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LDSS-2 is a common-user wide-field multiaperture spectrograph mounted at the Cassegrain focus of the 4.2m William Herschel Telescope.
LDSS stands for `Low Dispersion Survey Spectrograph'. LDSS-2 is the second such instrument. The first, known as LDSS or LDSS-1, is a non-common-user instrument at the AAT. LDSS-2 excels at obtaining spectra of large numbers of faint objects. Its high throughput and excellent sky subtraction lead to spectacular performance at very faint limits while its relatively large field of view and multiobject capability provide a large multiplex advantage for statistical projects.
LDSS-2 is similar to LDSS-1 but is easier to operate, having a greater level of automation. It also has a different optical design to cope with the WHT's f/11 Cassegrain focal ratio and to provide better images to exploit the superior seeing on La Palma. The optics of LDSS-2 form a focal reducer based on the design of Charles Wynne and Sue Worswick, and can be used for direct imaging over an field of view.
LDSS-2 operates as follows. The telescope is focussed onto a multiaperture mask held in an 8-position wheel. The light then passes through various apertures cut in the mask and enters the collimator which converts the input f/11 beam into parallel light, before passing it through either a filter and/or a grism. The light is then focussed by the camera onto an external detector with a final focal ratio of f/2. The camera can be moved along the optical axis slightly to allow the images to be optimized for different wavelengths of interest. Removable Hartmann masks are provided in the filter and grism wheels to aid focussing.
Four grisms are provided of which 3 can be mounted in the grism wheel at the any one time. These cover a range of spectral resolution , where . Up to 7 filters can be mounted in the filter wheel at once. Three broad-band filters (the BVR Harris set as used by NOAO) are provided together with 3 other filters designed for use in spectroscopic mode to suppress unwanted orders.
By using clear positions in the aperture and grism wheels, LDSS-2 can be used to give direct images in the chosen filter passband over a very wide field of view. It thus doubles as a high throughput, wide-field imager. This capability is essential for target acquisition and allows astrometry for mask manufature to be obtained in near real-time.
Fig.1 shows a schematic of LDSS-2 and how it
relates to the telescope system and
Fig.2 shows the main elements of the optical system (excluding the reconfigurable components).
Table 1 summarizes the main properties of LDSS-2.