SPECIAL REPORT: UIST - a new 1-5µm imager/spectrometer for UKIRT

Suzanne Ramsay Howat

UKATC, Edinburgh, UK.

Introduction

A new instrument combining imaging and spectroscopy over the 1-5µm wavelength region will be delivered to UKIRT early in 2002. The UKIRT Imager Spectrometer (UIST) will be available for "shared risks" observations during the latter half of semester 02A.

UIST has as its primary scientific goal the exploitation of the improved image quality now routinely delivered by UKIRT. Combining the functions of the existing UKIRT cameras with enhanced spectroscopic modes and a 1024x1024 pixel array, UIST offers great improvements in sensitivity and observing efficiency for the UKIRT community of astronomers. UIST employs one of the latest 1-5µm arrays (the 1024x1024 pixel ALADDIN array from the Raytheon Infrared Centre of Excellence) which have 16-times more pixels than CGS4. The UIST design not only provides moderate resolution spectra over broad wavelength ranges with a 2arcmin long slit mode, but also permits the introduction of spectroscopic modes new to UKIRT. The most novel of these, and arguably the most powerful, is the integral field mode, which provides 3D spectroscopy in a common-user cryogenic instrument for the first time.

UIST in the lab in Edinburgh

UIST is being built at the UK Astronomy and Technology Centre (ATC) in Edinburgh using many of the design principles that have been successfully used on CGS4, SCUBA and Michelle (see Figure 2 below). In August, an important milestone was reached when the UIST team obtained laboratory "first light" on the 1024x1024 science-grade array (shown above). This all-important first full image signals that the instrument is in the last stages of testing before delivery to the Joint Astronomy Centre. The opto-mechanical components of the instrument are fully assembled and the electronics and software are capable of handling the output from the 1million pixels of the array. The electronic system is based on the Michelle EDICT array controller, extended to handle the larger number of pixels from the UIST array. The UIST software and observation control for the astronomer will fit into the ORAC system already in use at the telescope; indeed, the instrument control and data acquisition system should immediately be familiar to astronomers who have visited the telescope in recent years.

FIGURE 2: UIST under test in the lab at the ATC. The instrument is cooled using liquid nitrogen for operation at 60K to reduce the infrared background.

During the past year at the ATC, many of UIST's functions have already been confirmed using the engineering grade array. The ALADDIN arrays consist of four electrically independent quadrants of 512x512 pixels. Information gleaned from observations with this array have given early indication that the imaging and spectroscopy specifications will be met. Preliminary tests of instrument flexure have also been carried out which demonstrate that the internal components of UIST are free from flexure. The instrument will undergo full flexure testing on its telescope mounting trusses in September. Acceptance tests, when JAC staff will visit Edinburgh to accept the instrument for use at the telescope, are planned for November.

UIST scientific specifications

UIST will offer the following scientific modes:

• imaging with pixel scales of 0.12 arcsec or 0.06 arcsec over the full 1-5µm range (see Table 1 for available filters)
• long slit spectroscopy (with a 2 arcmin-long slit) with low (R~1500) and moderate spectral resolution (R~3500)
• cross-dispersed JHK spectroscopy with R~2000 and a 10-20 arcsec slit
• polarimetric capabilities for imaging and spectroscopy with a 20 arcsec field using IRPOL2
• integral field spectroscopy over a (rotatable) 3.3x6.0 arcsec region of the sky.

The plate scale for imaging is remotely selectable from lenses located on a cooled wheel so that UIST can respond rapidly to changing seeing conditions or science requirements. The set of filters for broad and narrow band imaging are listed in Table 1 (the full complement of filters is not yet determind -- there will be more!). Acquisition of objects for spectroscopy, by first imaging and then adopting the spectroscopic instrument configuration, will simplify the use of narrow slits for high spatial resolution spectroscopy.

The dispersing elements in use with UIST are grisms which offer a fixed wavelength range. Grisms are available to cover the full 1-5µm range with two resolving powers offered for JHK. A list of the grisms and their measured wavelength ranges and resolutions is given in Table 2. Polarimetry is provided via the combination of a Wollaston prism in the UIST cryostat and IRPOL2. We expect that the Wollaston may be used with any of the grisms indicated in Table 2. The field of view for polarimetry is 20 arcsec. The same field is available for imaging- and spectro-polarimetry.

UIST Integral field spectroscopy

UIST's newest mode will be the integral field mode. UIST uses an image slicing design to provide spectroscopy of a ~3.3x6.0 arcsec (rotatable) area of the sky, sampled every 0.24x0.48 arcsec. The image slicing mirror re-formats the rectangular input field into a 'staggered' slit (illustrated in Figure 3) which then passes through the rest of the UIST optical system as if it were a long slit.

FIGURE 3: A schematic showing how a ~3.3x6.0 arcsec patch of sky is "sliced" onto a staggered slit which then projects 17 (of these 18) spectra onto the array (the spectrum from slit 1 is off the top of the array). Tests indicate that 14 of the 18 slices are usable (note the weaker spectra from slices 2-4 [slice 1 is missing]). Each slitlet is a 0.24x6.0 arcsec strip of the field (the slices are two 0.12 arsec-pixels wide, so 14 slices correspond to 3.3 arcsec). The IFU spectral image on the right is of an Argon arc lamp observed at K. The ~3.3x6.0 arcsec box (excluding the first 4 slices which are shaded) is the usable area on sky.

The integral field mode, optimised at K band, will be available with any of the grisms though the throughput and performance at L and M will be limited by diffraction losses. An example IFU arc spectrum is shown in Figure 3. Data reduction for the IFU spectral mode will be a significant challenge. UIST on-line reduction through ORAC will produce a datacube from the IFU spectral images which can be manipulated using the 'Datacube' software developed for STARLINK by Alasdair Allan.

Predicted Sensitivities

These estimated sensitivities are based on the sky backgrounds measured with UFTI. Please note that these figures are meant only as a rough guide; more accurate figures will be made available on the UIST home page at JAC. Use ONLY the figures on the UIST home page for telescope proposal preparation.

Imaging sensitivities (with the 0.12 arcsec camera) assume a 50% throughput for UIST and a read noise of 20 electrons.

The sensitivity for spectroscopy (given here for R~3000) depends more closely on the slit width chosen and the delivered image quality. These estimates in Table 4 assume that 60% of the flux falls in a two pixel slit.

More detailed estimates of sensitivity will become available shortly; please keep an eye on the UIST web links from the UKIRT home page ( http://www.jach.hawaii.edu/UKIRT/instruments/uist/uist.html). In the meantime, please contact Suzanne Ramsay-Howat at the ATC (skr@roe.ac.uk) or the UIST instrument scientists at the JAC, Sandy Leggett (s.leggett@jach.hawaii.edu) or Chris Davis (c.davis@jach.hawaii.edu) with specfic questions.