The University of Wisconsin Astronomy Department and the Space Astronomy Lab at UW are designing
an SHS spectrometer for the WIYN 3.5-meter telescope on Kitt Peak and the SALT 10-meter telescope in
South Africa. The new device will be mated to the Sparsepak, (Bershady et al, 2004, 2005) and/or the
Hydra fiber array at WIYN, and fed by either the prime focus image at SALT or the High Resolution
Spectrograph fiber-feed at SALT. The spectrograph will produce spectra at a reciprocal dispersion, R =
25,000 in 20 orders, each order covering an average wavelength band 250 km/s wide, for a total
wavelength range of 5000 km/s. Spectra from approximately 82 fibers will be resolved. Once the system is
proven at WIYN, and because the aperture size for this spectrometer does not scale with telescope size, we
will be able to test this same prototype at the SALT 10-meter telescope. This will be the first application of
this technique to large aperture astronomical observations.
Ultraviolet astronomy is an important tool for the study of the interplanetary medium, comets, planetary upper atmospheres, and the near space environments planets and satellites. In addition to brightness distributions, emission line profiles offer insight into winds, atmospheric escape, energy balance, currents, and plasma properties. Unfortunately, the faintness of many target emissions and the volume limitations of small spacecraft and remote probes limit the opportunities for incorporating a high spectral resolution capability. An emerging technique to address this uses an all-reflective form of the spatial heterodyne spectrometer (SHS) that combines very high (R >105) spectral resolution and large étendue in a package small enough to fly as a component instrument on small spacecraft. The large étendue of SHS instruments makes them ideally suited for observations of extended, low surface brightness, isolated emission line sources, while their intrinsically high spectral resolution enables the study of the dynamical and spectral characteristics described above. We are developing three forms of the reflective SHS to observe single line shapes, multiple lines via bandpass scanning, and precision spectro-polarimetry. We describe the basic SHS approach, the three variations under development and their scientific potential for the exploration of the solar system and other faint extended targets.
This paper describes the characteristics and performance of a novel spatial heterodyne spectrometer designed to measure the extremely faint [OII] 372.6 nm (λ3726 Å) and 372.9 nm (λ3729 Å) emission lines from the warm (10,000 K) ionized component of our Galaxy's interstellar medium. These [OII] lines are a principal coolant for this wide spread, photoionized gas and are a potential tracer of variations in the gas temperature resulting from unidentified interstellar heating processes that appear to be acting within the Galaxy. In the basic SHS system, Fizeau fringes of wavenumber-dependent spatial frequency are produced by a Michelson interferometer modified by replacing the return mirrors with diffraction gratings; these fringes are recorded on a position sensitive detector and Fourier transformed to recover the spectrum over a limited spectral range centered at the Littrow wavenumber of the gratings. The system combines interferometric and field-widening gains in tandem to achieve 10,000-fold sensitivity gains compared to conventional grating instruments of similar size and resolving power. SHS systems also have relaxed flatness tolerances (20-50 times compared to Fabry-Perots) and do not require precision imaging to achieve diffraction-limited spectroscopic performance. Defects can largely be removed in data processing.
Early results from our [OII] SHS system confirm the superb performance of the SHS technique for measurements of spatially extended faint emissions, including the first detection of [OII] emission lines extending out to 20 degrees from the Galactic equator ([OII] intensities ranged from tens of rayleighs near the Galactic plane to less than one rayleigh at high latitudes; the [OII] line profiles show structure indicating emission along the lines of sight from both the local interstellar gas and more distant gas in the Perseus spiral arm).
Large gains in the sensitivity of Fabry-Perots for geocoronal research have been achieved at the University of Wisconsin employing the technique of CCD annular summing spectroscopy. Earlier 'demonstration observations' of this technique lead to a significant new understanding of geocoronal hydrogen excitation. This paper will outline a new ground-based observing program which is building on these earlier observations in order to obtain definitive data regarding the physical processes which govern the abundance and transport of atomic hydrogen in the earth's atmosphere. Two double-etalon Fabry-Perot spectrometers have been installed at the University of Wisconsin's Pine Bluff Observatory (WI) for the purpose of making a systematic series of high spectral resolution (R approximately equals 100,000) line profile, and intensity observations of geocoronal hydrogen nightglow. For the first time it will be possible to obtain coincident observations of geocoronal hydrogen Balmer-alpha and Balmer- beta with sufficient signal-to-noise for detailed line profile studies. Because the geocoronal Balmer-beta emission is about one tenth the intensity of Balmer-alpha, the fitness of this line has frustrated past attempts to determine its profile; however, gains in sensitivity afforded by the annular-summing technique make these new observations possible. It is anticipated that these simultaneous observations will provide a means by which to isolate previously observed perturbations to the Balmer-alpha line, the components of which may arise from both contributions due to quantum mechanical fine structure and the non-Maxwellian dynamics of the hydrogen exosphere. Each of these instruments employs the annular summing technique in which the Fabry-Perot's annular fringe pattern is imaged onto a low noise CCD chip. Using the property that equal area annuli correspond to equal spectral intervals, software is used to divide the CCD image into equal area annular bins, whereby the Fabry-Perot interference pattern is converted into a useful spectral profile. This paper will describe the instrumentation, and how it relates to the planned observational program.