Instrument Name:FORS2
Telescope:Very Large Telescope
Telescope Aperture:8m
Telescope Port:Cassegrain
Commissioning Date:01.04.1999
Polarimetry Type:Imaging and spectropolarimetry
Stokes Parameters:I Q U V
Wavelength Range:3500-9000 A
Modulator Type:Superachromatic Rotating Retarder
Modulator Type Other:
Modulation Frequency: Set a fixed positions
Analyzer Type:Wollaston Prism
Analyzer Type Other:
Accuracy Zero Point: 0.03% to 0.1%, depeding on instrument mode
Accuracy Scaling:
Wavelength Range:3500-9000
Field of View:6.8 arcmin x 6.8 arcmin
Resolution:Site seeing is often as good as 0.8" or better
Pixel Scale:0.125 arcsec (1x1) or 0.25 arcsec (2x2)
Pixel Dimensions:15 um
Sensor Type:CCD
Limiting Magnitude:
Coronograph Type:
Coronograph IWA:
Adaptive Optics:No
DM, Number of Actuators:
DM Type:
WFS Type:
WFS Update frequency:
Limiting Magnitude:
Wavelength Range:3500-9000
Spectral Resolution:from ~150 to 3500 depending on grism and slit width
Limiting Magnitude:
Instrument Image:52090fadb5c4a-eso9857a.jpg
Instrument Image Caption:FORS at the Cassegrain focus of the VLT Unit 1 "Antu" (Credit: ESO)
Instrument Reference Paper:Appenzeller, I., Fricke, K., Furtig, W., et al. 1998, The Messenger, 94, 1
Known Issues:Instrumental polarisation off axis
(see Patat & Romaniello, 2006, PASP, 118, 146)
Cross-over from linear to circular polarisation (Bagnulo et al., 2009, PASP, 121, 993)
Instrument Contact:
Instrument Website:
Planned Upgrades:
Science Description:FORS is attached at a 8m telescope. It can do imaging polarimetry and low resolution spectropolarimetry, and has good capabilities in the continuum, therefore it has as vast range of applications, including surveys of stellar magnetic fields, studies of solar system objects, quasars, AGNs, etc. Because of the low spectro-resolution, FORS is not suitable for Zeeman Doppler imaging.
Target Types:
Sample Image/Result:52090fadb676f-HD94660.jpg
Sample image Caption:The polarised spectrum of the magnetic Ap star HD 94660 observed with FORS. The top panel shows the observed flux F (black solid line, in arbitrary units, and not corrected for the instrument response), the V/I profile (red solid line centred about 0), and the null profile (blue solid line, offset by -0.75% for display purpose). The null profile is expected to be centred about zero and scattered according to a gaussian with sigma given by the V/I error bars. The V/I error bars are represented with light blue bars centred about -0.75%. The slope of the interpolating lines in the bottom panels gives the mean longitudinal field from V/I (left bottom panel) and from the null profile (right bottom panel) both calculated using the H Balmer lines only. The corresponding field values values are -1885 +/- 90G and -192 +/- 62 G, respectively. Note that the field is calculated from the null profile only for quality check purpose, and it is expected to be consistent with zero.
Sample Science Papers: