ASHI, an All Sky Heliospheric Imager Designed to Maximize the Scientific Return from Structures Passing the Spacecraft

We have conceived, designed, and evaluated components for an All-Sky Heliospheric Imager (ASHI), suitable for flight on future space missions. ASHI is currently manifested on a DoD Space Test Program ESPA ring in geosynchronous orbit where high satellite communication downlink rates are relatively easy to provide. As a simple, light weight (<5kg), and relatively inexpensive instrument, the ASHI system has the principal objective of providing a minute-by-minute and day-by-day near real time acquisition of precision photometric maps of the inner heliosphere. The instrument's optical system is designed to view a hemisphere of sky starting a few degrees from the Sun. A key photometric specification for ASHI is 0.1% differential photometry in one-degree sky bins at 90 degrees elongation that enables the three dimensional (3-D) reconstruction of heliospheric density extending outward from near the Sun. The ASHI system, unlike other operating or planned heliospheric imagers, is intended to maximize the scientific return of heliospheric structures that pass the spacecraft. This will allow density structure measured in-situ to be extended outward into the surrounding plasma. This is especially important at Earth where recent high-resolution Solar Mass Ejection Imager (SMEI) analyses have shown some CMEs with a corrugated structure when they pass Earth. We anticipate that velocity can also be determined from the 2-D sequence of images by following the motion of the background structures using correlation tracking techniques, and also using 3-D reconstruction techniques along with density. This should also help determine the extent to which this corrugated structure extends to CME velocity. An instrument similar to that of the full space-borne ASHI system has been tested this year on a NASA-sponsored topside balloon flight, and it is our intent that a successful space-borne flight make inroads into this result found in the SMEI analyses.