An all-sky heliospheric imager

1. An all-sky heliospheric imager All-sky coverage (when two are employed) 0.1% differential photometry Robust, light-weight design Low background-light contribution When bright background-light contributors are kept away from FOV and optics do not protrude from protected volume

2. Basic concept – see models here • A smooth corral blocks stray light from background sources below FOV • Light must multiple-diffract over surface to reach protected volume • Non-protruding optics provides a virtual image of a hemisphere of sky • A conventional CCD camera views virtual sky • Two such cameras placed on opposite sides of a spacecraft provide whole-sky coverage

3. Corral concept: laboratory tests

4. Principal corral features • Lightweight, simple, robust (resembles a foxhole rather than a castle…) • Does not depend upon high-tech surfaces, nor is surface performance subject to normal contamination • Precise alignment is unnecessary when corral surface extends a bit beyond minimum required • ~ 1 m size permits viewing within 2° of Sun

5. Principal optics features • Non-protruding design • Toroidal mirror covers most of sky • That portion of sky occulted by mirror separately covered by lens in center • Virtual image of sky recorded by conventional CCD camera system • Stray light residue controlled by field stop, Lyot stop and final imaging system

6. Cutaway schematic of corral/optics (left)Conventionally turned optic (right)

7. Hemispherical-Imager Mirrors A diamond-turned lightweight mirror has been fabricated with the tool moving in a circular path, that avoids the “jaggies” that ordinarily plague such strongly curved mirrors. Manufactured by Bach Research, who also made the SMEI diamond-turned mirrors. Conventionally turned & polished Diamond-turned

8. Schematic optics cross-section

9. Mirror interior design