VIMOS Integral Field Unit
The VIMOS Integral Field Unit is made of 6400 (80x80) fibres, coupled to microlenses. Here we give some details about spectral format and spatial geometry.
Grisms and spectral format
Six grisms are available in IFU mode. The grisms are the same as used in MOS mode, but spectral resolution and wavelength coverage are slightly different. The fibre to fibre distance on the detector is 5 pixels, corresponding to 1" on sky. The fibre FWHM is about 4 pixels. Therefore the spectral resolution is ~1.25 times the spectral resolution corresponding to a 1" slit in MOS mode.
|Grism||Order sorting filter||Wavelength coverage
common to all quadrants
|LR blue||OS blue||400-670 nm||220||5.3||4|
|LR red||OS red||590-915 nm||260||7.3||4|
|HR blue (NEW)||Free||370-535 nm||1440||0.71||1|
|HR orange||GG435||525-740 nm||2650||0.6||1|
|HR red||GG475||645-860 nm||3100||0.6||1|
IMPORTANT NOTE (1) A new set of four holographic HR_blue grisms has been installed in VIMOS on March 15 2012. Compared to the previous set, the peak efficiency has increased by about 65% and has moved to approximately 440~nm. The old set is not available anymore. Note that the resolution for this grism is given for the undeviated wavelength of 401.6 nm, which corresponds to 2100 at 587.5 nm, the reference wavelength for the old HR_blue grism.IMPORTANT NOTE (2) A new set of four holographic HR_red grisms has been installed in VIMOS on October 5 2005. Compared to the previous set, the efficiency has increased of about 50%. We remind that before that date, there was no HR_red grism in quadrant 4. The grism HR_orange was instead automatically inserted in quadrant 4 when requesting the grism HR_red. The efficiency curve of the old HR Red grisms is here.
IMPORTANT NOTE (3) Because the pseudo-slits are centered slightly differently in each quadrant, the spectral coverage changes from quadrant to quadrant (and it is different compared to the MOS mode). The spectral coverage for each quadrant derived from the automatic reduction of standard star observations with VIMOS pipeline is listed in the following table.
|LR blue + OS blue||390 - 670 nm||395 - 675 nm||400 - 680 nm||400 - 680 nm|
|LR red + OS red||570 - 915 nm||580 - 930 nm||590 - 930 nm||585 - 930 nm|
|MR + GG475||485 - 1015 nm||490 - 1020 nm||490 - 1020 nm||490 - 1015 nm|
|HR blue (NEW) + free||370 - 537 nm||370 - 535 nm||370 - 550 nm||370 - 563 nm|
|HR orange + GG435||525 - 745 nm||525 - 745 nm||525 - 745 nm||525 - 740 nm|
|HR red + GG475||635 - 860 nm||635 - 865 nm||635 - 860 nm||645 - 875 nm|
IFU Spatial geometry
The field of view is a square, and the spatial sampling is continuous.
The dead space between fibres/microlenses is below 10% of the fibre to fibre
distance. At the entrance of the IFU, a focal elongator offers two different spatial
samplings (magnifications): 0.67" per fibre, and 0.33" per fibre.
With the low resolution grisms (LR blue and LR red), the entire IFU FOV can be used (i.e., 80 fibres each side). Four parallel slits of fibres per quadrant (called "pseudo-slits") provide 4 stacked horizontal spectra along the dispersion direction. The field of view corresponds to 54" x 54" (when the magnification is set to 0.67" per fibre) or to 27" x 27" (when the magnification is set to 0.33" per fibre). A shutter can be used to partially mask the IFU head, leaving available a 40 x 40 fibre square. In this case, the FOV corresponds to 27" x 27" (when the magnification is set to 0.67" per fibre) or to 13" x 13" (when the magnification is set to 0.33" per fibre).
With the medium and high-resolution grisms (MR, HR blue, HR orange, and HR red) , the spectra of a single pseudo-slit will cover the entire length of the CCD. Therefore, only one pseudo-slit can be used. The IFU head will be partially masked by a shutter, so that only a square of 40 x 40 fibres is used. The field of view is four times smaller than with the high-resolution grisms, i.e.: 27" x 27" (with the magnification set to 0.67" per fibre) or 13" x 13" (with the magnification set to 0.33" per fibre).
All this is summarized in the following table.
|Field of View
|Number of fibres||Spectral Multiplex|
|LR blue, LR red||OFF||0.67"/fibre||54" x 54"||80 x 80||4|
|LR blue, LR red||ON||0.67"/fibre||27" x 27"||40 x 40||1|
|LR blue, LR red||OFF||0.33"/fibre||27" x 27"||80 x 80||4|
|LR blue, LR red||ON||0.33"/fibre||13" x 13"||40 x 40||1|
|MR, HR blue, HR orange, HR red||ON||0.67"/fibre||27" x 27"||40 x 40||1|
|MR, HR blue, HR orange, HR red||ON||0.33"/fibre||13" x 13"||40 x 40||1|
Instrument flexures of different importance in the different quadrants strongly affect the data. To minimize their impact on the science, at a prize of increased overheads, MOS and IFU observations need to have attached a NightCalib template. Since August 2010 VIMOS uses an active flexure compensation system. For IFU mode all corrections are done via a lookup table thus reducing the accuracy compared to MOS mode.