Lessons learned from SMEI. and their potential implications for WASSS B.V. Jackson & A. Buffington 12 May 2010.
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Lessons learned from SMEI
and their potential implications for WASSS
B.V. Jackson & A. Buffington
12 May 2010
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).
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.
Launch 6 January 2003
1 gigabyte/day; now ~3 terabytes
Simultaneous images from the three SMEI cameras.
[Jackson, B.V., The Solar Mass Ejection Imager (SMEI) Mission, Solar Phys., 225, 177, 2004.]
SMEI orbit difference images
Figure 3. STEREO HI-1,2 instruments.
Figure 4. Wide-angle imager + corral. (PERSEUS proposal, see also Buffington, 1998; 2000)
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.
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.
Mirror interior design (Diamond-turned mirror manufactured at Bach Research, Boulder)
The Diamond-turned mirror is at UCSD where tests show its stray light properties