(note : although webcams and video cameras are equipped with a real CCD sensor, for reasons of simplicity the term CCD designates in this text the specialized cooled cameras for astronomy)

Here are two images of the Straight Wall that I made several years apart with the same telescope. The left one has been taken with an astronomical CCD camera (KAF-0400), the right one is a combination of 27 images taken with a webcam (Philips Vesta Pro). Although the right image has been taken in far worse seeing conditions (Airy pattern strongly distorted and shaken), its resolution is clearly better. The same observation can be made on the webcam image of Saturn of October 31 2002 that shows the Encke division much better than my CCD images of the previous years do. Yet one could think that the considerably higher price of the CCD would be for it a guarantee of much higher performances. But things are not so simple...

The difference of quality between these two images is perhaps liked partially to the slightly higher sampling for the webcam image, but it is certainly not the main explanation. The first superiority of a webcam, and more generally of a video camera, is its image rate that tremendously increases the probability of obtaining good raw images in the presence of turbulence, which is almost always the case. With a full frame every 10 to 30 seconds, the CCD has no chance in the face to the 5 to 10 images per second of the webcam. When the webcam image above has been taken, the turbulence did not left more than about thirty exploitable images on more than a thousand taken during several minutes. In CCD, the probability to fall on one of these good images would have been virtually null.

16-bit or 8-bit?

The 16-bit digitization of a CCD camera corresponds to a dynamic of 65536 (216) shade of gray, that seems to give to it a decisive advantage over webcams and standard video cameras that only have a digitization on 256 shades of gray (8-bit). But this reasoning does not take one thing into account: the noise.

There are different sources of noise, the first that comes to the mind is the electronic of the camera but another one is very important in planetary imaging : the light of the object itself. Everyone knows that light is composed of photons, but it is less known that they come in a mess, like raindrops, and the consequence is that if one counts during one second the number of photons coming from a very stable source (for example a sun-like star observed from space), this number will vary from one second to the next. The variation of the measure is a noise, the photon noise, and its value is a function of the square root of the number of photons: more light means a better signal-to-noise ratio but also more noise.

In a CCD camera of good quality, for a bright object like a planet or the Moon, this photon noise is the dominant noise in the image. The computation, for an amateur camera equipped with a Kodak sensor (KAF-xxx), of the quantity of noise corresponding to a planetary image correctly exposed shows that a 10-bit or even a 9-bit digitization would be sufficient, the rest is useless luxury since the last 6 or 7 bits are lost in the noise. This result is confirmed by a reduction of the number of bits (image below), starting from 16 bits raw images: at 10 bits no damage is visible. At 8 bits, the image becomes noisy because it is the digitization noise that is now dominant.

On the strict level of digitization, one may consider that a combination of several video images (2 to 8) gives a results that is not lower than a CCD image. However, we must take into account the noise of the webcam and the image compression: in practice, the experience shows that the combination of 1 to 3 dozens of images taken with a webcam gives an excellent result by comparison with a CCD image. Even if more images must be combined than in CCD, considering the highest number of raw images, the situation remains to the advantage of video.

Color or monochrome sensor?

The sensor of webcams of a color sensor composed of a mosaic of pixels sensitive to blue, green or red. For the Sun and the Moon, color is useless and one may think that a monochrome sensor would allow to work with a lower sampling and to decrease the exposure time (a monochrome sensor is generally much more sensitive than a color one). However, we must notice that a webcam used in black-and-white mode already gives excellent results, even if its sensor remains a color sensor (in black-and-white mode the 3 color layers given by the camera are averaged).

Why such a price difference between CCD and webcam ?

Comparing the prices of a CCD camera and a webcam, like comparing their weight or their color, is meaningless: the price difference is above all a question of the quantity produced. If astronomical CCD cameras were produced in Asia by hundreds of thousands like webcams, their cost would be much lower than today. We must also take into account the fact that a webcam contains no cooling system since it is not destined to long exposures. Moreover, the sensor of a webcam (interline type) allows to do without a mechanical shutter, costly and source of vibrations. A CCD Camera is above all optimized for deep-sky imaging where it work wonders by comparison with a webcam, but it is not wise to consider that the price rate between CCD and webcam is representative of the difference of planetary performance. In this case, for planetary imaging we can take the comparison between a Formula 1 car and a 4-wheel drive vehicle; the former is immensely more expensive, but it is not certain that Michael Schumacher, even with all his know-how, could easily win the Paris-Dakar rally with his Ferrari.

And the ease of use?

In video, the high rate of images presents another advantage: the use of a webcam or a video camera is much easier than a CCD camera because centering and focusing can be made almost directly, and that gives superior comfort and ease of training.

Considering its conviviality and its relative tolerance toward atmospheric instability, a webcam allows to obtain more easily and more often results that are equal or superior to those of CCD cameras.