THUMPER - a Two HUndred Micron Photometer for the JCMT

Under the very best atmospheric conditions at Mauna Kea, a narrow atmospheric window opens at 200 microns, with a peak transmission of around 25 per cent or more during very dry weather. Figure 1 shows the predicted atmospheric transmission for different levels of precipitable water vapour.

Figure 1: The predicted zenith atmospheric transmission at JCMT for various levels of precipitable water vapour between 0.1 and 1.0 mm.

THUMPER is a novel instrument, currently being built at Cardiff University, which has been designed to make use of this window and make 200 micron observations from the ground - something that has never been done before. THUMPER will have a resolution of 7 arcsec, with sensitivities comparable to that previously only attainable using airborne facilities such as KAO.

The 200 micron photometer system uses a very simple scheme to make the design, fabrication and operation as straightforward as possible. The optical layout is shown in Figure 2, and uses two mirrors with a corrector plate on the entrance window of the cryostat (as with SCUBA), to remove spherical aberration from the beam. The mirrors focus the beam into a 7-element array of Winston cones which act as feedhorns to couple the radiation from the telescope onto the detectors.

Figure 2: THUMPER optics and focal plane array of 7 detectors in a hexagonal arrangement.

The detectors are stressed Ge:Ga photoconductors operating at liquid Helium temperature (around 3.7 K at the altitude of Mauna Kea). A 200 micron filter with a passband carefully tailored to match the atmospheric window is located in front of the array of feedhorns. The detectors are read out using transimpedance amplifiers with cooled JFETs.

THUMPER will operate simultaneously with SCUBA by means of a dichroic beam-splitter in front of the SCUBA window which will allow the long wavelength (longer than 200 microns) radiation to pass unhindered into SCUBA (with 98 per cent transmission) and the short wavelength radiation to be reflected into THUMPER. Hence THUMPER will operate in `serendipity' mode throughout all SCUBA observations, in exactly the same way as SCUBA currently takes data at 450 microns simultaneously with 850 microns. Data acquisition will take place through spare SCUBA channels, allowing the system to be used easily in conjunction with SCUBA. Software will be provided through the standard SURF interface so that to the user, THUMPER will simply appear as an additional array within SCUBA.

THUMPER will provide powerful new data for the study of many different types of astronomical sources, ranging from young stars, protostars and pre-stellar cores to evolved stars and even nearby galaxies. It will help to answer many questions relating to such sources, since its operating wavelength lies close to the peak emission of cold dust (10-30K). Used in conjunction with SCUBA at the same resolution it will provide a unique facility for JCMT.

Previously, to obtain such broad wavelength coverage, it has been necessary to combine SCUBA data with data from airborne instruments or satellites such as ISO. Figure 3 shows the pre-stellar core L1544 at 200 microns seen by ISO and at 850 microns seen by SCUBA. Note the very different scales on the axes of the images. THUMPER will provide 200 micron imaging like ISO, but at the resolution of SCUBA.

Figure 3: Images of the L1544 pre-stellar core at 200 microns taken with ISO at around 80 arcsec resolution and at 850 microns taken with SCUBA at 14 arcsec resolution. THUMPER will be able to take 200 micron images with 7 arcsec resolution.

The combination of 200 micron THUMPER data together with 350, 450 and 850 micron SCUBA data - all at very similar resolution - will help to disentangle variations of temperature, density and optical depth in cold dust sources and lead to new understanding of the astrophysics of these objects.

THUMPER is due to be commissioned on JCMT early in 2003. In the first instance it will operate in PI mode as a private instrument, due to the anticipated problems with characterising the atmosphere and calibrating data at this new wavelength for ground-based astronomy. However, the THUMPER team are happy to collaborate with members of the community on individual projects. Eventually it is hoped to offer THUMPER as a common-user instrument on JCMT.

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