Instrument's Characteristics
The two FORS instruments are designed as focal reducer multi mode instruments for the Very Large Telescope. The wavelength range of operation is between 330nm and 1100nm. Two spatial resolutions and hence field sizes can be selected by exchange of the collimators. The resulting field of view is 6.8'x6.8' with the standard resolution collimator (SR) and 4.2'x4.2' with the high resolution collimator (HR) for the mosaiced CCDs in operation on both FORSes.Instrument Optics
The main instrument optics consists of two remotely exchangeable F/15 collimators of focal length f1233mm (COLL_SR) and f616 mm (COLL_HR) giving a 90 mm and a 44.5 mm diameter pupil. Together with the f280mm, F/3.1 camera the reduction factors are 4.4 and 2.2 respectively. The plate scales on the detectors are 0".25 and 0".125 for the SR and HR collimators with the standard 2x2 binned readout modes. There are no significant differences in image quality between unbinned SR mode and binned HR mode (both would provide plate scales of 0".125), but for seeing of 0".3 and better the HR collimator is expected to improve the image quality.
Detectors
The recently
upgraded FORS1 detector system consists of two 2kx4k E2V CCDs
(15µm pixel size). The E2V CCDs provide much higher response in the
blue and UV wavelength range below 600 nm, but suffers from strong fringing
above 650 nm. The vignetting is expected to be the same as experienced with
the MIT mosaic of FORS2 (see below).
The FORS2
detector system consists of two 2kx4k MIT CCDs (15µm pixel
size). The MIT CCDs provide much higher response in the red
wavelength range beyond 650nm, with impressively low fringe
amplitudes. The response in the UV below 400nm drops however
significantly. In standard resolution SR-mode the CCD mosaic will be
larger than the unvignetted field
in the focal plane. In high resolution HR-mode the corners of the
CCDs are vignetted by the
camera optics.
FORS1+2 components
| Filters | Grisms | FORS1 Polarimetry | MOS | FORS2 MXU | LADC | ECU |
Standard Instrument Configuration
Both instruments are operated
in the pre-defined configuration as specified in the
FORS User Manual
for the 3 broad band filter wheels. The configuration is fixed for the
subsequent observing periods and only filters and grisms which are
installed can be requested in proposals, setup requests of visiting
astronomers or service mode observing blocks. Filter/grism combinations can
be requested if the filter and the grism is not in the same wheel. Narrow and
Intermediate band filters will be installed into the interference filter
wheels and can be requested at any time.
Observing Modes
All modes are offered in visitor and service mode with the exception of the high time resolution spectroscopic mode (HIT-S) which requires a special instrument setup.
| FORS1 observing modes: | ||
|---|---|---|
| IMA | imaging | fast |
| OCC | imaging with occulting bars | FIMS |
| LSS | longslit spectroscopy | fast |
| MOS | multi-object spectroscopy (movable slits) | FIMS - SR collimator only |
| IPOL | imaging polarimetry | fast |
| PMOS | multi-object spectropolarimetry | FIMS & fast** - SR collimator only |
| ** fast mode PMOS observations are only possible for a single target on 1 slit | ||
| FORS2 observing modes: | ||
|---|---|---|
| IMA | imaging | fast |
| OCC | imaging with occulting bars | FIMS |
| LSS | longslit spectroscopy | fast |
| MOS | multi-object spectroscopy (movable slits) | FIMS - SR collimator only |
| MXU | multi-object spectroscopy (masks) | FIMS - SR collimator only |
| HIT-I | high time resolution imaging | fast - SR collimator only |
| HIT-S | high time resolution spectroscopy | fast - SR collimator only - visitor mode only |
| HIT-MS | high time resolution multiple shift mode | fast - SR collimator only - visitor mode only |
All observing modes marked with "FIMS" will require interactive mask preparation with the FORS Instrument Mask Simulator fims.. All modes marked with "fast" can be directly prepared with the phase 2 proposal preparation tool p2pp. In some cases both options can be selected. MXU, MOS, PMOS and HIT modes are only supported with the standard resolution (SR) collimator.