Fabry-Perot spectrometers were installed at the McMath-Piece solar telescope at the National Solar Observatory (Kitt Peak, AZ) to study the effects of the solar wind on the ionized tail of the comet Hale-Bopp and to make spectroscopic observation of atomic components of the comet`s atmosphere. The spectrometers, developed at the University of Wisconsin (Madison, WI) by Frank Scherb and Fred Roesler and by Ron Oliversen at NASA`s Goddard Space Flight Center (Greenbelt, MD), use highly sensitive charge-coupled-device (CCD) arrays as detectors.
"A comet has two distinct tails," says Scherb. "The most visible is the yellowish curved tail comprised of dust." A bluish (to the eye) ion tail contains ionized water and other ionized molecules, such as carbon monoxide and carbon dioxide. "The ionized tail, which is very straight and long, is pushed away from the comet by the solar wind, much like a wind sock," says Scherb. "With our observations, we hope to develop models of what is happening, so that future comets may be used as probes to study the effects of the solar wind in remote parts of the solar system."
A 15-cm, dual-etalon Fabry-Perot spectrometer was installed at the 0.8-m west auxiliary telescope for extensive observations of the H2O molecular ion velocity structure in a 6-arcmin field of view centered on the comet nucleus. This instrument was tuned to the 6158-Å emission line of ionized water and used to observe the interaction of the solar wind with the cometary ions produced in the coma. Velocity maps were produced by obtaining images tuned off the H2O molecular ion line, at the rest wavelength, and at several Doppler-shifted wavelengths chosen to study how the solar wind accelerates ions along the plasma tail.
The instrumental spectral resolution was about 0.4 Å or 20 km/s, and the observations consist of "data cubes," or a sequence of images of the H2O molecular ion emission line at velocity steps of 12.5 or 25 km/s, covering a range from -75 km/s to +75 km/s in the comet reference frame.
A second dual-etalon Fabry-Perot spectrometer was installed at the north port of the main solar telescope to make spectroscopic studies of the neutral species of oxygen, hydrogen, and carbon. It used 6-cm etalons in a spectral multiplexing mode where the Fabry-Perot fringes are imaged onto the CCD camera. The spectral resolution is about 3 km/s, and the field of view on the sky is 6 arcmin. The data will be used to monitor the cometary water production rate as a function of heliocentric distance and to study photochemical processes in the coma, especially processes involving the production of carbon atoms in an excited state.
A third instrument, the Wisconsin H-Alpha Mapper (WHAM), is installed semipermanently at Kitt Peak for a program of observations unrelated to the comet. In the spectrographic mode it achieves about a fiftyfold sensitivity gain over conventional scanning instruments, which use photomultiplier tube detection, by taking advantage of the high quantum efficiency and low read noise of CCDs and their multichannel capability when operating in the annular summing mode. 1During the comet observing program, the WHAM was occasionally diverted from its official observations to map the velocities and distribution of H2O molecular ions within 30 arcmin of the comet head.
1. Coakley et al., Appl. Opt. 35, 6479 (1996).