From the Telescope to the Web
The images coming straigh from the telescope are quite different from the final product that will either be published on the web as a picture for the public, or used by the astronomers to study the comet. The various steps of this process are presented below.
The "raw" image, as it appears on the control screen at the telescope, shows all the features of the CCD detector that is used in the instrument (more details can be found at O. Hainaut's dedicated page). On this image, one sees the black gap between the 2 CCD composing the camera, and the field mask defining the countour of image, and the vignetting caused by an internal part of the instrument, that appears as a black area on the right.
Correcting the Image
The individual images are then corrected for all these instrumental effects, and several images can be combined together, resulting in a longer exposure time. The image at Figure 2 shows the combination of 9 individual exposures, that have been cleaned, then shifted in order to co-align them on the comet and finally added together. Note the background star that appear as little trails. As the comet moves along its orbit around the Sun, it appears in motion with respect to the background stars. As the images are re-centered on the comet, the stars appear as trails.
Seeing in Colour
The CCD cameras used in the astronomical instrument only measure the intensity of light reaching them. In other words, they produce black-and-white images. In order to reconstruct the colour information, the images are taken through colored filters. While an image as seen by a human eye can be reconstructed from only 3 separate colours (which is why the colour TV screens are consituted by little pixels emmitting red, green and blue light), the astronomical images are routinely taken through 5 different filters: Red, Green, Blue, but also Ultraviolet and Infrared. Combining these filters would then build an image that has much more information than a human eye could see. This extended color range permits the astronomers to expand their studies. For a nice picture to be published on the Web, three of the filters are generally combined to construct a RGB image such as the above one. In this case, the colours of the comet are extremely soft: the dust cloud constituting the coma of the comet just reflect the sun light.
In order to put in evidence the faint part of the coma, the image can be further enhanced. The intensity scale (i.e. the image intensity corresponding to the real intensity) can be stretched to boost the low light level, but without saturating the brightest part of the image. This would be the final image normally going on the web.
Calibrating the Image
For the astronomers, the previous colour image is not the most interesting way to represent the data. Further image processing and enhancement methods are applied to the data in order to reveal fainter or smaller features.
For instance, it is interesting to look in the coma for "jets" and "fan" structures that would reveal active area on the surface of the comet nucleus. To do so, a mean luminosity profile of the comet (i.e. the brightness starting at the center and moving outward) is extracted from the original image, then is subtracted from the comet. This removes the component of the coma that is "circular", leaving only the asymetric parts. In this resulting image, where the intensity is represented using an arbitrary colour scale, various bright and dark sectors are apparent. It will be interesting to study their evolution. Also, the image has been calibrated in kilometers so that the real size of the comet can be estimated. For instance, the frame shown here is just slightly larger than the Earth!