Dark Spot on EVE SAM Images
The Solar Aspect Monitor (SAM) on NASA’s Solar Dynamics Observatory - Extreme ultraviolet Variability Experiment (SDO-EVE) is a pinhole camera. It utilzes a metal foil filter and a small, pinhole aperture, to image the Sun in soft X-rays (0.1 – 7 nm) on a corner of one of the EVE CCD detectors. Some astute observers have noticed that sometimes there is a dark spot just off the limb of the Sun. This is due to a blemish on the CCD, not an actual object in space. The first image below (Fig. 1) is a flat field of the portion of the CCD where the SAM images are located. The white spots and streaks are transient charged particle hits during the flat fielding. The dark spot is a permanent feature of the CCD response, basically insensitive pixels. The instrument is aligned and pointed such that the dark spot is off the disk of the Sun (see Fig. 2), so the spot cannot be seen against the black of deep space. Sometimes, however, there is an appropriately placed active region on the limb of the Sun with material in the upper atmosphere of the Sun (the corona) that is hot and emitting X-rays that SAM can detect. If that off-limb signal falls in the area of the CCD blemish, then the dark spot becomes readily visible (see Fig. 3).
Figure 1. A flat field (uniform illumination) of the SAM portion of the EVE CCD detector. The blemish is shown as a dark collection of pixels.
Figure 2. A typical SAM solar image with the CCD blemish off the solar disk not visible against black space.
Figure 3. A SAM solar image with well-placed off-limb solar X-ray emission. The CCD blemish is readily visible against the bright background.