Results from the First SCUBA Polarimeter Tests

Polarimetry tests were recently carried out with SCUBA in the laboratory at ROE using the spare UKT14 polarimeter module. An electrically isolated mounting plate was used to attach the polarimeter directly in front of the SCUBA window after removal of the chopper wheel and laser assembly. The source signal was provided by a chopped blackbody viewed through the JCMT telescope simulator and the polarimeter was operated in the step and integrate mode. The output from two selected pixels were demodulated simultaneously using lock-in amplifiers and sampled with a PC based analogue to digital converter. This same PC was used for data reduction and analysis.

Microphonics tests

The microphonic effects on the bolometers produced by the movement of the waveplate were found to be minimal (much less severe than those experienced with UKT14) with a settling time well within a second. For this trial no special efforts were made to vibrationally isolate the mounting plate from the cryostat. When the waveplate was set to spin continuously, some induced noise was observed during the acceleration stage but was negligible in the constant velocity phase. It is therefore expected that a continuously spinning mode of observation may be possible.

Instrumental polarisation

A chromatic half-waveplate of 70 mm clear aperture was used to measure the IP across the long wavelength array at 850 micron. This caused some beam truncation since it was undersized with respect to the SCUBA window (89 mm). The IPs measured at the central pixel and at four extreme off-axis pixels (in the four cardinal directions) were in the range 0.8 - 1.7% with position angles between 0 and 45 .

Polarisation modulation efficiency

Two prototype achromatic half-waveplates were tested: the short-waveplate usable at all array wavelengths (350 - 850 micron) and the long-waveplate for the photometric wavelengths (1100 - 2000 micron). The measured polarisation modulation efficiencies were greater than 90% at all wavelengths. Reflections accounted for most of the transmission losses occurring at the waveplates (which will be reduced by antireflection coating), except at 350 and 450micron where absorption begins to be significant (5 - 15%). The QMW photolithographic analysers used proved to be perfect polarisers.

Measurement of a simulated faint partially polarised source at 850 micron

A sample of fluorogold (for which the percentage polarisation and position angle at 850 micron were well known from previous laboratory experiments at QMW) was placed in front of a small object plane aperture to simulate a partially polarised source. The blackbody was located some distance behind this aperture. A measurement carried out at high signal level (blackbody temperature 1200K) yielded, after IP correction, the expected percentage polarisation for this fluorogold sample of 2.7 ± 0.2% with position angle 43.5 ± 2.1 . The blackbody temperature was then lowered to its minimum level (~360K with some instability) to give an estimated source flux of ~500mJy which gives a polarised flux of ~14mJy. After an integration time of 80 minutes the polarisation obtained was 2.8 ± 0.8% with position angle 47.5 ± 7.7 , in excellent agreement with the high level value.

Conclusions

Polarimetry will be possible with SCUBA using essentially the same data acquisition and analysis methods used for the current UKT14 polarimeter and will yield a considerable improvement in polarimetric sensitivity.

Sye Murray & Ramón Nartallo, QMW

Wayne Holland & Walter Gear, ROE