The Low Dispersion Survey Spectrograph (LDSS)

LDSS allows multi-object spectroscopy of ultra-faint stars and galaxies in the magnitude range 20-23. It has a 12.3' diameter field, and light is fed through a simple but efficient spectrograph onto the IPCS or a CCD. The novel design of LDSS is described by Wynne and Worswick (1988 Observatory 108,161). LDSS uses lenses throughout to maximize efficiency, and has a transmission grating as the dispersion element. The spectrograph was designed to work in the blue, and is optimised for the spectral range 3700-7500Å.

The summary of LDSS properties given below is taken from Colless et al. (1990 MNRAS 244, 408), who also discuss techniques for multi-slit fabrication, acquiring fields at the telescope and data reduction as well as showing examples of spectra taken with the system.

Two grisms are currently available, with dispersions of 165Å/mm and 870Å/mm. LDSS has four modes of operation:

Direct
With the grism removed, LDSS can be used as a simple f/2 focal reducer for broad-band or narrow-band imaging over a 12.3' field of view. In 1"  seeing the FWHM spatial resolution is ~50um (1.3", which is well matched to typical detector pixel sizes of 20-30um. This mode can be used to obtain photometry/astrometry of deep spectroscopic targets, for field acquisition and for calibration purposes.
Long slit
Long slit aperture masks can be made for any desired slit width. A slit width of 1.7" gives FWHM spectral resolutions of ~ 13Å and ~ 70Å at 165 and 870Å/mm respectively.
Slitless
With no aperture mask, spectra of all objects in the field may be obtained simultaneously, though overlap and sky contamination reduce the performance. The resolution is also seriously degraded if the seeing is worse than about 2".
Multislit
This is the primary mode of LDSS as a survey instrument. Using an aperture mask with slits 10-25u in length, between 20 and 100 targets (depending on the dispersion and the detector) may be observed simultaneously.

The LEXT software package available on AAO and Starlink computers is used in conjunction with LDSS and has a triple role:

• It facilitates the design of LDSS masks, taking the user from a raw astrometric catalogue of potential targets to an LDSS multi-slit mask design which maximizes the number of objects observable in a given field under various observational and instrumental constraints.
• It assists in the complex process of field acquisition at the telescope.
• It aids in the extraction and reduction of the individual spectra obtained with LDSS on the telescope using the multi-slit mask parameters and instrumental information defined by the LDSS configuration.

LDSS observers planning to use long slit or multislit mode should contact the LDSS support scientist at AAO well before their run to arrange for masks to be made.