SOFIE instrument includes a sun sensor which provides precise knowledge
of the location of the solar image relative to the science
The SOFIE sun sensor (SS) uses a radiation hardened focal plane array (FPA) image sensor with 1024 x 1024 pixels. The FPA field of view is 2.04 degrees in azimuth and 2.025 degrees in elevation. Individual FPA diodes are 15 microns square, and subtend roughly 7.14 arcsec at tangent. Incoming light is dispersed by the sun sensor optics according to the Airy disc function shown in Figure 1. This blur actually increases the ability to determine the solar edge location. The sun sensor center wavelength is 701.4 nm with a bandwidth of 23 nm (FWHM) (Figure 2). A file of relative spectral response (RSR) data for the sun sensor is available below. Atmospheric attenuation in the SS bandpass is due primarily to water vapor, ozone, and molecular (Rayleigh) scattering. Figure 3 shows calculated atmospheric transmission and refraction angle profiles for the SS bandpass. Atmospheric refraction causes the solar image to become compacted and displaced upward, as illustrated in the sunset movie below.
Figure 3. Top: Calculated limb path transmission for the sun sensor considering ozone and H2O absorption and Rayleigh (molecular) scattering for summer conditions at 60°N latitude. A breakdown of the attenuation by absorber is available here. Bottom: The refraction angle versus altitude calculated for the sun sensor wavelength and summer at 60°N latitude (solid line). A fit to these data (dashed line) allows refraction angles to be easily approximated as a function of altitude (km).
Figure 1. The sun sensor Airy disc function, or "blur circle". The Airy disc data are available below.
Figure 2. The SOFIE sun sensor RSR based on lab measurements of flight spare neutral density and bandpass filters, combined with the detector RSR reported by the manufacturer. The above curve does not include other intervening optics, however, they should not introduce spectral content. Vertical lines indicate band center and half power points. Sun sensor RSR data are available below.
Theoretical basis for the sun sensor (word document).
Conceptual movie of sunset from space, illustrating image refraction and attennuation (Quicktime).