FORS2 has 9 long-slits with fixed widths of between 0.3'' and 2.5''.

Moveable Slitlets (MOS) mode

FORS2 has a set of 19 pairs of arms that can be moved into the focal plane to form slitlets with user-defined widths.

Spectroscopic Mask (MXU) mode

In addition, FORS2 offers the possibility to insert in the focal plane a mask where slits of different length, width and shape can be cut with a dedicated laser cutting machine. Up to 10 masks can be mounted in a mask unit insidethe instrument and each mask can have up to 470 slits, depending on the grism and filter used. The FIMS tool (see below) must be used for Phase 2 preparation of the mask cutting files. Performance in this mode is equivalent to that of the standard MOS mode.


The polarimetric modes allow the measurement of linear and circular polarization, both for direct imaging (IPOL) and spectroscopy (PMOS). The position angle and degree of the linear polarization or of the circular polarization of an object are determined by using a remotely controlled rotatable lambda/2- or lambda/4-plate in front of the Wollaston prism.

Note: a field-dependent instrumental polarization pattern was discovered in the FORS1 linear polarization mode. This spurious polarization field shows a high degree of axial symmetry and smoothly increases from less than 3e-4 on the optical axis to 7e-3 at a distance of 3 arcmin from it (V band). A similar issue exists with FORS2.


Two detector systems are available for FORS2. The first is a detector consisting of a mosaic of two 2kx4k MIT CCD (15 µm pixels), which provides excellent red sensitivity (> 750nm) and very low fringing. The second is a mosaic of two 2kx4k E2V CCDs (15 µm pixels), which was formerly the FORS1 (post-upgrade) detector. This is very sensitive in the blue range (< 500 nm) but shows a lot of fringes above 650 nm. The E2V detector is only available in Visitor Mode and must be requested at phase I.

Summary tables

Below we summarise in two tables the operational modes and performance of FORS2. Those tables are intended as quick feasibility references only, proposers should refer to the detailed information(e.g. User's Manual, Exposure Time Calculator).

FORS2 imaging modes

Instrument Mode
Direct Imaging (E2V)
U=25.9 B=27.6 V=27.3 R=26.6 I=25.8
Direct Imaging (MIT)
U=24.5 B=27.1 V=27.0 R=26.7 I=25.7 z=24.7


The direct imaging "Mag-limit" is the broad band magnitude calculated for a point source of zero colour (A0V star) which would give a S/N of 5 in one hour with dark sky, clear conditions, a seeing FWHM of 0.8'' and an airmass of 1.2. The U, B, V magnitudes are calculated using the broadband filters of the standard instrument configuration.

FORS2 spectroscopic modes

Instrument Mode
Rs = λ/Δλ
Longslit Spectroscopy [1]
MOS - movable slits [2]
MXU - exchangeable masks
[1] In longslit spectroscopy the slit is chosen out of a set of 9 slits with fixed width between 0.3'' and 2.5''.
[2] In multiobject spectroscopy one may have 19 slitlets of length alternating between 20'' and 22''.


The spectroscopic Mag-limit given in the table above are the R-band magnitudes of a point source of zero colour which would give a S/N of5 per pixel at 650nm (grisms 150I and 600R) in the continuum in one hour with dark sky, clear conditions, a seeing FWHM of 0.8'', an airmass of 1.2, and using a 1.0" slit and the SR-collimator. The two limits given are for the two representative resolutions. The limits on spectropolarimetry are those which for linear/circular polarisation allow a 1% accuracy in determination of degree of polarisation for one hour of total integration time.

FORS Instrumental Mask Simulator (FIMS)

To prepare precise target acquisitions at Phase 2, ESO provides the FIMS software tool. FIMS is required when using FORS2 in several spectroscopic modes, and is also used to prepare occulting bar imaging and spectropolarimetry observations. Phase 1 proposers who wish to justify their time request by optimising movable or MXU slitlet positions during Phase 1, may find it useful to download and install FIMS. Please refer to the FIMS page for instructions on how to install FIMS and to the FIMS User's Manual on how to use FIMS.


Accurate Astrometry or Pre-imaging Required

Highly accurate relative astrometry is required for any observing mode which in Phase 2 will make use of FIMS or blind offset acquisitions. The mask preparation with FIMS requires input images which are astrometrically corrected within the definitions and precision given below. DSS images will, in almost all cases, not be suitable for the task.

In general the relative astrometry must be known to better than 1/6 of the slit widths all over the field of view. Relative astrometry here means that the slit positions must be known relative to those of reference stars in the field of view with the given precision.

If images of adequate quality are not available, Phase 1 proposers must apply for pre-imaging defined as a separate run in the Phase 1 proposal and should be clearly marked as pre-imaging in "instrument configuration" section of the proposal. Failure to do so will, in case the programme is approved for execution, result in the deduction of the time necessary for pre-imaging from the allocation destined to the main part of the project. As a rule, pre-imaging runs are carried out in Service Mode, even for programmes whose main (spectroscopic) runs are conducted in Visitor Mode.