The United Kingdom Astronomy Technology Centre

Executive Summary

Project Scientist : Dr. Wayne Holland (Joint Astronomy Centre, Hilo HI)
Project Manager : Dr. William Duncan (Astronomy Technology Centre, Edinburgh)
Co-Investigators : Dr. Kent Irwin (National Institute of Standards & Technology, Boulder, CO), Prof. Anthony Walton (University of Edinburgh, Edinburgh), Dr. Matt Griffin (Queen Mary & Westfield College, London), Prof. Peter Ade (Queen Mary & Westfield College, London)

We propose to build a new continuum bolometer camera for the James Clerk Maxwell Telescope (JCMT). The instrument ("SCUBA-2") will capitalise on the success of the existing SCUBA camera and maintain the JCMT at the forefront of world submillimetre astronomy for the next decade. With a much larger field-of-view and sky background limited detectors, SCUBA-2 will be able to map several hundred times faster than SCUBA to the same noise level. All areas of astronomy are expected to benefit: large-scale cosmological surveys of various sizes and depths to probe elliptical galaxy formation, and the star formation history of the Universe. Closer to home, we expect SCUBA-2 to make tremendous advances in studying our own galaxy – particularly in providing a census of local star-formation activity (young protostars and pre-stellar cores – the first stages of star formation).

The JCMT potentially offers access to a large unvignetted field-of-view of nearly 100 arcmin2. By comparison, SCUBA ultilises only about 5 arcmin2 of this available area. The field-of-view of the new camera will cover as large a fraction of this area as possible with a minimum goal of 64 arcmin2. We propose to fill the re-imaged focal plane of the telescope with state-of-the-art transition edge sensors (TES). Unlike SCUBA, which requires the telescope secondary mirror to "jiggle" to produce a fully-sampled map, SCUBA-2 will instantaneously sample the sky in a way akin to CCDs or infrared arrays. Furthermore, we expect the per-pixel sensitivity of SCUBA-2 to be better than SCUBA by at least 50%. DC-coupled pixels will also remove the necessity to chop the secondary mirror to remove the large sky-offset. This will potentially allow more large-scale source structure to be visible, easier and more accurate flat-fielding (using the sky), better calibration (correcting for transmission variations and sky-noise levels), and even the exciting possibilities of correcting for submillimetre "seeing" (by using shift-and-add methods with rapid frame corrections to the secondary mirror). Image reconstruction techniques for two-beam chopping also tend to propagate noise, and so with only a single beam on the sky this problem will be minimized.

The baseline design for the instrument is relatively simple. It will operate at two fixed wavelengths (there will be no moving filter drum), although other wavelengths may be possible with a hardware change. In the heart of the instrument will be two arrays of TES detectors operating at 850 and 450 mm. There will be approximately 30,000 pixels in total (cf. 131 in the current SCUBA) and they will be cooled to 250mK to provide excellent sensitivity and background limited performance. The TES detectors will have low microphonic sensitivity (a problem with the current SCUBA) and excellent linearity over the passband of operation. The detector technology is expected to be applicable to other wavelengths (e.g. the infrared) and there is potential for industrial exploitation. The instrument will be cryogen-free, utilising closed-cycle 3He systems to cool the arrays and pulse-tube cryofridges to cool the optics and filters. This will give an enormous operational budget saving (the current SCUBA cryogen costs alone are close to $2000 a week!).

The SCUBA-2 project will be a collaboration between the UK Astronomy Technology Centre (UKATC), the National Institute of Standards and Technology (NIST), Queen Mary and Westfield College (QMW) and the Joint Astronomy Centre (JAC). The ATC will be responsible for the overall project management, as well as instrument integration and testing. The TES arrays and readout electronics will be designed and constructed by NIST (possibly with the assistance of the University of Edinburgh). QMW will offer their expertise (gained from many years of submillimetre technology development) in the areas of array baffling, filters, and system testing. The JAC, as the eventual recipient of the instrument, will provide the Project Scientist (at least in the definition phase), and will be involved from the early stages of the project in a number of areas.

The proposed design of SCUBA-2 is not without risk. There are complexities in a number of areas - primary amongst these are the multiplexed signal readouts and high data rates. Other problems are shielding from magnetic fields, baffling the array from stray light, and more processing power needed for array control. These are not considered insurmountable problems.

SCUBA-2 will allow the JCMT to fulfill its ultimate potential. The improved sensitivity and large field-of-view will allow the productivity of the JCMT to increase many fold - especially in a time when submillimetre interferometry with the SMA and the arrival of the first heterodyne arrays to the telescope, will mean less time available for continuum astronomy. Perhaps the most exciting prospect that SCUBA-2 will offer is in the statistical significance of large-scale surveys. For example, the observing time to reach the galaxy confusion limit at 850mm in good submillimetre weather will drop from ~50 hrs (e.g. the Hubble Deep Field study by SCUBA) to around 2hrs (with a much bigger area covered). Suddenly, we would also have the prospects for the first high-resolution submillimetre survey of the Galactic Plane (taking only about a week instead of several years…), the prospects of investigating the Initial Mass Function for stars down to limits of only a few Jupiter masses, and the ability to study the extended cold dust distribution in an unbiased selection of external galaxies.