Comet 46P/Wirtanen is a small short-period comet with a current orbital period of 5.4 years. It was the original target for close investigation by the Rosetta spacecraft, planned by the European Space Agency, but an inability to meet the launch window caused Rosetta to be sent to 67P/Churyumov–Gerasimenko instead.
On December 16 Comet 46P/Wirtanen made is closet approach to Earth (7.4 million miles). This is a composite image made of 2 data sets. “The stars only“ data set was an RBGx22x180 or 66 images, 22 for each color channel…. 22 red, 22 blue and 22 green each exposure was 3 minutes long. This was the image used to capture the stars without the comet.
The “Comet no stars” data set was RBGLx10x180 or 40 images 10 red, 10 green, 10 blue and 10 luminance each eas 3 minutes long. This was the image used to create the come with out stars.
The 2 final images were combined to create this composite image seen above.
Because the comet is moving faster than the stars in the sky from our vantage point, it is necessary to capture 2 sets of images, one tracking the stars and one tracking the comet.