4.5. Narrowband Spectroscopy

An additional hybrid method, combining elements of both the narrowband imaging and serendipitous long-slit search techniques, involves obtaining many limited-wavelength spectra through a medium-width (OH avoidance) filter (e.g., Crampton 2000). With a uniform placement of 30 2" wide, 9' long parallel slitlets, Crampton (2000), using the Canada-France-Hawaii Telescope atop Mauna Kea, probes 9 arcmin² of sky per mask observation with a narrowband filter plus low-dispersion grism. The survey is designed to search for Ly at 6.38 < z < 6.64, corresponding to a region of both low OH emission and low H₂O absorption. This hybrid approach has the advantage of providing spectra of all targets over a short wavelength region. This is important for distinguishing high-redshift Ly, which can be identified both by line profile and by the presence of a strong continuum decrement across the line. Lower redshift emission features might also be identified from the presence of secondary lines, such as [O III] 4959 near [O III] 5007, or the double profile of [O II] 3727, 3729. Finally, as mentioned in Section 4.4, the higher resolution of spectroscopy relative to narrowband imaging is more sensitive to the typical emission-line widths, making this hybrid approach an efficient method for probing the distant universe.