Comet SWAN in all-sky map from SOHO
One of the images that was used by Australian amateur Michael Mattiazzo to discover comet C/2020 F8 (SWAN). The image was taken by the Solar Wind ANisotropies (SWAN) instrument aboard the ESA/NASA Solar and Heliospheric Observatory (SOHO).
It is a full-sky map showing differences in ultraviolet brightness between 10 and 11 April 2020. Most spots are just residuals of star signals that have not been completely eliminated in the subtraction process. They are easily discarded, however, because they stay fixed in the sky, while the comet (indicated by the labelled white arrow) changes position from day to day.
On all SWAN maps, there are two large black areas. These are regions that are too bright to be observed. One is centred on the Sun (indicated here with a yellow star), the other on parts of the SOHO spacecraft that reflect sunlight. These areas are often surrounded by bright areas, because the stray light generated by the sun shade and SOHO is variable and so cannot be fully eliminated by the difference process.