SCUBA: An introduction

SCUBA is a submillimetre continuum array receiver, with a field of view 2.3 arcmin in diameter. It has two hexagonal arrays of bolometric detectors (or pixels); the Long-Wave (LW) array has 37 pixels, while the Short-Wave (SW) array has 91 pixels. Both arrays are used simultaneously by means of a dichroic beamsplitter, which in SCUBA's prime mode corresponds to observing at 450 microns on the SW array and 850 microns on the LW array. Each of the pixels has diffraction-limited resolution, corresponding to about 7.5 arcseconds at 450 microns, and 14 arcseconds at 850 microns. The arrays can be used to make maps, in either the "jiggle-map" or "scan-map" modes, or as photometers, where only the central pixel on each array is used. A polarimeter can also be attached to the front window of SCUBA which then allows polarimetric photometry and jiggle-map modes.

(SCUBA's original design allowed for observations also at 350, 750, 1100, 1300 and 2000 microns, but due to engineering problems which began in 1997, these windows are not currently available.)

SCUBA is cooled to below 100mK by means of a dilution refrigerator, which means SCUBA's sensitivity is limited by the photon noise from the sky and telescope background at all wavelengths. The background power is further limited by a combination of single-moded conical feedhorns and narrow-band filters. This, in combination with the JCMT's 15 m dish, provides SCUBA observers with unprecedented sensitivity at submillimetre wavelengths.

A more detailed description continues here.