ASHI: An All Sky Heliospheric Imager to Provide Space Weather Forecasting Using Thomson-Scattered Sunlight

We have conceived, designed, and are now evaluating components for an All-Sky Heliospheric Imager (ASHI), to fly on future NASA missions. ASHI's principal objective is the minute-by-minute and day-by-day acquisition of a precision photometric map of the inner heliosphere. The instrument's optical design views a hemisphere of sky starting a few degrees from the Sun. Two such instruments on a single spacecraft, or a single instrument if the spacecraft has a 180 degree rotation, can view nearly the whole sky. A key photometric specification for ASHI is 0.1% differential photometry in a one-degree sky bin at 90 degrees elongation. This enables the three dimensional (3-D) reconstruction of heliospheric density starting from near the Sun and extending outward, with updates as heliospheric structures approach, and pass the spacecraft. Velocity can also be ascertained from the imagery in 3-D, by following the motion of the background structures using correlation tracking techniques. SMEI analyses have demonstrated the success of this technique: a similar analysis for ASHI data will yield an order-of-magnitude improvement in 3-D density reconstructions (better than 2 x 2 degrees in latitude, and longitude, and a 2-hour time resolution near the spacecraft). Here we will present the latest results of the ASHI laboratory evaluations, and nighttime full-sky tests of the instrument data sets.