1. Lessons learned from SMEI and their potential implications for WASSS B.V. Jackson & A. Buffington 12 May 2010

2. . Figure 1. Surface brightness versus solar elongation for zodiacal and star light (from Allen's Astrophysical Quantities, Cox, 2000), and of various sky brightnesses from observations using the Helios photometers and SMEI. One S10 is the sky brightness of one 10th magnitude solar-type star spread over a square degree.Also shown is an ambient heliospheric medium having a density of 10 cm-3 at 1 AU and an inverse-square density drop off with solar distance. The SMEI stray light value is from Buffington, Jackson and Hick (2005), the STEREO HI-2 upper limit is from Eyles et al. (2009).

3. Figure 2. Sky maps of zodiacal light (corona) and sidereal sky as measured by SMEI in red light.The zodiacal light (top panel) matches well the table presented in Astrophysical Quantities (Cox 2000) and, more recently, in Kwon et al. (2004).The sidereal sky (bottom 3 panels) shows the Milky Way Galaxy and numerous individual stars. Note the differing coordinate systems for these: Sun-centered Ecliptic for the zodiacal light, and Right Ascension and Declination for the starlight. In a year’s time, the zodiacal light moves along a sinusoidal path over the sidereal map.

4. Background light levels vs solar elongation • At 5º away, the corona is ~10-9 as bright as the Sun, at 30º away, 10-11. • To enable clean subtraction of this, and of the attendant starlight, stray light must be reduced below these values (probably well below…) • To enable seeing below 1 S10, unaccountable time-variable stray light must be < 10-14.

5. Three optical schemes for wide-angle photometric measuring of the sky • SMEI – Solar Mass Ejection Imager (Eyles et al., 2003; Jackson et al., 2004) • Scans most sky in a single orbit (102 min) • Labyrinthine baffle reduces sunlight ~10-10 • Optics/data analysis further reduce this to ~10-15 • Wide bandpass, fast optics for max. photoelectrons • 3º×60º field of view, 10th magnitude star gives about 1000 photoelectrons per 4-second exposure • 0.1% photometry enables 3σ measurement of one S10 change within a 1º×1º sky bin per orbit

6. STEREO HI-2 • Staring field of view ~60º dia. , simple-lens optics • Combination corral-style and labyrinthine baffle • HI-2 parameters from Eyles et al. 2009 • FOV 70º • Aperture diameter 7 mm • Focal Length 22 mm • Views 18.7º to 88.7º from Sun • Pixels 2048 x 2048, 13.5μ • TM bin size 0.07º (2x2 binning) • Bandpass full (400 – 1000 nm) • Individual frame integration: 50s • Onboard summed & binned: 99 • Onboard cleaning of contaminants Figure 3. STEREO HI-1,2 instruments.

7. Wide-angle Imager + corral • Smooth corral removes sunlight & background from outside a hemisphere field of view • Primary optics produces virtual image of sky • Secondary optics images sky onto detector • Background light sources must be kept out of “field of regard” • Enables looking within a few degrees of Sun’s disk • Smooth corral is simple & robust • Strongly curved optics can be diamond-turned for low scattering • Very large FOVs up to 180º are enabled, but at a potential cost of aperture area Figure 4. Wide-angle imager + corral. (from Perseus proposal), see also Buffington 1998, 2000

8. Figure 5. Laboratory-measured stray-light rejection from a curved (radii R = 1.0 m and 0.5 m) surface. Note the insensitivity of the performance to surface smoothness (from Buffington, 2000). The corral potentially provides nearly all the stray-light rejection that is needed, but also needs ~1 m of “lever arm” to enable looking a few degrees close to the Sun. If we can’t get this large a lever arm, either can’t look as close to the Sun, or must use smaller aperture.

9. WASSS Design Challenges & Parameters • Stray-light control – direct & reflected sunlight • This is the driving design parameter! • Sun disk must be kept out of the “field of regard” • Illuminated S/C objects, ditto • Moon near FOV may be a problem, loss of FOV • LEO orbit may suffer from particle contamination (SSA & auroral ovals) • LEO orbit may suffer from auroral light • Scanning versus staring: option choice

10. Challenges, continued #2 • FOV size, Angular resolution, detector properties • Fine angular resolution enables resolving smaller objects, but may require longer integration time to reach a given photometric accuracy • Number of pixels: is Readout noise a problem? • Is subpixel structure a problem for the photometry? • Focal ratio: “fast” optics versus difficult PSF’s • For a given detector size, effective aperture diminishes as FOV is increased, given focal ratio remains the same • What photometric accuracy is required, to remove sidereal & zodiacal background to desired level? • Wide dynamic range for detector • Sky brightness varies over several decades • Could control by individual-pixel readout or with filters • Problem with Moon in FOV due to full-well overflow

11. Challenges, continued #3 • How much temporal loss of sky coverage is OK? • What level of photometric precision and calibration (pre- & during flight) is needed? • How much onboard data analysis is required, and what will it then deliver to the pipeline for on-the-ground analysis? • What further on-the-ground analysis is required? • Is a mechanical shutter necessary? • Is periodic explicit dark-current calibration needed? • If so, how well, how often? • What S/C pointing accuracy/stability is needed, in order to remove the bright, bumpy sidereal background from the sky images?

12. Summary • Stray-light control should be “front and center” in the design • How close to the Sun can be viewed is governed by how well the stray light can be controlled, which in turn is governed by how large the stray-light control system can be

13. Supplemental SlidesReferences • Buffington, A.: 1998, Appl. Opt. 35, 6669. • Buffington, A.: 2000, Appl. Opt. 39, 2683. • Buffington, A., Jackson, B.V., Hick, P.P.: 2005, Proc. SPIE 5901, 590118, 1. • Cox, A.N.: 2000, Allen’s Astrophysical Quantities, fourth edition, New York. • Eyles, C.J., Simnett, G.M., Cooke, M.P., Jackson, B.V., Buffington, A., Hick, P.P., Waltham, N.R., King, J.M., Anderson, P.A., Holladay, P.E.: 2003, Solar Phys. 217, 319. • Eyles, C.J., Harrison, R.A., Davis, C.J., Waltham, N.R., Shaughnessy, B.M., Mapson-Menard, H.C.A., Bewsher, D., Crothers, S.R., Davies, J.A., Simnett, G.M., Howard, R.A., Moses, J.D., Newmark, J.S., Socker, D.G., Halain, J.P., Defise, J.M., Mazy, E., Rochus, P.: 2009, Solar Phys. 254, 387. • Jackson, B.V., Buffington, A., Hick, P.P., Altrock, R.C., Figueroa, S., Holladay, P.E., Johnston, J.C., Kahler, S.W., Mozer, J.B., Price, S., Radick, R.R., Sagalyn, R., Sinclair, D., Simnett, G.M., Eyles, C.J., Cooke, M.P., Tappin, S.J., Kuchar, T., Mizuno, D., Webb, D.F., Anderson, P.A., Keil, S.L., Gold, R.E., Waltham, N.R.: 2004, Solar Phys., 225, 177. • Kwon, S.M., Hong, S.S., Weinberg, J.L., 2004. An observational model of the zodiacal light brightness distribution. New Astronomy 10, 91-107.

14. The most severe restriction placed on a heliospheric imager is the very large difference between the signal intensity and the brightness of the Sun, the latter of which will also illuminate the spacecraft bus and portions of the imager (Figure 1). This large range of brightness challenges, and usually precludes, direct laboratory testing except under very limited circumstances (e.g. Figure 3 below). In a heliospheric-imager design there are two important considerations in the elimination of stray light, the field of view and the “field of regard”. The first is simply the region of the sky that falls on the image plane of the detector, while the second is the region outside this which, because of its proximity to the field of view, must also not have extremely-bright objects present. For the Helios 90˚ photometer, a SMEI camera unit, and the STEREO HI-2, the former quantity is respectively, 3˚, 3˚×60˚, and 70˚. All of these designs employ a baffle system to reduce light from objects within the field of regard but beyond the field of view. The baffles of Helios, SMEI, and STEREO are multistage labyrinthine designs that employ multiple vanes to reduce light multiple-scattering and/or diffracting over baffle edges (Buffington, Jackson, and Korendyke,1996; Buffington, Jackson, and Hick, 2003; Eyles et al., 2009) in order to prevent stray light from getting through the instrument’s main aperture in the first place. It is best if an instrument defines its own field of regard, but this is possible for a very wide-angle viewing design only if the instrument’s location on the spacecraft bus is somewhat above a horizontal plane that extends outward from the instrument aperture, and so that the field of regard does not include the Sun or, in the case of an instrument in Earth orbit, both the Earth and the Sun.