Taking a picture with the Hubble Space Telescope isn't quite like taking a picture with a point-and-shoot camera. The cameras on the telescope don't produce color images automatically, but make black-and-white images through color filters. Color images are assembled from a number of exposures taken during Hubble orbit around the Earth.
The color image of Comet ISON described in a previous blog post is a composite of five exposures taken on April 30, 2013. All of the images were made with the Wide Field Camera 3 UVIS instrument (WFC3/UVIS) during one orbit of Hubble around the Earth. Three exposures of 440 seconds each were made using the V band filter (technically known as F606W), which transmits yellow/green light, and two exposures of 490 seconds each in the I band filter (F814W) which transmits red and some near infrared light.
These exposures were made while the telescope tracked the stars. Because of the motion of the comet and the motion of HST in its orbit around the Earth, the comet trailed slightly relative to the stars during and between these exposures. This is not the way comets are usually observed. Normally we would track on the comet to keep it stationary in the camera during the exposure. However, in this case we wanted to produce an image of the comet against a background clearly showing stars and galaxies.
The separate images from each filter were combined to result in two images.
Applying different colors to each image and combining them produces a color composite image. In this case we applied colors appropriate to the two filters: blue/cyan for the V and red/orange for the I. In other cases we have three images and would usually use the primary colors red, green and blue.
When these images are combined, the result is a full color image.
But now it's pretty obvious that there's something a little fishy with the comet. Since the comet moved between the individual exposures, it is not uniform color, as we expect. So in order to produce an image that better reflects what the comet really looks like, we removed the comet from the images in which it did not overlap but kept a single exposure of the comet shown as gray/white.
More information about this image, full-size versions, and other formats are available from NewsCenter.
The stars and galaxies in the background sky appear in different colors since their images overlap in the exposures in different filters and their different physical properties produce light of different colors (red stars are cooler, blue stars are hotter, for example). Since there were not two separate overlapping images of the comet in both colors, it appears neutral (gray) in the final composite image. But we also know that comets shine by reflected light from the sun and have very little intrinsic color, so appear gray/white in images made in visible light.
If you're interested in the original data used to produce this image, it's available from the Mikulski Archive for Space Telescopes (MAST).