This instrument is a special-purpose design to accomplish observations that other missions, such as the Hubble Space Telescope (HST), are not able to perform. The use of a pre-dispersing prism to prevent long wavelengths from entering the spectrograph permits a long-aperture approach to echelle spectroscopy, and the chosen combination of imaging and dispersion scales provides high spectral resolution of emission line profiles with a several arc sec wide aperture for good sensitivity. For comparable spectral resolution the HST/STIS uses a 0.2 arc sec aperture, which provides 375 times less solid angle on the sky than a 3 x 5 arc sec region observed by our rocket telescope. For comparison, the ratio of telescope areas HST/rocket is roughly 50 times. With equivalent efficiencies, HST would need 4 hours of observing time to achieve the same S/N that the rocket obtains in 5 minutes. STIS also experiences order overlap of longer wavelength light from the echelle grating (wavelengths which never enter our spectrograph due to the pre-dispersing objective prism).
The VeSpR telescope is a Cassegrain design with a Dall-Kirkham figure, and with a 35 cm diameter ellipsoidal primary and a spherical secondary mirror providing 1-2 arc sec image quality within a few arc min of the optic axis. The telescope delivers a f/21 beam to the focal plane with a plate scale of 26 arc sec/mm. The spectrograph has been designed and constructed for UV echelle line profile measurements with long-slit imaging. The Ebert-Fastie configuration employed in the design of the spectrograph has many characteristics well suited to the science needs of this mission. Symmetric off-axis reflections from a single collimating mirror are employed to remove aberrations: the spatial resolution is limited by the telescope and the spectral resolution by the grating and aperture characteristics. Use of a paraboloidal collimator, has produced 2 arc sec image quality with minimal astigmatism along the central 2-3 arc min. A 2 deg. magnesium fluoride (MgF2) objective prism mounted forward of the aperture plate is used to disperse the converging beam from the telescope: with the 1216 Å image focused onto the primary aperture, other wavelengths are excluded from the spectrograph producing a very low level of scattered light on the detector. The spectrograph uses a replica of a prototype grating for the HST STIS. For a 5 arc sec aperture width, the measured resolution in the last flight was 0.055 Å FWHM. We have included a camera to re-image the focal plane onto a separate detector, providing an image of the telescope field of view minus the light which passes through the spectrograph aperture. The Venus image on the aperture plate will be sufficiently dispersed by the objective prism that the Ly α image will be clearly separated from the longer wavelength continuum and emissions.