The desired signal from Thomson scattering is very faint, and this places stringent requirements on the performance of the instrument. The background starlight intensity is close to that of the zodiacal light at elongations ≳ 90◦. However, a typical large CME is well below this level. Thus in order to extract the signal from CMEs and other transients of similar brightness, the instrument must be capable of accurate photometry and have a dynamic range of at least four orders of magnitude. Thus the CCD camera has a 16-bit analog-to-digital converter, which gives an instrument dynamic range in the region of 50,000.
The tomographic analysis requires 0.1% differential photometry and background light reduction below the one-S10 (the equivalent brightness of a 10th magnitude star in a square degree) level. This translates to a 10−15 surface brightness reduction relative to the solar disc. SMEI achieves this reduction in stray light through the design of the bafﬂe (10−10) and the optics chamber (10−5) (Bufﬁngton, Jackson, and Hick, 2002).