IFU: Optical Parameters

UIST uses an image slicing design to provide spectroscopy of a 3.3arcsec x 6.0arcsec (rotatable) area of the sky, with a plate scale of 0.24"x0.12". The image slicing mirror comprises 18 segments or "slices" that are each 0.24"x6.0" in size; 14 adjacent slices are useable, giving a spatial coverage on the sky of 3.3arcsec x 6.0arcsec.

The mirror slices re-format the rectangular input field into a 'staggered' slit (see Figure 1 below) which then passes through the rest of the UIST optical system as if it were a long slit. This "long-slit" data may then be broken up and reformated to reconstruct 3.3arsecx 6.0arcsec images of the target, one for every resolution element of the grism (so up to 1000 images!).

To put it another way, its like observing fourteen 6"-long, 0.24"-wide, parallel slitlets simultaneously; these "mini-slits" are observed side-by-side, thereby covering an area of 3.3" x 6.0"

Figure 1: A schematic showing how a ~3.3x6.0 arcsec patch of sky is "sliced" onto a staggered slit which then projects 14 (usable) spectra onto the array. Each adjacent slitlet covers a 0.24" x 6.0" strip of the field (the slitlets are each two-pixels, or 0.24", wide, so 14 slitlets correspond to 3.3 arcsec). The IFU spectral image on the right is of an Argon arc lamp observed at K. The ~3.3" x 6.0" box (excluding the first 4 slitlets which are not used) is the usable area on sky.

As with all UKIRT instruments, sophisticated pipeline software (written by Stephen Todd and further developed by Brad Cavanagh) is available at the summit (described later in this document) to help "reconstruct" your IFU data. The DR should ultimately yield a data cube, comprising a stack of 2-D images spanning the full wavelength range available from the grism used. The data shown below were obtained during early UIST commissioning and reduced with an early version of the software.

Figure 2: The active galaxy NGC1068, imaged using the Integral Field Unit. The greyscale image in the foreground shows the galactic nucleus in white light (integrated in wavelength). Above is an image at a single wavelength (H2 2.12 microns - this is one of the hundreds of images that add up to make the white-light image), and to the right is a spectral slice showing a spectrum across the entire nucleus (this time the image is collapsed in one spatial direction).