VIMOS IFU components:
numbering scheme and mapping

We describe here the conventions used by the IFU pipeline recipes to identify IFU fibers, IFU masks and pseudo-slits.

VIMOS has four IFU masks that are numbered as the VIMOS quadrants to which they correspond (i.e. counterclockwise, with the same convention used by the cartesian plane). Each mask hosts 4 IFU pseudo-slits . The pseudo-slits are numbered from 1 to 4 as shown in the figure:

IFU masks and pseudo-slits. The pseudo-slit 1 is more separated than the others, it is at the bottom in mask 1 and 2, and on the top in mask 3 and 4. Each pseudo-slit hosts 400 fibers, divided into 5 blocks of 80 fibers each. Each pseudo-slit produces a sequence of 400 spectra that are counted successively from left to right.

Each pseudo-slit corresponds to a specific 20x20 region of the IFU head as shown in figure:

IFU head and pseudo-slits position. The position of all the pseudo-slits on the IFU head is shown here. The position of each individual fiber is listed in the IFU tables (see below). The dimension of the IFU head is 80x80 and each pseudo-slit covers a 20x20 portion. In High Resolution and Medium Resolution observations only the four central pseudo-slits (pseudoslits number 2) are used, resulting in a single sequence of 400 spectra per quadrant. Instead, in general, in Low Resolution all the 16 pseudo-slits are used, resulting in 4 sequences, each of 400 spectra (i.e. a multiplexing factor of 4)

Note on fiber visiblity

The theoretical number of 400 fiber spectra per pseudo slit is not reached. Two fibers in the middle of each block of 80 fibers are typically missing because they are vignetted by the IFU head shutter. There is also vignetting present at one border of each CCD. The actual number of lost fibers cannot be predicted since the positions of the fibers on the raw images vary within 2 to 5 pixels. Depending on the quadrant, 20 to 40 fibers are typically lost on each pseudo slit.

IFU tables
The pipeline recipes ( vmifucalib, vmifustandard and vmifuscience) work with one quadrant at the time and, in addition to other products, they all produce extracted fiber spectra images.
Correspondence between raw and extracted spectra position: The raw spectra starting from the left side of each pseudo-slits are extracted and stored successively in the product image starting from the bottom first row. In Low Resolution, where each quadrant frame contains 4 pseudo-slits, the product images have 1600 rows with the 400 spectra from pseudo-slit1 at the bottom and the 400 spectra from pseudo-slit 4 at the top. In High and Medium Resolution, where each quadrant frame contains the spectra from one pseudo-slit only, the product images have 400 rows. Redder wavelengths are toward the top in the raw images and toward the right in the product images.
Correspondence between each row (i.e. each spectrum) of the extracted spectra product image with the fibers position on the IFU head: It is described in the
IFU tables, that have the following configuration:

column name	description
ROW	extracted spectra image row, counted from the bottom frame starting from 1.
L	X coordinate on the IFU head, counted from left, ranging from 1 to 80
M	Y coordinate on the IFU head, counted from bottom, ranging from 1 to 80

There is one set of 4 IFU tables (one per-quadrant) for High and Medium Resolution observations, and one set of 4 IFU tables for Low Resolution observations. The IFU tables are included in the Service Mode packages and can be downloaded here:

IFU-tables Low Resolution (download: ifutablesLR.tar)
IFU-tables High/Medium Resolution (download: ifutablesHR.tar)

VIMOS IFU components: numbering scheme and mapping

VIMOS IFU components:
numbering scheme and mapping