Astron. Astrophys. 325, 745-754 (1997)

2. Method and description of the observations

We have obtained observations of spectral Ly emission from five different directions at five positions of the earth in its orbit. The spectral observations were carried out with the GHRS, echelle A, spectrograph on-board the Hubble Space Telescope using the large science aperture (LSA) at 1216 Å. Meanwhile it has become clear that the GHRS Echelle A spectra are more than sufficient to resolve the velocity structure in the interplanetary hydrogen flow at a satisfactory signal/noise level (see Clarke et al. 1995). These spectra by their specific profiles in different line-of-sight directions thus are the key to distinguish between authentically solar effects and interface effects modulating the dynamics of hydrogen in the heliosphere. The nature of the point spread function (PSF) for diffuse monochromatic emission has already been examined in some detail (Clarke et al. 1995). For this purpose the scattering wings and the occurrence of grating ghosts within a few Å  of the line center have to be taken into account. The general characteristics of the GHRS gratings are reported by Cardelli et al. (1990 , 1993), and the data given there are applicable to point source observations. When observing diffuse emission the resulting PSF then can be synthesized as the sum of point sources filling each of the eight diodes corresponding to the 2 arcsec aperture. The result of this synthesis is a nearly rectangular PSF with a full width half maximum of 0.07 Å and a scattered light level in the wings larger by the ratio of increased area of the emission region in the aperture. Here the large aperture was required to obtain a sufficient level of signal-to-noise.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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