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M.W. Buie, W.M. Grundy (Lowell Obs.), S.D. Kern (Univ. AZ)
The high-spatial resolution of HST combined with its location above the Earth's atmosphere presented a compelling opportunity to collect separate spectra of Pluto and Charon at all sub-Earth longitudes. Such observations are needed to permit detangling the relative spectral contributions of Pluto and Charon in the growing collection of ground-based observations.
We obtained data on the Pluto-Charon system with the NICMOS instrument on the Hubble Space Telescope which were finally released to us on 1998 June 15. Our strategy was to obtain the highest possible signal-to-noise ratio spectra of Charon within a single HST visibility window (visit). All our data were taken with the G206 grism which provides spectral coverage from 1.4 to 2.5 \mum with a dispersion of 0.0115~\mum/pixel. We have data at four sub-earth longitudes: 45, 135, 225, and 315 degrees taken on 1998 Mar 17, 1998 Jun 7, 1998 Mar 27, 1998 May 28, respectively. Each visit comprised a total integration time of 35.2 minutes over 11 dithered positions. The scheduling for each observation was strongly constrained by the requirement of maximizing the Pluto-Charon separation by an off-nominal roll of the spacecraft. As a result, only one of the four spectra were taken during NIC3-camera ``campaigns'' where the telescope focus was optimized for the NIC3/G206 camera/grism combination. Despite the non-optimal focus, our data clearly resolve the two objects with only minor amounts of overlap between each object's PSF. Given the goal of extracting high quality data on Charon, a careful two source extraction from the data is required.
We will present our spectra of Charon and Pluto together with a discussion of the new data reduction techniques we have developed for the optimal extraction of point-source spectra (blended and unblended) from NICMOS grism data. This work was supported by GO-07818.01-96A, HF-01091.01-97A, and the NSF REU Program grant to Northern Arizona University.