The standard filter complement for WFCAM comprises five broad-band filters, Z, Y, J, H, and K, and two narrow-band 1% filters, H2 1-0 S1, and Br-g. The manufacturer for the Z filter is Research Electro-Optics, Inc. The manufacturer for the remaining 6 filters is NDC Infrared Engineering. The eighth filter holder is blank, for darks. The specified bandpasses for the eight filters are shown in Table 1. The specified filter profiles are plotted in Fig. 1, which also shows the wavelength dependence of the atmospheric transmission, for typical conditions. The actual total system response curves, for the ZYJHK bands, i.e. the transmission curves of the manufactured filters, multiplied by atmospheric transmission, instrument throughput, and detector q.e., are provided as ascii tables with the Hewett et al. synthetic photometry paper (link 5. to the left).
The WFCAM J, H, and K bandpasses follow the specification of the Mauna Kea consortium. The design of the MK filter set is described by Tokunaga et al. (2002, PASP, 114, 180). The WFCAM Z filter has a similar effective wavelength to the SDSS z' filter, and the WFCAM Y filter fills the gap between Z and J. An explanation of the design of the bandpasses for the Z and Y filters is provided below.
Table 1. Bandpasses of the 7 WFCAM filters. The cut-on and cut-off wavelengths are specified at 50% of peak transmission. The sharpness of the band edges is quantified by [wav(0.8)-wav(0.05)]/wav(0.05), and is specified to be <0.025 (here wav(0.8) is the wavelength where the transmission is 80% of peak).
Figure 1. The specified transmission profiles of the WFCAM filter set, normalised to the peak transmission, plotted against wavelength. The five broad-band filters are plotted in red, and the two 1% narrow-band filters are plotted in green. The lilac line plots the atmospheric transmission (normalised to the continuum) for typical conditions.
|The bandpass of the WFCAM Z filter (0.83-0.925µm) was designed to provide a reasonable match to the effective wavelength of the SDSS z' bandpass, but with a rectangular profile, and avoiding the atmospheric absorption band near 0.95µm. The two bandpasses are compared in Fig. 2. The specified WFCAM Z profile is shown in red. The array quantum efficiency is approximately flat over this wavelength range. The SDSS z' bandpass is defined by the long-pass z' filter and the falling q.e. of the CCDs towards longer wavelengths, multiplied by the atmospheric transmission. The normalised z' system throughput is shown as the grey dash-dot line. The effective wavelengths of the WFCAM Z and SDSS z' bandpasses are 0.876µm and 0.887µm, respectively (effective wavelength as defined by Schneider et al. (1993) ApJ, 264, 337).||
Figure 2. WFCAM Z filter. The red line plots the specified profile of the WFCAM Z filter, normalised to the peak transmission. The dash-dot grey line is the normalised system throughput for the SDSS z' passband (Fan et al. (2001) AJ, 121, 31). The lilac line plots the atmospheric transmission (normalised to the continuum) for typical conditions.
|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 two bandpasses are compared in Fig. 3. 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)).||
Figure 3. 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.