VIMOS General Description

VIMOS is a multi-mode wide-field optical instrument mounted at theNasmyth focus B of UT3. VIMOS allows imaging in UBVRIz andmulti-object low (R ~ 200) to high (R ~ 2500) resolution spectroscopy.The wavelength range covered in the two spectroscopic modes goes from 360 to 1000 nm. VIMOS is made of 4 quadrants of 7'x8' each separated by a gap of ~ 2'.

For MOS observation masks are prepared using the Mask Manufacturing Unit (MMU) on Paranal.The typical number of slit per quadrant is 150 at high resolution and up to 750 in low resolution.

VIMOS is also equipped with an integral field unit (IFU) with 6400 microlenses coupled to fibers, with a choice of two spatial samplings (magnifications): 0.67'' per fiber or 0.33'' per fiber. With the lower spectral resolution settings, the IFU covers on the sky a 54'' x 54''FOV (using the 0.67'' magnification), or a 27'' x 27'' FOV (using the 0.33'' magnification). A quarter of these fields are covered at intermediate to higher spectral resolutions.

The following table describes the operational modes and basic performances of VIMOS. This tableis intended for quick reference. Informations about the performance of the instrument can be obtained using the Exposure Time Calculator .

Notes:
In MOS mode, with the LR and MR settings, the actual spectral range depends also on the order separation filter used (the values given in the table refer to the standard filters). With the HR settings, the actual spectral coverage depends also on the position of the slit in the field of view. The ranges given in the table refers to the slit at the field center. For the new HR blue it corresponds for a slit in Q1 positioned approximately at y=1220 in the acquisition image.
Spectral resolution refers to what given by a 1'' slit width in MOS mode, and by 1 fiber in IFU mode.
Spectral multiplex represents the number of slitlets that can be accommodated along the dispersion direction.

VIMOS Observations requirements

Limits in Imaging observations

Imaging observations with VIMOS involving offsets greater than 30" inamplitude very often produce the need to change the guide star during the execution of the OB, resulting in large operational overhead; OBs involving offsets larger than 30" are not allowed. In case of large offset patterns it is recommended to prepare separate OB's with the appropriate guide star for each offset position. Please take that into account in the overheads computations and observing time request during Phase 1 applications.

VIMOS Mask Preparation Software

For Multi Object Spectroscopy observation VIMOS uses masks of invar. A software package called the "VIMOS Mask Preparation Software" (VMMPS) is released to the user for slit definition and positioning during the Phase 2 preparation. This software can be downloaded from the tool sectionon this page menu.Using VMMPS, the users can define, rectangular slits of widths larger than 0.6" and shorter than 30".Tilted slits can also be defined.

Masks need to be prepared well in advance for both service and visitor modes

Pre-imaging

Pre-imaging with VIMOS is mandatory in both visitor and service modesfor observation in MOS mode even when targets come from a pre-definedcatalog. Pre-imaging is done only in service mode, accordingly separate runs must be requested for pre-imaging in the Phase 1 proposals .

Pre-imaging must be carried out with the R filter

Pre-imaging is carried out typically 2 month in advance of the spectroscopic follow up. User asking for MOS observation should verify that the pre-imaging can be done in advance, and that enough time for the preparation of the masks and execution of the following spectroscopic observations still remains.

Limits in MOS observations

VIMOS is not equipped with an Atmospheric Dispersion Corrector,neither in imaging nor in spectroscopic mode. Special operations procedures have therefore to be taken as to minimize the losses induced by atmospheric effects (field differential refraction and chromatic dispersion). A series of plots illustrating this atmospheric effects is presented in the following link . In this page you will also find references to a more complete study of this effect.

As a consequences of this effects, MOS observations as well aspre-imaging will be taken at a pre-determined position angle on the sky, with slits oriented N-S. This corresponds to a field orientation of 90 degrees. Any other request of field orientation should be clearly explained in the proposal and requested as a waiver during the Phase 2.

A further consequence of these limitations is that MOS observations (including pre-imaging) will be executed only withing 2 h of the meridian for the N-S orientation, and 3h from the meridian for the E-W orientation. Motivated exceptions can be requested by the users through a waiver. If PILMOS was used for mask preparation we allow only N-S orientation.

Specific instructions for MOS observing runs in visitor mode are given here.

Night time Calibrations in spectroscopic modes

In both spectroscopic modes, instrumental flexures introduce flatfield residuals and wavelength calibration offsets if calibrations are taken at a very different rotator angle to the science data. In order to minimize these effects night time screen flats and arcs calibrations are mandatory for all IFU settings. Because of the VIMOS Upgrade Project underway, during P85 and P86it will be mandatory to include at least 3 flats and 1 arc attached night calibrations for all MOS settings. See the latest VIMOS User Manual for details.The overhead of this night calibrations have to be taken into account properly inthe calculations of the total overhead.

In MOS and IFU mode, only one filter+grism combination per OB is permitted.Users who want to observe the same targets with different filter+grism combinations are requested to submit separate OBs for differentfilter+grism combinations, and to consider the respective overheads.

VIMOS Overheads

The following table contains the values used for the estimation of the overhead during the Phase I preparation of VIMOS observations.

NOTE: Overhead times are approximated to the nearest integer number. There might be differences with the more reliable P2PPestimates.

Examples

IMG: Suppose that we want to do imaging in 5 filters, taking 1 exposure each filter (shutter time for each exposure is [Exp time] ). The total telescope time is obtained calculating:

MOS: Suppose that we want to do MOS spectroscopy taking 3 exposures per OB (shutter time for each exposure is [Exp time] ), and attaching only wavelength calibration (mandatory with LR blue and HR blue).The total telescope time is obtained calculating:

Note that in MOS mode the actual overheads will depends also on theexposure time used for the acquisition. The numbers given in thetable refer to a shutter time for acquisition of 1 m (repeated twice).

IFU: Suppose that we want to do IFU spectroscopy, taking 3 exposures per OB(shutter time for each exposure is [Exp time] , wavelength calibration and screen flats are mandatory).The total telescope time is obtained calculating: