Rapid Imaging Planetary Spectrograph

Abstract

The Rapid Imaging Planetary Spectrograph (RIPS) was designed as a long-slit high-resolution spectrograph for the specific application of studying atmospheres of spatially extended solar system bodies. With heritage in terrestrial airglow instruments, RIPS uses an echelle grating and order-sorting filter to obtain optical spectra at resolving powers of up to R ∼ 127,000. An ultra-narrowband image from the reflective slit jaws is captured concurrently with each spectrum on the same electron-multiplying charge-coupled device detector. The “rapid” portion of RIPS’s moniker stems from its ability to capture high frame rate data streams, which enables the established technique known as “lucky imaging” to be extended to spatially resolved spectroscopy. Resonantly scattered emission lines of alkali metals, in particular, are sufficiently bright to be measured within short integration times. RIPS has mapped the distributions of Na and K emissions in Mercury’s tenuous exosphere, which exhibits dynamic behavior coupled with the planet’s plasma and meteoroid environment. An important application is daylight observation of Mercury with solar telescopes, as the synoptic context of the exosphere’s distribution comprises valuable ground-based support for the upcoming BepiColombo orbital mission. As a conventional long-slit spectrograph, RIPS has targeted the Moon’s surface-bound exosphere, where structures in line width and brightness are observed as a function of tangent altitude. At the Galilean moons, RIPS can study the plasma interaction with Io and place new constraints on the sputtered atmosphere of Europa, which in turn provides insight into the salinity of Europa’s subsurface ocean. The instrumental design and construction are described herein, and these astronomical observations are presented to illustrate the performance of RIPS as a visiting instrument at three different telescope facilities.