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). 

UKIDSS WFCAM Y filter profile

WFCAM Y filter. The red line plots the specified profile of the WFCAM Y filter, normalised to the peak transmission. The dash-dot grey line is the normalised profile of the Y filter of Hillenbrand et al. (2002, PASP, 114, 708). The orange line shows the spectrum of the sky (units unspecified). The lilac line plots the atmospheric transmission (normalised to the continuum) for typical conditions.

(credit: UKIDSS website)