FIRST LIGHT WITH ROVER

The ROVER spectral-line polarimeter has just seen first light, although not yet at JCMT. It 'roved' to the IRAM 30m telescope in Spain in May 2003, under the care of Jane Greaves and Sye Murray - whom those of long memories may recall from the commissioning of the UKT14 and SCUBA polarimeters. We observed at 3mm wavelength, in a variety of conditions ranging from fog to snow to glorious sunshine (it was daytime!). Some of the first results are shown below.

The main aim of this observing run was to prove, once and for all, that millimetre spectra-line polarimetry gives real science results and is not contaminated by instrumental polarization effects. This has been a long-standing criticism since the early 1980's when the first attempts were made to measure weakly polarized lines from molecular clouds. JCMT results published over the last few years have been very impressive, but there are a number of subtle effects like sidelobe polarization that are hard to quantify. So, we took advantage of a unique opportunity to compare the polarization measured with a waveplate device (ROVER) with the same polarized source seen with a 'correlation polarimeter'. IRAM have been developing observations in this mode, using a pair of receivers accepting the two orthogonal planes of light and extracting the correlated (polarized) component. Our idea was that if the two polarimeters produce the same signal, by completely different methods, then it must be 'real'. There is probably no other telescope where this could be tried, and we pushed the system to the limits, including sampling the spectra at an impressive rate of 32 Hz!

Figures 1 and 2 show the results we obtained for the SiO Maser in the envelope of the star R Leo. Can you tell which is taken with which instrument?? (They are so close, at first we thought we'd analysed the same data twice by mistake!). The sharp-of-eye may notice that there is a flat line in the ROVER data - this is circular polarization which we can't measure with a half-wave plate. Also, the percentage of linear polarization is slightly less than with the IRAM 'Xpol' results, because the data are not yet corrected for the polarizing efficiency of the plate. Apart from that, the shape of the percentage and angle of polarization curves is exactly the same - just the proof that has been sought for so long.

Figures 1 & 2 - ROVER and Xpol spectra of R Leo in the SiO v1,J=1-0 line. From top to bottom: total intensity, percentage of linear polarization, percentage of circular polarization, direction of linear polarization.

We obtained a few other science results - most importantly, detecting polarized CO from Mira giant stars, which has never been done before - and also did some work on optimising observing modes. It turned out the simplest mode was the best: we simply took data as fast as the correlator could sample, while simultaneously spinning the waveplate. Each sample then corresponds to a small range of plate angles, and can be analysed as though it is all one fixed angle (which is what we get with SCUBA polarimeter data, for example). To analyse the data, all you have to do is compare the time stamps of the spectra and the polarimeter cycles - all done by logging and with a single TTL pulse. This continuous spinning observing mode (another 'first') was very efficient and is the way we plan to observe also at JCMT.

The instrument performed beautifully and the only real problem we had was fitting it in the receiver cabin! I would very much like to thank the ATC team who built the polarimeter (especially Ian Laidlaw and Brenda Graham), all the IRAM staff who helped us, and the Directors who allowed us to take the risk of removing one of the delicate beam-splitters so we could slide the polarimeter module in instead. Our IRAM colleagues Clemens Thum and Helmut Wiesemeyer also did heroic work with getting everything set up and having reduction software all ready so that we saw the polarization results immediately they came in. We are all now very pleased with the data (and also all very fit after running up to the cabin to check the polarimeter rotation - it's a long way to go, halfway up a 30m!).

The final stages of the ROVER project are to design and test the JCMT waveplate - hopefully this will be achromatic covering both the A and B bands, so there will be no need to swap plates. ROVER should be coming out in spring 2004 to be included in the late stages of ACSIS commissioning, and will be available to the community as soon as possible thereafter. And when HARP-B comes online, ROVER will also provide the world's only imaging spectral polarimeter for the submillimetre regime. The scientific advances we can then make should be as dramatic as those made with the SCUBA polarimeter.

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