FLAMES is the multi-object, intermediate and high resolution spectrograph of the VLT. Mounted at the Nasmyth A platform of UT2, FLAMES can access targets over a large corrected field of view (25 arcmin diameter). It consists of three main components:
- A Fibre Positioner (OzPoz) hosting two plates: while one plate is observing the other positions the fibres for the subsequent observations, therefore limiting the dead time between one observation and the next to less than 15 minutes, including the telescope preset and the acquisition of the next field.
- A medium-high resolution optical spectrograph, GIRAFFE, with three types of feeding fibre systems: MEDUSA, IFU, ARGUS.
- A link to the UVES spectrograph (Red Arm) via 8 single fibres of 1 arcsec entrance aperture.
Special observing software (FLAMES OS) coordinates the operation of the different subsystems, also allowing simultaneous acquisition of UVES and GIRAFFE observations with the observing modes listed in Table below. For combined observations, the exposure times for UVES and GIRAFFE do not need to be the same. Note that it is not possible to observe simultaneously in two GIRAFFE modes, or to observe the same target simultaneously with the two spectrographs.
- GIRAFFE is a medium-high (R=5500-65100) resolution spectrograph for the entire visible range, 370-950 nm. It is equipped with two gratings and several filters are available to select the required spectral range. Five additional fibres allow simultaneous wavelength calibration of every exposure. Each object can be only observed in one, or a fraction of a single echelle order at once. The fibre system feeding GIRAFFE consists of the following components:
- The MEDUSA fibers, which allow up to 132 separate objects (including sky fibres) to be observed in one go. Two separate sets of MEDUSA fibers exists, one per positioner plate. Each fiber has an aperture of 1.2 arcsec on the sky.
- The IFU: each IFU (deployable Integral Field Unit) consists of a rectangular array of 20 microlenses of 0.52 arcsec each, giving an aperture of 2 x 3 arcsecs. For each plate there are 15 IFU units dedicated to objects and another 15 dedicated to sky measurements. In the latter, only the central fibre is present.
- ARGUS: the large integral field unit ARGUS is mounted at the centre of one plate of the fibre positioner and consists of a rectangular array of 22 x 14 microlenses. Two magnification scales are available: ``1:1'' with a sampling of 0.52 arcsec/microlens and a total aperture of 11.5 x 7.3 arcsec, and ``1:1.67'' with 0.3 arcsec/microlens and a total aperture of 6.6 x 4.2 arcsec. In addition, 15 ARGUS sky fibres can be positioned in the 25 arcmin field.
GIRAFFE is equipped with one 2k x 4k EEV CCD (15 micron pixels), with a scale of 0.3 arcsec/pixel in MEDUSA, IFUs and ARGUS direct mode, and a scale of 0.15 arcsec/pixel in the enlarged ARGUS mode. GIRAFFE is operated with 39 fixed setups (31 high resolution + 8 low resolution modes).
- UVES is the high resolution spectrograph at UT2 of the VLT. It was designed to work in long slit mode but it has been possible to add a fibre mode (6 to 8 fibres, depending on setup and/or mode) fed by the FLAMES positioner to its Red Arm only. Only the three standard UVES Red setups are offered, with central wavelength of 520 (6 fibres=1+5 only), 580 and 860 nm respectively (see the manual for details). The spectra are not flux calibrated.
The standard readout mode of FLAMES-UVES is 225 kHz (unbinned) which ensures low readout noise. As of P76 a high-speed readout mode (625 kHz, unbinned, low gain) with increased readout noise but less overheads is offered in visitor mode only.
With an aperture on the sky of 1 arcsec, the fibres project onto 5 UVES pixels giving a resolving power of 47000. For faint objects and depending on the spectral region, one or more fibres can be devoted to recording the sky contribution. In addition, for the 580 nm setup only, a separate calibration fibre is available to acquire simultaneous ThAr calibration spectra. This allows very accurate radial velocity determinations. In this configuration, 7 fibres remain available for targets on sky.
Full list of instrument setups in the FLAMES User manual
|Spectrograph||Mode||N. Of Objects||Aperture ('')||Resolving Power (*)||Spectral Band [nm] (**)|
|UVES||Red Arm||7 + 1 calibration||1.0||47000||200|
|GIRAFFE||MEDUSA buttons||130 (w. sky fibres)||1.2||12000 - 24000||λ/12 to λ/24|
|GIRAFFE||MEDUSA buttons||130 (w. sky fibres)||1.2||7000||λ/9.5|
|GIRAFFE||IFU||15 (+15 sky fibres)||2 x 3||19000 - 39000||λ/12 to λ/24|
|GIRAFFE||IFU||15 (+15 sky fibres)||2 x 3||11000||λ/9.5|
|GIRAFFE||ARGUS||1||11.5 x 7.3 or 6.6 x 4.2||19000 - 39000||λ/12 to λ/24|
|GIRAFFE||ARGUS||1||11.5 x 7.3 or 6.6 x 4.2||11000||λ/9.5|
(*) In HR mode, the resolution and spectral coverage depend on setting used.
(**) λ is the desired observing wavelength.
The operation of FLAMES requires the observer to have his/her own targets coordinate list, with a relative astrometric accuracy better than 0.3 arcsec, when preparing their Phase 2. Bad astrometry will translate into large losses at the fibre entrance and therefore in poor performance of the instrument. The minimum object separation is 11 arcsec. The Fibre Positioner is able to place the fibres with an accuracy better than 0.1 arcsec.
During Phase 2 it is necessary to run dedicated software (FPOSS) that assigns the fibres to the selected objects. Considerations regarding the relevant astrometric effects induced by the atmosphere can be found in the FLAMES User Manual. Ideally the VLT guide star chosen should have R-band magnitude between 9 and 11 and should always be in the same coordinate system as the main targets.
All required standard FLAMES calibrations are acquired by the observatory staff during daytime. No nighttime calibrations are foreseen except for screen flatfields (so-called attached flatfields) if additional flatfielding accuracy is required.
For ARGUS mode, screen flatfields are taken for service mode programmes in twilight together with a spectrophotometric standard star with no cost to the user.