Overview
VIMOS General Description
VIMOS is a multi-mode wide-field optical instrument mounted at the Nasmyth focus B of UT3. VIMOS allows imaging in UBVRIz and multi-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 table is intended for quick reference. Informations about the performance of the instrument can be obtained using the Exposure Time Calculator .
VIMOS Modes and Setups
| |
|
|
Wavelength |
Spectral |
Dispersion |
Spectral |
|---|---|---|---|---|---|---|
| Mode |
Scale |
FOV |
range (nm) |
Resolution |
A /pix |
multiplex |
| IMG UBVRIz |
0.205''/pix |
4 x 7' x 8' |
-- |
-- |
-- |
-- |
| |
|
|
|
|
|
|
| MOS LR Blue |
0.205''/pix |
4 x 7' x 8' |
370 - 670 |
180 |
5.3 |
4 |
| MOS LR Red |
0.205''/pix |
4 x 7' x 8' |
550 - 950 |
210 |
7.3 |
4 |
| MOS MR |
0.205''/pix |
4 x 7' x 8' |
500 - 1000 |
580 |
2.5 |
1 |
| MOS HR Blue |
0.205''/pix |
4 x 7' x 8' |
410 - 630 |
2050 |
0.5 |
1 |
| MOS HR Orange |
0.205''/pix |
4 x 7' x 8' |
520 - 760 |
2150 |
0.6 |
1 |
| MOS HR Red |
0.205''/pix |
4 x 7' x 8' |
630 - 870 |
2500 |
0.6 |
1 |
| |
|
|
|
|
|
|
| IFU LR Blue |
0.67''/fibre |
54" x 54" |
400 - 670 |
220 |
5.3 |
4 |
| IFU LR Red |
0.67''/fibre |
54" x 54" |
590 - 915 |
260 |
7.3 |
4 |
| IFU MR |
0.67''/fibre |
27" x 27" |
490 - 1015 |
720 |
2.5 |
1 |
| IFU HR Blue |
0.67''/fibre |
27" x 27" |
415 - 620 |
2550 |
0.5 |
1 |
| IFU HR Orange |
0.67''/fibre |
27" x 27" |
525 - 740 |
2650 |
0.6 |
1 |
| IFU HR Red |
0.67''/fibre |
27" x 27" |
645 - 860 |
3100 |
0.6 |
1 |
| IFU LR Blue |
0.33''/fibre |
27" x 27" |
400 - 670 |
220 |
5.3 |
4 |
| IFU LR Red |
0.33''/fibre |
27" x 27" |
590 - 915 |
260 |
7.3 |
4 |
| IFU MR |
0.33''/fibre |
13" x 13" |
490 - 1015 |
720 |
2.5 |
1 |
| IFU HR Blue |
0.33''/fibre |
13" x 13" |
415 - 620 |
2550 |
0.5 |
1 |
| IFU HR Orange |
0.33''/fibre |
13" x 13" |
525 - 740 |
2650 |
0.6 |
1 |
| IFU HR Red |
0.33''/fibre |
13" x 13" |
645 - 860 |
3100 |
0.6 |
1 |
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).
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" in amplitude 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 I 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 section on 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 modes for observation in MOS mode even when targets come from a pre-defined catalog. 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 as pre-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 II. Further consequences of this limitations is that the observation for MOS (again including for pre-imaging) will be taken only withing 2 h of the meridian. Motivated exceptions can be requested by the users through a waiver.
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 flat
field 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
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 different filter+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.
VIMOS Overheads for P82
|
Action
|
Time in minutes
|
|---|---|
|
Telescope Preset
|
6
|
|
IMG Acquisition + Instrument setup
|
3
|
|
MOS Acquisition + Instrument setup
|
15
|
|
IFU Acquisition + Instrument setup
|
10
|
|
CCD Read-out for all 4 quadrants (IMG,MOS,IFU)
|
1
|
|
Change of Filter (IMG)
|
3
|
|
Attached flats + arc (IFU,MOS)
|
5-8
|
|
Attached arc (MOS)
|
4
|
NOTE: Overhead times are approximated to the nearest integer number. There might be differences with the more reliable P2PP estimates.
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 the exposure time used for the acquisition. The numbers given in the table 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: