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	Spectral resolution (1 fibre)	Dispersion (Å/pix)	Multiplex
LR blue	OS blue	400-670 nm	220	5.3	4
LR red	OS red	590-915 nm	260	7.3	4
MR	GG475	490-1015 nm	720	2.5	1
HR blue	Free	415-620 nm	2550	0.51	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_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 (2) 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.

Grism	Q1	Q2	Q3	Q4
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 + free	415 - 620 nm	415 - 620 nm	415 - 620 nm	415 - 620 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.

Grisms	Shutter	Spatial Sampling arcsec/fiber	Field of View (arc sec2)	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

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. The following page show the flexures as a function of rotator angle.