THUMPER: a 200-μm photometer for ground-based astronomy

Atmospheric modelling predicts that a window at 200-mum occurs under very dry conditions at high altitude sites. The transmission can reach up to 30 % in the driest conditions, but also exists for as many as 80 nights per year at Mauna Kea. A 200-mum photometer, THUMPER, is currently under construction at Cardiff University for use at the JCMT to exploit this atmospheric window. THUMPER consists of a seven-element hexagonal array of stressed Ge:Ga photoconductors cooled to liquid helium temperature. Initial laboratory testing suggest an NEFD of 20 Jy/rootHz should be possible, under conditions of 0.5-mm pwv. A dichroic splits the beam between SCUBA and THUMPER, allowing simultaneous observations with THUMPER effectively acting as a third SCUBA array. Photometric measurements at 200-mum, in conjunction with SCUBA, will provide valuable information on cold dust sources in the temperature range 10 to 50 K. Since SCUBA fails to sample the peak of the Planck function at these temperatures, it is not possible to differentiate between temperature and density variations across a source using SCUBA data alone. THUMPER will provide these additional data at the same spatial resolution as SCUBA. This will provide an unprecedented combination of wavelength coverage and resolution when imaging sources such as protostars and pre-stellar cores.