1. B. V. Jackson 1, A. Buffington 1, P. P. Hick 1, H.-S. Yu 1 Mario M. Bisi 2, and David F. Webb 3 Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Drive #0424, La Jolla, CA 92093-0424,USA 1 Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Drive #0424, La Jolla, CA 92093-0424, USA 2 RAL Space, STFC Rutherford Appleton Laboratory, Harwell Oxford, England, UK 3 Institute for Scientific Research, Boston College, Newton, MA 02459, USA ASHI – A Light-weight All Sky Heliospheric Imager Design for the DSCOVR Follow-on Abstract The zodiacal-light photometers on the twin Helios spacecraft, the Solar Mass Ejection Imager (SMEI) on the Coriolis spacecraft, and the Heliospheric Imagers (HIs) on the Solar-TErrestrial RElations Observatory (STEREO) twin spacecraft all point the way to optimizing future remote-sensing Thomson-scattering observations from deep space. In the future, such a light-weight instrument could be provided by two very wide-angle (hemispheric) viewing systems incorporated with other instrumentation at L 1 on the DSCOVR follow-on. The specifications for this instrument system include viewing the whole sky from a few degrees of the Sun, to as near anti-solar as the Moon and Earth permit from L 1, with an instrument mass of about 2.5 kg per system and ten minute integration times. Moreover, to enable useful 3-D reconstructions of solar mass ejections and co-rotating structures from the imaging system's data, a key photometric specification is 0.1% differential photometry in a one square degree sky bin. Here, we review this instrument and the optics and baffle techniques that enable this light- weight system. 1. The All-Sky Heliospheric Imager (ASHI) Instrument 3. ASHI optics cross section The optics cross section depicts the multi-optic aspect (see Buffington et al., 1998) used for ASHI (characteristics given in the next Section’s Table). The imaging system allows a nearly linear placement of the mirrored exterior and inverted interior images from the optical system onto a flat CCD plane. An example of a composite and reconstructed image (right) is from the front cover of Applied Optics 39, #16 (see Buffington, 2000). The Solar Mass Ejection Imager (SMEI) heritage (Jackson et al., 2004) was used provide this advanced ASHI design as described in Jackson et al. (2010). 2. The ASHI Field of View Two optical systems as shown here (in Sections 1 and 3 left) provide an ASHI field of view, that is nearly all-sky. A small 4º- wide band of the sky near the Sun is excluded from this remote view. The Earth and Moon will blank out portions of the CCD that can be oriented such that these locations are at the far east and west ends of the all sky view (white dashed lines). This image was adapted from the SMEI (Solar Mass Ejection Imager) composite all sky image presented in National Geographic Magazine, July 2004, pps. 14-15. The ASHI instrument provides a hemispheric view of the sky from L 1. Bright spacecraft appendages must be kept at least 2º below the “field of regard” (the 180º portion of sky above the instrument corral). This can be accomplished by building the spacecraft bus so that each imager faces away from the center of the bus, and having a sun-pointed 3-axis stabilized system that keeps the Sun 2º below the instrument horizon. 4. ASHI instrument specifications (The UCSD main library building)

2. ASHI full-up instrument optical tests giving night-sky sky though-put from a cooled CCD camera and the model instrument mounted on the roof of the UCSD laboratory. For this test, a hand-buffed mirror and central optic was used to provide the images of Sections 2 and 4. 5. ASHI instrument tests Images of the night sky taken from the ASHI set-up to the left. 6. ASHI diamond-turned optic tests An L 1 ASHI, is designed to accommodate some stray-light brightness from the brighter background features within the field of regard. At L 1, these features include light from the Moon and Earth that will on occasion fall onto the mirrored optic. Direct light from these sources on the CCD image plane will need to be eliminated from the response either by masking that region of the CCD or by selective read-outs of different quadrants of the CCD. To insure that scattered light from these unwanted bright sources will not scatter around the mirrored system, a diamond-turned optic or similar non-scattering optical mirror system is required for ASHI. Above: An ASHI optic before diamond turning and (for the interior) black anodizing. The internal mounting brackets are shown. This size mirror was chosen to provide 0.1% photometry, within an hour’s integration time and a 1° × 1° sky bin at about 90° elongation from the Sun, at 1 AU. Below: Diamond-turned optic from Bach research, Inc. (see Buffington et al., 2009) References Buffington, A., Hick, P., Jackson, B.V. and Korendyke, C.M., 1998, ‘“Corrals, hubcaps and crystal balls”: some new designs for very-wide-angle visible-light heliospheric imagers’, Proc. of SPIE, Vol. 3442, 77-86 Buffington, A., 1998, ‘Very-wide-angle optical systems suitable for spaceborne photometric measurements’, Applied Optics 37, 4284-4293 Buffington, A., 2000, ‘Improved design for stray-light reduction with a hemispherical imager’, Applied Optics 39, 2683-2686 Buffington, A., et al., 2009, ‘Fabrication and Test of a Diamond-turned Mirror Suitable for a Spaceborne Photometric Heliospheric Imager’, Proc. of SPIE, Vol. 7438, 74380O-1-12, doi: 10.1117/12.825362 Jackson,B.V., et al., 2004, ‘The Solar Mass-Ejection Imager (SMEI) Mission’, Solar Phys., 225, 177-207. Jackson,B.V., et al., 2010, ‘A Heliospheric Imager for Deep Space: Lessons Learned from Helios, SMEI, and STEREO’, Solar Phys. 265, 257–275 Above: Schematic plan diagram of the stray-light measurement setup within the UCSD clean room. The mirror being tested remains within the HEPA Workstation and is illuminated as shown by a laser beam. The Aperture Screen removes light outside of the main beam. Scattered light on the Viewing Screen is viewed by the camera. Specular-reflected light passes through a hole and is absorbed into the SMEI Prototype Baffle. Above: Diamond-turned optic stray-light measurement perpendicular to (left) and parallel to (right) the diamond-turned furrows using the setup above. This mirror provides sufficient reduction near the image location of either the Sun and Earth from L 1 to prevent this light from creeping around the mirror and illuminating nearby portions of the image. The final optics enclosure’s interior vanes and non-reflecting surfaces will reduce any residue of scattered/reflected light from the mirror. Left: Scattering from the baffle system (Buffington, 2000). This reduction is required primarily to remove scattered sunlight near the dark sky when viewing within a few degrees of the solar surface. The tested reduction within 2º of the Sun’s disk from the ASHI prototype baffle is sufficient to allow a view of Thomson-scattered light from this 2º all the way out to as distant from the Sun as 180º (antisolar). 7. ASHI baffle tests