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).

	IFU: Grisms

The integral field mode is optimised for use in the H- and K-bands, but is available with most of the spectroscopy grisms . The throughput and performance at L and M will be limited by diffraction losses.

Unfortunately, with the IJ grism we are unable to focus the instrument - use with the IJ grism is therefore NOT recommended. We have a similar focus problem with the JH grism, though the situation is less severe. Users may use the JH grism with the IFU, provided the reduced resolution is acceptable.

Note also that because the spectra are initially staggered on the array, the resulting wavelength coverage given by the "scrunched" or spectrally-aligned data is slightly less than with normal long-slit spectroscopy. In other words, about 4% (~35 pixels) is chopped off the red and blue ends of each spectrum.

The spectral resolution with the IFU is equivalent to a 2-pixel slit. See the long-slit spectroscopy pages for further details.

	IFU: Sensitivity

IFU versus long-slit - should I, shouldn't I...?

The IFU is optimised for use in the H and K bands, though it may also be used at longer and shorter wavelengths. To characterise its performance, during commissioning, observations of the same bright standard star were obtained with the 2-pixel-wide long slit and then with the IFU. The same exposure times were used with each set-up; all grisms were checked. The stellar continuum was (optimally) extracted from a differenced pair of raw, long-slit data. A spectrum from a differenced pair of raw, IFU spectral images was extracted in the same way; in this case the strongest continuum spectrum seen on the array was used. The single IFU spectrum should therefore be comparable to the "peaked-up" 2-pixel long-slit spectrum (provided the source is centred on one IFU mirror slice) and the relative signal strengths should be a measure of the losses associated with the IFU optics. These losses are quantified in the table below.

The increase in relative transmission at longer wavelengths is due to the increase in reflectivity of the aluminium IFU mirror segments (with a smaller contribution from the lessening effect of scattering). Diffraction will reduce the overall throughput at longer wavelengths (although the relative throughput shouldn't decrease since this should be the same for the long-slit and the IFU).

Note also that, although the signal through the IFU is attentuated, so is the source of the noise: the IFU spectra in the above table are noticably less noisey than the long-slit data. So even for 50% light loss, the S/N on a given spectrum will only decrease by 0.70.

The IFU and Point Sources

Although the additional optics (and mask) used with the IFU do clearly result in some light loss, these losses may in some cases be regained because the IFU is a 2-dimentional spectrograph. The IFU may even be the best option for point sources if the seeing is bad and moderate spectral resolution is required (the alternative - use of a wider long-slit - would of course reduce the spectral resolution). Averaging adjacent rows and/or spectra from adjacent slitlets will in many cases (poor seeing and/or extended sources) improve S/N. The pixel scale along each 6"-long slitlet is 0.12"; the width of each slitlet is 2 pixels, or 0.24" (14 slitlets, or slices, gives the 3.3" width of an IFU image), so averaging adjacent rows in the scrunched spectral image would give square 0.24" x 0.24" pixels. Note, however, that averaging rows also introduces more read-noise, so please be wary of this with faint point/compact sources at shorter wavelengths.

IFU Sensitivities

For IFU sensitivity estimates for telescope proposals please use the values listed below:

• For Extended, continuum sources - Use column 3
• For Extended, line-emission sources; line spectrally RESOLVED - Use column 4
• For Extended, line-emission sources; line spectrally UNRESOLVED - Use column 5

• The surface brightness sensitivities are per resolution element (as opposed to per spectral pixel, used for point/continuum sources on the long-slit pages: note that per resln. element is more appropriate for line emission sources [line in a resln element], while per pixel is more appropriate for continuum flux spread across the whole wavelength coverage).
• The sensitivities assume nodding to blank sky, i.e. 30 minutes represents 15 minutes on source and 15 on sky.

• Values were therefore derived from the 4-pixel long-slit spectroscopy sensitivities, with the following assumptions:
  • 4pix to 2pix (x sqrt-2): The long-slit sensitivities were derived from observations obtained with a 4-pixel slit, whereas an individual spectrum on an IFU image is through a 2-pixel slit (with double the spectral resolution). For the sensitivity on an individual IFU spectrum, the figures from the long-slit spectroscopy page have therefore been reduced by a factor of sqrt-2, or by 0.4 mag. However, the quoted 4-pixel long-slit performance could probably be regained by averaging adjacent slices from the IFU spectral image and/or by binning over 2 pixels in the dispersion direction.
  • Nod to sky (x sqrt-2): The long-slit sensitivites assume nodding up-and-down the slit. This will not be possible with the IFU in most cases, so sensitivities were reduced by a further sqrt-2 (or 0.4 mags).
  • Transmission (x 1/0.8): To account for the transmission losses associated with the additional IFU optics (described earlier), we have also reduced sensitivities by ~1/0.8 (or 0.25 mag) E.G. if the IFU throughput in the K-band is 60% of the throughput for ordinary long-slit spectroscopy, then the S/N in an IFU spectrum will be the square root of 0.6 times the S/N expected with long-slit spectroscopy (assuming Poisson noise from the sky and source itself dominates the noise).

IFU "white-light" images

The white-light image produced by the DR essentially represents the collapse (along the dispersion axis) of the individual slices in the scrunched spectral image. These 1-D image strips are then displayed side-by-side to give a 3"x6" image. Note, however, that this image will not be as deep as a "normal" image of the source, because of the additional read-noise added into the data from the 1024pixels in the dispersion direction. The best spectrum in the scrunched image should, however, be "comparable" to a spectrum from the 2-pixel slit.

Units for PATT proposals...

When estimating exposure times for a patt proposal, please give surface brightnesses in W/m2/arcsec2 (or mags/arcsec2); a flux of, say, 10{-18} W/m2 will only be detectable if it is confined to a reasonably small area. A bright object that is extended over many arcseconds may not be a good target for the IFU!

	IFU: Read Modes and Noise

Readout modes and noise performance with the IFU will be the same as for UIST in spectroscopy mode .

	IFU: Saturation and Sky Counts

Again, these will be essentially the same as with the long-slit spectroscopy mode, except that

• the background limited exposure times on the long-slit spectroscopy page are for the 4 pixel slit; the slits in the mask used with the IFU are 2 pixels wide (and generally give twice the spectral resolution), and
• there are additional losses associated with the IFU optics discussed earlier

Consequently, at least twice as long will be needed for background-limited performance with the IFU!