SALT/PFIS Observer's Guide
2. Instrument Overview
2.1 Optical Design
PFIS resides at the prime focus of SALT. The Spherical Aberration Corrector (SAC) delivers a F/4.2 beam providing a flat, 8 arcminute diameter field of view at the focal plane, with a plate scale of 4.5 arcsec/mm.
2.1.1 Collimator and Camera
PFIS is an all-refractive collimator and camera system, optimized for spectroscopic performance in the 320-900 nm wavelength range. The collimator has 9 lenses in 5 groups plus a fold mirror before the last doublet. In the 150 mm diameter collimated beam are the shutter and the dispersers, which include one of six gratings (five volume phase holographic (VPH) gratings and a standard transmitting surface relief grating) or a double-etalon Fabry-Perot system. The camera (F/2.2, 8.6 arcsec/mm) has 9 lenses in 4 groups, with the final lens, a field flattener, being the dewar window. All optical surfaces are spherical except for the first surface in the camera, which is an asphere.
The collimator is designed to work in the 320-1700 nm range, to accomodate a future near-infrared camera. Air-glass interfaces in the collimator will be coated with either a MgF2/Solgel hybrid or just a MgF2 antireflection coating. Camera surfaces will be coated with a MgF2/Solgel hybrid or multi-layer antireflection coating.
To compensate for image error introduced by possible differences in filter thicknesses and uncompensated thermal effects, the camera will have an active focus system. Focusing will be accomplished by moving the singlet and the triplet in the camera together. Additionally, the camera will need to be refocused for each configuration, as the imaging was not optimized for all wavelengths simultaneously. However, a fixed focus position can be set for each configuration. Then final focus error due to filters/thermal effects can be removed.
PFIS will have a complement of five volume phase holographic (VPH) gratings and one standard surface-relief transmission grating. The gratings were chosen to provide spectroscopic capability over the entire wavelength range of the detector and resolving powers from R=500 to R=5500 (with typical slit widths).
The VPH gratings provide high diffractive efficiency and significantly reduced scattered light as compared to standard surface-relief gratings. Also, VPH gratings can be tuned to shift the diffraction efficiency peak to a desired wavelength. The use of such gratings requires the camera to be able to articulate, to accommodate various grating tilts. The grating will reside on a rotatable stage, and the entire camera will articulate about the same axis as the grating rotation, so that the grating is always used in a Littrow configuration.
2.1.3 Fabry-Perot Etalons
PFIS will implement a double etalon system, allowing resolving powers from R=500 to R=12500 over a full octave in wavelength (430 - 860 nm). On a telescope the size of SALT, this provides a high dispersion, high spatial resolution diffuse-object capability with a net efficiency that exceeds slit spectroscopy by more than an order of mantidue, since the detector pixels are more efficiently multi-plexed.
The system will have three spectral resolution modes: low (R=500 - 1000, tunable), medium (R=2500), and high (R=12500). Low-resolution mode will use a single etalon, with an interference filter to select the desired interference order (corresponding to wavelength). The medium- and high-resolution modes will use two etalons in series, with the low-resolution etalon and its filter selecting the desired order of the medium- or high-resolution etalon, respectively. Two of the three etalons will be installed at any time; the low-resolution etalon will reside on PFIS at all times, while the one or the other of the medium- and high-resolution etalons will be installed.
2.1.4 Polarimetric Optics
Polarimetric observations will utilize a ``wide-field'' design, in which a polarizing beamsplitter in the collimated beam takes the central half of the field and splits it into two orthogonally polarized fields, the ``ordinary'' (O) and ``extraordinary'' (E) beams. A polarization modulator preceding the beamsplitter modulates the polarization state with time, and the difference between the intensities of the O and E images as a function of time yields the polarization.
The beamsplitter is an array of calcite Wollaston prisms, inserted in the collimated beam just before the first camera element. This ensures that there is no vignetting of the split beam, which would compromise the polarimetric precision. Additionally, by placing the beamsplitter after the dispersers, both the O and E beams have the same wavelength gradient in Fabry-Perot mode.
The modulator consists of two rotating superachromatic mosaic waveplates, a 105 mm half- and a 60 mm quarter-waveplate. The modulator should be ahead of any optical elements with polarization sensitivity, like the fold mirror and the dispersers.
2.2 Detector CCD array
The detector subsystem comprises a cryostat containing a 3x1 mini-mosaic of CCD chips. The chips are E2V (formerly Marconi) 44-82 CCDs (Data Sheet) with 2k x 4k x 15 micron pixels. The mosaic is housed in an evacuated cryostat and thermally connected to the cold end of a Cryotiger, which cools the chips sufficiently to render dark current insignificant while minimizing QE reduction. The detectors are managed by an SDSU III CCD controller, which is in turn controlled by a PC.
2.3 System Efficiency
Below is the predicted throughput performance of the PFIS optical system. This plot excludes the CCD QE and any of the dispersers or polarimetric optics. We expect the throughput to be in the 75-82% range across the entire visible camera wavelength range.
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