THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1997

4.2. Instrumentation Development

All elements of the programme outlined in the last Report have made progress, albeit with delivery slippages in two of the three instruments.

4.2.1. UKIRT Fast-Track Imager

The Fast-track imager, UFTI, will use a large-format (10241024) HgCdTe array sensitive from 0.85 to 2.5 microns to provide imaging through J,H,K and narrowband filters over a 92 arcsec field of view with high spatial resolution (0.09 arcsec pixels), to exploit the markedly sharper images of the upgraded telescope. The imager has been under development at Oxford University but suffered several setbacks towards the end of the year. Problems with fabrication of large vacuum components caused a programme slippage in October. At the end of the year the late delivery of detector arrays and, in early 1998, of the array controller, all conspired to move the anticipated time of first light from 1 April 1998, the target date at the time of the first UKIRT Board meeting in May 1997, to late June by the end of the year. ³

4.2.2. Michelle

Michelle, the largest and most complex instrument yet destined for UKIRT, also had its problems in 1997. This powerful imager-spectrometer is one of the most ambitious astronomical instruments ever built. As set out in more detail in the last report, Michelle will provide diffraction-limited imaging through broad- and narrow-band filters, and spectroscopy at spectral resolutions between 250 and 30,000, in the atmospheric windows between 7 and 25 microns wavelengths. It will employ an array of 320240 arsenic-doped silicon (Si:As) blocked-impurity-band (BIB) detectors, operating at temperatures around 5 K, which will be achieved by a Joule-Thompson refrigerator. Michelle will be shared with Gemini.

In 1997 Michelle had a number of successes as well as several problems. Early in the year staff shortages at ROE caused some slippage, while the detailed opto-mechanical design of the large aluminium mirrors proved more complex than expected. Throughout 1997 there was concern about the availability and capabilities of the Santa Barbara Corporation (now Raytheon) 320240 Si:As arrays. The project developed two levels of fallback options, a 256256 option from Rockwell, and a 128-square array which was tested by lending the engineering version to a collaborating group to use for astronomy. ⁴

A second cliff-hanger was provided by the central portion of the vacuum vessel, the largest of three sections. At the commencement of the project no supplier was known who could fabricate such a large vaccuum-tight structure in aluminium (stainless steel, the more common material on these scales, would have been much too heavy for Michelle). Manufacture by casting was therefore proposed, a supplier identified and an order placed. However the component was too large for processing to eliminate the porosity frequently encountered with cast aluminium, and the outcome was uncertain. Suspect areas of the casting were repaired by welding, but concerns were still acute. Since a company capable of fabricating the unit in aluminium had by then been identified, an order for a fabricated backup unit was placed in November 1997. Early in 1998 the cast unit failed its vacuum test and the fabricated version was adopted.⁵

4.2.3. The UKIRT Imager Spectrometer UIST

The project formally commenced at the beginning of the 1997/98 financial year, although considerable preparatory work had been undertaken before that stage. The overall design of the instrument took form rapidly, and a successful preliminary design review was held in December 1997. The design is based on the 10241024 InSb ALADDIN arrays recently developed at SBRC. To curtail cost and development time it deliberately concentrates on exploiting the accumulated expertise and experience at ROE. Filter wheels, focus units, etc. utilise pre-existing designs to a maximum degree. The design is all-transmissive, using grisms as dispersing elements in spectroscopic mode. It will be delivered with a pixel scale of 0.12 arcseconds per pixel and upgraded thereafter by the installation of a lens exchange wheel to provide 0 06 per pixel for adequate sampling of the better images. The focus mechanism has used a design for another cryostat and has already been manufactured. Array procurement has required to be accelerated in order not to delay delivery; fortunately it has proved possible to obtain the requisite movement of funds forward in the project.

In its spectroscopic modes UIST will be a highly versatile instrument offering suites of grisms to provide:

• Long-slit (120) spectroscopy with 1-, 2-, 3- and 4-pixel-wide slits at resolutions R (for a 2-pixel-wide slit) of 1600, when a whole atmospheric band is covered at once, and 4000, covering about half a band but with high enough resolution to minimise the impact of OH lines in the J,H and K bands. Grisms providing resolutions of 2000-3000 will offer excellent sensitivity in the longer-wavelength bands.
• A short-slit (20) cross-dispersed mode using the lower-dispersion (R1600) grisms to cover two atmospheric bands (I+J, H+K, K+L?) at once.
• A low-resolution mode using the cross-disperser to provide long-slit (120) spectra in the perpendicular direction on the array. These will extending across the I, J, H and K windows at once at a spectral resolution R200 (with a 2-pixel slit).

4.2.4. University of Hawaii Adaptive-Optics system (UH-AO)

The possible installation of the University of Hawaii Adaptive Optics system on UKIRT next year, and its delay because of secondary mirror print-through, was discussed in the last report. This system is now intended to go on Gemini in 1999 as a near-first-light facility and it is unlikely to be available on UKIRT until well into 2000.