From:  UKIDSS website, technical section - filters.

The WFCAM Y filter (0.97-1.07µm) occupies the clean wavelength range between the atmospheric absorption bands at 0.95µm and 1.14µm. The bandpass is similar to, but slightly narrower than, the Y bandpass of Hillenbrand et al. (2002, PASP, 114, 708). The WFCAM Y filter (shown red) has a slightly redder cut-on wavelength than the Hillenbrand Y filter (shown grey dash-dot), in order to be clear of the atmospheric absorption band. The cut-off wavelength of the WFCAM Y filter is at a significantly bluer wavelength, and largely avoids the strong sky emission lines near 1.1µm. Not only does this reduce the sky background, but it improves the Y-J colour discrimination between high-redshift quasars and T dwarfs (as shown by Warren and Hewett 2002).