J. C. Brandt

The Goddard High Resolution Spectrograph: In-orbit performance

The in-orbit performance of the Goddard High Resolution Spectrograph onboard the Hubble Space Telescope (HST) is presented. This report covers the pre-COSTAR period, when instrument performance was limited by the effects of spherical aberration of the telescopes primary mirror. The digicon detectors provide a linear response to count rates spanning over six orders of magnitude, ranging from the normal background flux of 0.01 counts diode -1 s-1 to values larger than 104 counts diode-1 s-1. Scattered light from the first-order gratings is small and can be removed by standard background subtraction techniques. Scattered light in the echelle mode is more complex in origin, but it also can be accurately removed. Data have been obtained over a wavelength range from below 1100 A to 3300 A, at spectral resolutions as high as R = lambda/delta-lambda = 90,000. The wavelength scale is influenced by spectrograph temperature, outgassing of the optical bench, and interaction of the magnetic field within the detector with the earths magnetic field. Models of these effects lead to a default wavelength scale with an accuracy better than 1 diode, corresponding to 3 km s-1 in the echelle mode. With care, the wavelength scale can be determined to an accuracy of 0.2 diodes. Calibration of the instrument sensitivity functions is tied into the HST flux calibration through observations of spectrophotometric standard stars. The measurements of vignetting and the echelle blaze function provide relative photometric precision to about 5% or better. The effects of fixed-pattern noise have been investigated, and techniques have been devised for recognizing and removing it from the data. The ultimate signal-to-noise ratio achievable with the spectrograph is essentially limited only by counting statistics, and values approaching 1000:1 have been obtained.

The Goddard High Resolution Spectrograph: Instrument, goals, and science results

The Goddard High Resolution Spectrograph (GHRS), currently in Earth orbit on the Hubble Space Telescope (HST), operates in the wavelength range of 1150-3200A with spectral resolutions (lambda/delta-lambda) of approximately 2 X 103, 2 X 104, and 1 X 105. This paper describes the instrument and its development from inception, its current status, the approach to operations, representative results in the major areas of the scientific goals, and prospects for the future.

The Goddard High Resolution Spectrograph: Post-COSTAR Characteristics

ABSTRACT We review changes to the characteristics of the Goddard High Resolution Spectrograph (GHRS) which resulted from the installation of the Corrective Optics Space Telescope Axial Replacement (COSTAR) on the Hubble Space Telescope. The introduction of two new optical elements into the light path altered the spectral distribution of the light, decreasing the amount of light striking the instrument by about 30% at wavelengths greater than 1200 A and effectively eliminated all radiation at wavelengths less than 1130 A. However, at the longer wavelengths the improved focus offset this loss when the Large Science Aperture (LSA) was used and increased the overall throughput of the Small Science Aperture (SSA) by a factor of 2. The improved focus also enhanced the spectral resolution of LSA observations and improved the ability of the instrument to observe in crowded fields.

The disconnection events of 1986 April 13-18 and the cessation of plasma tail activity in comet Halley in 1986 May

The disconnection events (DEs) in the 1986 April 13-18 time period appear to be the last major ones of comet Halleys 1986 apparition. We determined the time of the first disconnection event in this sequence, April 13.9±0.1 days, from kinematic analysis of photographic images. We show here that at the time of this DE, comet Halley had just crossed a magnetic field sector boundary and was in a moderate-density solar wind region with a speed of approximately 350 km s -1 . A sequence of DEs began approximately 1 day after the April 13.9 DE and was easily visible on April 16 and 17. Based on IMP 8 data, we show that this sequence was associated with a complex magnetic structure of the solar wind, showing multiple reversals

Observations of 3C 273 with the Goddard High Resolution Spectrograph on the Hubble Space Telescope

The observations of the quasar 3C 273 taken with the Goddard High Resolution Spectrograph in 1991 February are presented here. We have included both the reduced raw data, and smoothed and deconvolved spectra. Also, a list of observed absorption lines is presented. The data comprise 11 spectra, including 1 low resolution observation and 10 medium resolution observations. The wavelength region covered ranged from about 1150 to 2820 A, but was not all inclusive. The procedures used to obtain and reduce the data, including corrections for fixed pattern noise, compensation for the effects of spherical aberration in the HST primary mirror, and objective detection of weak absorption lines, are described. We also have included a short discussion on the detection of galactic Ni II and Virgo cluster metal lines

COMET DE VICO (122P) AND LATITUDE VARIATIONS OF PLASMA PHENOMENA

Abstract In situ measurements by the Ulysses spacecraft have unequivocally established differences in solar wind properties at different solar latitudes for 1992 through 1996, and comets are exposed to these different environments depending on latitude. In the polar regions, a less time-varying appearance and a plasma tail orientation corresponding to a higher solar wind speed are expected. In the equatorial regions, a more time-varying appearance is expected as well as a plasma tail orientation corresponding to a slower solar wind speed. Disconnection events (DEs) are produced at crossings of the heliospheric current sheet (HCS) as established by the association of the comet Halley DEs in 1985–1986 with the HCS and by magneto-hydrodynamic simulations. Thus, it is expected to see DEs when comets are in equatorial latitudes, but not when they are in polar latitudes. Comet de Vico (122P), with its inclination of 85.3°, good brightness, and plasma activity, was an excellent test comet for latitude variations. Images of de Vico have been collected as part of the Ulysses Comet Watch, and plasma structures are seen from 18 September 1995 to at least 25 October 1995, corresponding to a range of about 90° in ecliptic latitude. The analysis confirms the picture described above with the boundary between the equatorial and polar regions at 20°–25° latitude, in agreement with the in situ measurements by Ulysses. The tail orientation is consistent with the different velocity regimes; DEs are seen only in the equatorial region; the comet in the polar region has a distinctly less disturbed appearance.

High Signal-to-Noise Ratio Observations of Weak Interstellar Absorption Lines Towards XI Ophiuchi With the Goddard High-Resolution Spectrograph Aboard the Hubble Space Telescope

We present an atlas and tabulation of weak interstellar absorption lines in the ultraviolet spectrum of £ Ophiuchi in four selected wavelength regions observed with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. The signal-to-noise ratio ranges from 150 to nearly 400, and the spectral resolving power exceeds 20 000, allowing 2cr detections of features as weak as Wx=0.8 mÂ. We report positive measurements of two lines of OH, and weak detections of P l, Tl n, and N v. Upper limits of WX<1 m are found for the molecules H20, HC1, SiO, NO , and CH2. Similar limits are found for heavy elements Te n, Co II, and Sb IL Three lines are present in our spectrum for which we have no identifications. They are found at wavelengths of = 1229.84, 1313.98, and 1314.23 Â. However, none of the features reported by previous authors as unidentified absorption lines in the wavelength regions we have observed are present in our data.

On the Existence of Small Comets and their Interactions with Planets

Arguments are presented for a substantial, unexplored population of comets with radii less than 1 km. Known examples confirm this population and extrapolation of any plausible size-distribution function indicates large numbers. However, their accurate numbers, orbital characteristics, and physical properties are unknown. Thus, even though the small comets may be the most frequent cometary bodies impacting the planets, a quantitative evaluation is not currently possible. We advocate an optimized, dedicated search program to characterize this population.