Spectroscopy: Slits

Spectroscopy: Slits


In spectroscopic mode UIST will only be used with the 0.12″/pixel plate scale. The instrument has a 16-slot slit wheel which contains field stops, polarimetry masks and slits for spectroscopy. The latter are listed below (see the Engineering pages for the full slit-wheel populations).

Which slits with which grism?

Please note that, because of focus limitations, new slits have been installed in UIST for use with ONLY the IJ and JH grisms. In the OT these are called 2_pix_wide_120s and 4_pix_wide_120s. The standard slits (1_pix_wide_120, 2_pix_wide_120, 4_pix_wide_120) should NOT be used with the IJ or JH grisms; likewise the 2_pix_wide_120s and 4_pix_wide_120s should NOT be used with the other grisms. Users will find that an incorrect combination can not be selected in the OT.

1 March 2003 – Present

SlitName in UKIRT-OTDescriptionUse with…
Long, 1-pixel-wide slit1_pix_wide_1200.12arcsec x 120arcsecall grisms except IJ and JH
Long, 2-pixel-wide slit2_pix_wide_1200.24arcsec x 120arcsecall grisms except IJ and JH
Long, 2-pixel-wide slit2_pix_wide_120s0.24arcsec x 120arcseconly the IJ and JH grisms
Long, 4-pixel-wide slit4_pix_wide_1200.48arcsec x 120arcsecall grisms except IJ and JH
Long, 4-pixel-wide slit4_pix_wide_120s0.48arcsec x 120arcseconly the IJ and JH grisms
Long, 5-pixel-wide slit5_pix_wide_1200.60arcsec x 120arcsecall grisms
Long, 7-pixel-wide slit7_pix_wide_1200.84arcsec x 120arcsecall grisms
Pol Mask, 2-pixel-wide slitpol_mask_spec_20.24arcsec x 20arcsec (x 2)all grisms except IJ and JH
Pol Mask, 5-pixel-wide slitpol_mask_spec_50.60arcsec x 20arcsec (x 2)all grisms

Spectral Resolution

The spectral resolution with each grism is given on the next page. Generally, the 2_pix_wide_120 slit gives roughly twice the resolution of the 4_pix_wide_120 slit. This is not the case with the IJ and JH grisms, however, where only moderate improvement (by ~30%) in spectral resolution is achieved with the narrower 2_pix_wide_120s slit.

The 1 and 2-pixel slits both give essentially 2-pixel resolution. See e.g. these portions of HK spectra: 1-pix spec, 2-pix spec.


The two polarimetry masks, pol_mask_spec_2 and pol_mask_spec_5, each have two aligned, 20″-long slits, separated by about 25″. These are used for spectro-polarimetry of an object and adjacent blank sky (see the Polarimetry web pages for further details.).

Recommendation on Slit Position Angles:

If a specific slit position angle is not required, we recommend using an east-west slit (i.e. p.a. set to -90 degrees in the ukirt-ot). When acquiring the source, the “acquisition image” will then appear with the same orientation on the Movie display as a normal “imaging-mode” image (i.e. N to the left and E up). Any tracking errors will also move the source “up and down” the slit, rather than “off” the slit (though such errors are extremely unlikely!).

When a specific position angle is needed, for angles greater than 90 degrees we recommend using NEGATIVE VALUES for the position angle, i.e. for an angle of 110 degrees E of N, enter -70 degrees in the OT. This will help the TSS with source acquisition.

With a position angle of -90 degrees, the TOP of the slit will be to the EAST; 0 degrees puts the top to the south. A table showing the relationship between position angle and slit orientation on the sky is available in another section.