THE ROBERT STOBIE SPECTROGRAPH (RSS)
The Prime Focus Imaging Spectrograph (designed and built for SALT by the University of Wisconsin-Madison and Rutgers University) – now renamed the Robert Stobie Spectrograph (RSS) in honour of the past SAAO Director and first Chairperson of the SALT Board, Dr Robert S. Stobie.
This instrument will be the priority first-light instrument of SALT. The instrument will be built at the University of Wisconsin, with Professor Ken Nordsieck as PI. The projects includes participation from Rutgers University (Fabry-Perot etalons and grating mechanisms) and SAAO (CCD detectors). RSS will exploit the improved blue/UV throughput of SALT as well as its access to a science field of 8 arcmin diameter. It will also be capable of multi-object (slitlets and Fabry-Perot option) spectroscopy (MOS), to resolutions of R ~ 12,000, and narrow-band imaging, as well as having a polarimetric capability.
The major observational 'niches' for RSS are:
- UV spectroscopy (instrument capable to 320 nm using CaF2, NaCl and fused silica elements).
- CaII H & K at rest, OII
3727
- Bowen lines: OIII 3133, 3343, 3444
- [NeV] 3340, 3425
- Redshifted UV features (e.g. MgII 2800 z = 0.1 - 0.4; Ly
z = 1.6 - 2.3) - Initial single visible beam capable from 320-850 nm
- Possible upgrade path to near-IR beam (but not for first-light)
- High throughput medium resolution (R to 10,000) MOS (>30 slitlets) spectroscopy over the 8' diameter FoV (= 110 mm diameter at focal plane).
- Fabry-Perot imaging spectroscopy (R = 300 - 10,000) over the 8' diameter FoV.
- Imaging polarimetric and spectropolarimetric modes into the UV (to 320 nm).
- Balmer jump
RSS will make use of recent technological advances to provide the highest possible efficiency spectroscopy using Volume Phase Holographic (VPH) gratings. They offer very high efficiency (~90%), and because they are always working at the Bragg condition (incident and diffraction angles equal), can be 'tuned' to different wavelengths by rotating the grating and camera. Fabry-Perot etalons, together with order exclusion filters, will give RSS imaging spectroscopy capability. In addition good narrow band imaging will be carried out over the entire science field. Large aperture polarimetric optics will also provide for spectropolarimetric capability in all the spectroscopic modes.
While RSS's first-light configuration supports visible observations (320-900nm), it is also planned to build in a future upgrade path for a second beam, operating in the near infrared (850-1700 nm).
RSS design details
The primary design challenge for RSS is the limited mass and volume envelope at prime focus, plus the limited access. RSS will be folded part-way through the refractive collimator to accommodate the payload envelope and to provide for a future infrared beam. The 150mm pupil will accommodate disperser wheels, containing grisms and VPH gratings, providing resolutions of R ~800-6000 with a 0.9 arcsec slit width (seeing FWHM is 0.9 arcsec). In addition, there is provision for a Wollaston polarizing prism and Fabry-Perot etalons (up to two inserted at one time), allowing for spectropolarimetry and and flexible single or dual-etalon Fabry_perot imaging with resolutions of 2500 to 13000.
The spectrograph will be 'articulated' to allow for tuning of the first-order Littrow VPH graings and for 'straight-through' grism and Fabry-Perot spectroscopy, and narrow band imaging. The all-refractive camera will be roughly f/2.2, to image the entire 8 armin diameter field, probably onto a 6144 × 4096 CCD mosaic (15 µm pixels).
The following schematic illustrate the imaging/Fabry-Perot and spectroscopy modes of RSS, respectively.
- Ha image in Fabry-Perot mode (left) of the galaxy Perseus A, and a narrow band image of the same object (right).
- More than 100 spectra at once
