The Wisconsin H Mapper (WHAM): A Brief Review of Performance Characteristics and Early Scientific Results

The Wisconsin H-Alpha Mapper

We have recently built a Fabry-Perot spectrometer called the Wisconsin H-Alpha Mapper (WHAM), which uses a low noise, high efficiency CCD camera as a multichannel detector behind a pair of 15cm diameter Fabry-Perots. The well known advantage of the Fabry-Perot spectrometer is that at a specified spectral resolving power it has a throughput 100 times larger than that of a grating spectrometer with a dispersing element of the same area. This angular throughput advantage over the grating spectrometer makes the Fabry-Perot well suited for high spectral resolution studies of faint extended emission line sources, particularly when coupled to an imaging detector such as a CCD, which can record many spectral elements simultaneously. In this case, when collimated rays passing through the Fabry-Perot are imaged on the detector, a ``ring spectrum'' is formed in an azimuthally symmetric pattern about the optical axis, with wavelength decreasing from the center to the edge such that equal spectral intervals correspond to a set of equal area, nested annuli on the detector (e.g., Reynolds et al 1990; Tufte 1997).

The spectral and spatial properties of the diffuse interstellar emission line background, determined from earlier observations, have defined the specific characteristics of the WHAM system. Because the interstellar emission is weak, the throughput was maximized by incorporating Fabry-Perot etalons with the largest available clear aperture (14.7 cm). Furthermore, the detector is a high quantum efficiency (78% at H), low noise (3 e rms) CCD camera. WHAM is a dual etalon spectrometer, which greatly reduces multi-order ghosts, especially those arising from the relatively strong atmospheric OH emission lines within the pass band of the interference filter. The tandem etalon design also suppresses the Lorentzian-like wings that are typical of single etalon systems, making it easier to detect the relatively weak and broad interstellar features in the wing of the ever present geocoronal H line. The spectral resolution of 12 km s matches well the 25 km s widths (FWHM) of the narrowest interstellar H emission components, and the 200 km s spectral window is set by the radial velocity spread of the emission components, which are typically within 50-70 km s of the local standard of rest (LSR). Broad band, antireflection (< 0.5%) coatings on most of the optical surfaces suppress extraneous reflections, which together with multilayer broad band reflection ( 90%) coatings on the etalons permit observations anywhere within the spectral region 4800 Å to 7200 Å. WHAM's 4.4 Å (200 km s) spectral window can be centered on any wavelength within this region by using a gas (SF) pressure control system to tune each etalon and a filter wheel to provide the correct isolating interference filter. The angular resolution of 1 is adequate to resolve some of the spatial structure known to exist in the H background at high Galactic latitudes (e.g., Reynolds 1993) and to create H maps comparable to existing 21 cm surveys.

  
Figure 1: Schematic diagram of WHAM showing the siderostat and the trailer that shelters the spectrometer. In this diagram the siderostat is pointing at the southern horizon.

A simplified diagram of the WHAM optical system is shown in Figure 1. Light from the sky is directed by the flat mirrors of a two axis siderostat horizontally through a 0.6 m diameter, 8.6 m focal length objective lens into a 2.5 m 2.5 m 6 m trailer that contains the spectrometer. The sky is imaged between the two Fabry-Perot etalons, whose 15 cm diameter define WHAM's 1 diameter beam on the sky. The light then passes through a series of spectral imaging lenses, a broad band (20 Å) interference filter, and a high speed camera lens, which images the ``ring spectrum'' onto the CCD chip. In this normal, spectral mode the sky is not imaged on the detector, only the average spectrum within the beam. This eliminates any possible confusion between spectral features and the spatial structure of the source, including stars, within the beam. With the insertion of additional lenses (not shown) into the optical path, the WHAM spectrometer can create, instead of a spectrum, a narrow band, monochromatic image of the sky at about 1 angular resolution within the 1 beam. The spectral width of the sky image can be adjusted by an iris diaphragm to any value between 10 km s (0.2 Å at H) and 200 km s (4.4 Å). The narrowest setting allows the imaging of individual velocity components within an interstellar emission profile, or with a series of exposures at different radial velocities, the creation of a complete data cube for the 1 field.

WHAM has been located at Kitt Peak, Arizona since November 1996. The siderostat, the CCD camera and LN dewar filling system, the etalon pressure tuning system, the interference filter wheel, the calibration light sources, the imaging optics carriage and iris, plus a number of environmental sensors provide information to and can be commanded from a single workstation. By incorporating and extending the remote observing subsystems developed for the 3.5 m WIYN telescope (Percival 1994), the entire WHAM facility, including opening and closing at the beginning and end of the observing night, is operated from a campus office at the University of Wisconsin in Madison, 2400 km from Kitt Peak.

© Copyright Astronomical Society of Australia 1997