UKIRT is flexibly scheduled, and it is important that PIs specify their requirements in terms of seeing, water vapour and sky brightness adequately for their observing programmes to be feasible. This document lists the effects of weather on various types of observing.

Seeing

This is a function of the science and PIs should simply specify what they require. However, some guidance as to the seeing behaviour may be useful:

(i) seeing currently improves markedly (by, say 0.3 arcseconds) during the first hour of the night, and thereafter decreases very slowly (by another 0.1 arcsecond) into the small hours of the morning. This variation is statistical in nature only, and cannot be relied upon to happen on any given night.

(ii) seeing is better in the summer and early autumn than it is through the winter months. This is again a statistical statement – the difference is small (of order 0.1 arcseconds on a median around 0.5 in the K band) and it is probably also a function of the Pacific ocean circulations – el Nino years (on a sample of one) may be better than others. The median seeing at UKIRT varied from 0.45 in the el-Nino year of 1998, to 0.55 arcseconds in 1999.

Water vapour

It is hard to place a hard and fast limit on the properties of water vapour absorptions on observed data, because the water lines are highly wavelength-dependent and variable. The UKIRT observing tool will initially allow observers to specify whether they require “dry” conditions, which at the telescope will be defined by a single tau cutoff of 0.09. This should be requested by PIs on programmes involving:

(i) spectroscopy and imaging in the three-micron window, particularly spectroscopy around 3.2 microns. Programmes concentrating on the long end of the window may be possible in worse conditions;

(ii) programmes of spectroscopy on the fringes of the 10-micron window and anywhere in the 20-micron band;

(iii) photometric programmes in the broad N filter and narrow filters in the Q band.

(iv) spectroscopy requiring contiguous coverage of the entire JHK window.

Further details of the statistical behaviour of the water vapour content above MKO are given here. (Available to Staff only via internal container)

Sky Brightness

Sky brightness in the near-infrared is dominated by photochemically-driven variations in the atmospheric OH emission. This is principally seen in the J and H bands, where the variation is pronounced and continual from sunset through to dawn. The J-band sky brightness shows the greatest variations, with a magnitude drop in the first hour after sunset and a further magnitude drop distributed through the rest of the night; a similar but slightly less dramatic decrease is seen in the H and K bands. Figures posted here show these variations in mag/sq arcsecond at zenith measured throughout an observing year. They are intended as a guide only, no attempt has been made to filter for poor weather or proximity to the moon (in practice it is time of the night which produces the greatest variations). In the thermal-IR (three microns and longer) the sky brightness is constant (and very high) and there is no particular preferred time of night.