These are just columns of pixels that do not work properly. There are several reasons for a column to be bad (ranging from electronic to mechanic), some columns are completely dead (dark), some are "hot" (bright), some have just a constant added to what they should normally show. A good CCD usually does not have more than a couple of them. They are quite easy to correct when they are isolated (one interpolates the neighboring columns), but nothing can be done when several are next to each other.
Some pixels are damaged: they cannot transfer the electrons when the image is read out. Therefore, all the pixels below such a bad pixel will be normal, but all those above it are lost, because their electrons are trapped in the bad pixel; this forms a partially bad column as illustrated in Fig.7.
Although it should not happen, sometime a foreign body can find its way into the CCD housing and end on the detector. These make dark marks on the images, that can usually be partially corrected by flat fielding the frames.
When a high energy particle hits the CCD, it loses its energy by knocking the atoms constituting the chip itself. That liberates many electrons that cause a bright spot on the image. These high energy particle can either be genuine cosmic rays (exotic particle produced by exploding supernovae, black holes, etc.), or just the product of the decay of some radioactive atoms present in the lenses just above the CCD. Cosmic rays are usually easy to recognize, because they are much sharper than stars (the high energy particle hits just a couple of pixels). If one is just planning to produce a nice picture for a web site, they are very easy to clean out. However, removing them without damaging the real objects can be more tricky, but is still possible.
The scale of some CCDs (size measure in arcsecond/pixel, i.e. what is the size of the pixel as projected in the sky) can be fairly large, especially the ones used by amateurs on small telescope. In that case, the image of a star will cover just a couple of pixels (we say that the images are under-sampled) and will look similar to a cosmic ray (c.f. Fig. 10). In that case, it is very important to take several images of the same field of view in order to remove the cosmic rays by taking a median of all the images: the result shows some light where there is light in most of the images, while a pixel hit by chance by a cosmic during only one of the exposures will remain dark.
Figure 10: The same field as in Fig.5 (with the same cosmic rays), with larger pixel to illustrate how difficult it can be to distinguish a cosmic ray from a star when the pixel size is under-sampling the images.
Each pixel can store only a certain amount of electrons (of the order of 100,000). If a pixel is illuminated by a bright star and/or if the exposure time is long enough, that pixel will fill up, and the electrons will start to fill the neighboring pixels: the CCD is saturated. When the image is read, all the extra electrons will be spread over the column containing the saturated pixels, making a saturation trail, as shown in Fig.11.
As a consequence, any bright and narrow feature on the same column as a bright star must be considered with suspicion: it most likely is a saturation trail.
As the light behaves like a wave, it is diffracted when it passes near an obstacle, i.e. a ray of light will be bended, the deviation being stronger if the ray passes closer to the obstacle.
In the case of a telescope, the most important obstacle is the spider, the cross-like support of the secondary mirror. Most telescopes have a 4-legged spider, which produces the typical cross-shaped diffraction pattern visible around the brightest stars. 1-legged spider produces a bar through the star, and some telescopes (like the Keck) have 6-legged spider, so the star images exhibit a 6-legged pattern.
It is important to note that the diffraction pattern is not necessarily symmetric: a cable around one of the legs of the spider will make the corresponding bar of the pattern much weaker (because the cable diffracts the light in random directions).
Also, every star in the field has a diffraction pattern, not only the brightest. If one co-adds many exposures in which just a couple of stars display a diffraction pattern, the result will show many more of these features around much fainter stars. Comets and other extended objects objects usually do NOT have diffraction crosses, because their fuzziness smears the diffraction pattern, which then blends in the object itself.
Figure 12: Complex diffraction pattern of the CFHT telescope: the main cross is not symmetric, and additional, fainter legs are present. The vertical mark is a saturation pattern.
Finally, the spider is the most common cause of the diffraction patterns, but there are other possibilities: the edge of the telescope, the light baffles, the edge of the mirror, etc... almost everything can cause diffraction patterns. They usually look less symmetric than the ones caused by the spider. This can also explain the presence of diffraction pattern in spider-less telescopes.
In some case, a mirror or field lens that has been carelessly
cleaned will cause diffraction-like patterns.
To image a moving object like a comet, the telescope is set to
compensate the motion, so that the comet will appear
un-trailed. Everything else in the field will be trailed. However, the
intensity threshold used to display the image can give the impression
that the brighter stars are less trailed than the fainter ones: the
elongation becomes negligible compared to the large apparent diameter
of the brightest star. The only way to be sure that an object is not
elongated is to display the image with proper threshold, or to measure
the elongation of the object using a software tool that takes into
account every light level. This has to be performed on the original images:
it cannot be done on a JPEG or a GIF copy, in which most of the
information is lost.
Figure 13.a: Some stars from Fig.1. The intensity thresholds are set for the brightest stars, which appear elongated.
Figure 13.b: Exactly the same images as X.a, with different thresholds. Fainter stars are now visible and elongated, while the brighter stars appear now circular, or un-trails
Tue Dec 10 19:09:33 1996