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