Imaging- and Spectro-Polarimetry with UIST

Introduction

The IRPOL module at UKIRT provides polarimetry between 1 and 5 microns via the combination of Wollaston prisms in the UIST cryostat and warm waveplates. UIST contains focal-plane masks for imaging polarimetry, coronographic imaging polarimetry and spectro-polarimetry. The Wollastons can be used with any of the broad or narrow-band filters for imaging-polarimetry (including Lp and Mp), or with any of the grisms for spectro-polarimetry (there is a Wollaston in each grisms wheel, so spectropolarimetry is now available with all grisms).

The IRPOL polarimetry module is described here.

(Coronographic-)Imaging-polarimetry

The available field-of-view through the imaging-polarimetry mask is 20 arcsec x 120 arcsec. 20 arcsec is just a little smaller than the e-/o- beam divergence. Hence, the e- and o-beams do not overlap. Imaging Pol of extended sources is possible by offsetting E-W and with blank sky frames taken off-source.

A similar mask is used for coronographic imaging polarimetry. The only difference is that two wires are available for use as an occultor. Wire thicknesses are 0.7 arcsec and 1.2 arcsec; the wires are extended across the lower of the two imaging polarimetry apertures (each is ~20 arcsec long). The position angle of the image plane can also be set so that extended sources are orientated orthogonal to the occulting wire.

If the coronographic mask is not acceptable, imaging polarimetry of very bright targets is also possible using a 512x512 pixel sub-array to facilitate shorter (sub-1-second) exposures. With this set-up we can not use the imaging-pol mask; instead a small-field aperture (1arcmin x 1arcmin) is used. Polarimetry is then possible on 6-7th magnitude sources (with the broad-band JHK filters) depending on the seeing. The lack of a mask means that e- and o-beam sky regions overlap, thereby doubling the background signal (and associated noise). Note also that high source counts (>~3000) push the array towards the non-linear regime, limiting the accuracy of polarisation measurements. Also, this is a non-standard polarimetry mode which requires a change to the UKIRT-OT software. Please contact Chris Davis (c.davis at jach.hawaii.edu) for further details.

Those wishing to do imaging-polarimetry in the L and M-band should contact Chris Davis (c.davis at jach.hawaii.edu) before preparing their proposal.

A full discussion of (coronographic-)imaging-polarimetry with UIST is given here.

Spectro-polarimetry

• Spec-pol is available with the following slits:
• 20" long-slit; 2-pixel (0.24") wide.
• 20" long-slit; 5-pixel (0.60") wide.
• Spec-pol is available with all available grisms:
• Low resolution: IJ, JH, HK, KL and M
• Moderate resolution: short-J, long-J, short-H, long-H, short-K, long-K, short-L, long-L

The narrow slit gives higher spectral resolution, though the 5-pixel slit is better matched to the typical (median) seeing at UKIRT. Note that, due to focus limitations, the IJ and JH grisms should not be used with the 2-pixel spectro-polarimetry slit For full details on spectral resolutions and sensitivities consult the main spectroscopy web pages.

A full discussion of spectro-polarimetry with UIST is given here.