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

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

Corral concept: laboratory tests

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

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

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

Hemispherical-Imager Mirrors

Conventionally turned & polished Diamond-turned

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.

Schematic optics cross-section

Mirror interior design