Imaging- and Spectro-Polarimetry with UIST
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
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 Tom
Kerr (t.kerr at jach.hawaii.edu) for further details.
Those wishing to do imaging-polarimetry in the L and M-band should
contact Tom Kerr (t.kerr at jach.hawaii.edu) before
preparing their proposal.
A full discussion of
(coronographic-)imaging-polarimetry with UIST is given here.
- 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,
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