FORS1 and FORS2 Overview
FORS1, FOcal Reducer/low dispersion Spectrograph 1
FORS1 is a multi mode (imaging, polarimetry, long slit and multi-object spectroscopy) optical instrument mounted on the UT2 Cassegrain focus. FORS1 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), on a new mosaic of two 2kx4k E2V CCDs (15 µm pixels). The corresponding field sizes are 6.8' × 6.8' and 4.25' × 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. With its new detector FORS1 is very sensitive in the blue range (< 500 nm) but shows a lot of fringes above 650 nm.
Polarimetry with FORS1
The polarimetric modes are implemented in FORS1 only. They 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.
FORS1 summary tables
Below we summarise in two tables the operational modes and performance of FORS1. Those tables are intended as quick feasibility references only, proposers should refer to the detailed information (e.g. User's Manual, Exposure Time Calculator).
|
Instrument Mode
|
Mag-limit
|
|
Direct Imaging
|
U=25.9 B=27.6 V=27.3 R=26.6 I=25.8
|
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 new, high-throughput filters.
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 3x10e-4 on the optical axis to 7x10e-3 at a distance of 3 arcmin from it (V band). The problem is still under investigation.|
Instrument Mode
|
Rs = λ/Δλ
|
Mag-limit
|
|
MOS - movable slits [1]
|
260-1700
|
R=24.0-22.8
|
|
Longslit Spectroscopy [2]
|
260-1700
|
R=24.0-22.8
|
|
Spectropolarimetry [3]
|
260-1700
|
R=19.2-17.2
|
[2] In longslit spectroscopy the slit is chosen out of a set of 9 slits with fixed width between 0.3'' and 2.5''.
[3] Only offered with the SR collimator.
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 of 5 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 again those which for linear/circular polarisation allow a 1% accuracy in determination of degree of polarisation for one hour of total integration time.
FORS2, FOcal Reducer/low dispersion Spectrograph 2
FORS2 is offered at UT1 with a detector consisting of a mosaic of two 2k×4k MIT CCD (15 µm pixels). Compared to FORS1, the FORS2 mosaic provides greatly improved red sensitivity (> 750nm), but reduced blue-UV response (< 500nm). The image scale in the default readout mode (2x2 binning) is 0.25''/pixel with the standard resolution (SR) collimator and 0.125''/pixel in the high resolution (HR) mode. The field of view in these two modes is, respectively, 6.8' × 6.8' and 4.25' × 4.25'. An unbinned CCD readout mode is only offered for applications that specifically require it and must be explicitly requested in the proposal.Apart from its detector system, FORS2 is mostly identical to FORS1. All information given here above for FORS1, with the exception of the important differences described in this section, are hence also true for FORS2. The important differences are:
- FORS2 is optimised for the red (> 650nm) and FORS1 for the blue
- FORS2 has no polarimetric capability.
- FORS2 offers an extended set of high-throughput volume phased holographic grisms.
- FORS2 offers, in addition to the MOS unit of movable slitlets, a mask exchange unit (MXU) which can accommodate up to 10 exchangeable slit masks for multiobject spectroscopy of approximately 80 objects each.
- FORS2 offers HIgh Time resolution (HIT) mode in imaging and spectroscopy.
Spectroscopic Mask Mode (MXU)
In addition to the standard MOS mode with 19 slitlets, 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 inside the instrument. 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 MXU is only offered with the standard resolution collimator.
HIT Mode
HIT mode is available with FORS2 in imaging (visitor and service) and spectroscopy mode (visitor only). Only one-shift mode is available, with a range on filters for imaging and the 600B and 300I grisms for spectroscopy. The available times for a full shift across the mosaic are 1, 4, 16, 64, 256 sec, providing time resolutions for 1" on sky from 2.3msec to 0.6 sec.
Starting with P77, also a multiple-shift (MS) mode is available. This mode is predominantly implemented for fast spectroscopy and allows a block of rows to be shifted together. In the MS mode, two user-defined slits can be used which place the spectra of the target and a comparison star onto a small region of the CCD. After a pre-defined "wait" time, the rows of the CCD are rapidly (in ~50 microsec) shifted, causing the exposed region to be moved into the "storage area" (the unexposed region) of the CCD and a new region to be illuminated. This "shift and wait" scheme continues until the first pair of spectra taken reach the limit of the storage region and the CCD is subsequently read-out in the normal way.
FORS2 summary tables
Below we summarise in two tables the operational modes and expected 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).
|
Instrument Mode
|
Mag-limit
|
|
Direct Imaging
|
U=24.5 B=27.1 V=27.0 R=26.7 I=25.7
z=24.7
|
|
HIT imaging
|
--
|
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.
|
Instrument Mode
|
Rs = λ/Δλ
|
Mag-limit
|
|
MOS - movable slits [1]
|
260-2600
|
R=24.2-23.3
|
|
MXU - exchangeable masks [2]
|
260-2600
|
R=24.2-23.3
|
|
Longslit Spectroscopy [3]
|
260-2600
|
R=24.2-23.3
|
|
HIT spectroscopy [4]
|
660-780
|
--
|
[2] MXU is only offered with the standard resolution collimator.
[3] In longslit spectroscopy the slit is chosen out of a set of 9 slits with fixed width between 0.3'' and 2.5''.
[4] In HIT resolution mode slit masks with widths between 0.5'' and 5.0'' are available.
The spectroscopic Mag-limit given in the table are the R-band
magnitudes of a point source of zero colour, which would give a S/N of
5 per pixel at 650 nm (grisms 150I and 600RI) 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.
FORS Instrumental Mask Simulator (FIMS)
To prepare precise target acquisitions at Phase 2, ESO provides the FIMS software tool. FIMS is required when using FORS1 or FORS2 in several spectroscopic modes, and is also used to prepare occulting bar imaging and spectropolarimetry observations with FORS1. 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.