Webcam astronomy
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A webcam observatory at school

Valuable observations can be made with Internet cameras – webcams. A typical webcam usually costs about €25 and after slight adjustments it can be used for very interesting observations. Even in the city, with pollution, using a small telescope or a zoom objective it is possible to catch interesting images of the Moon, Jupiter atmospheric bands and satellites, Saturn Rings, red-planet Mars, ...

 

 

Why observe?

Astronomical observations belong to the simplest and least expensive forms of examining and admiring the laws of nature. Unlike many other branches of physics, astronomy is a field where scientifically valuable observations may be carried out by amateurs. Beginners will surely focus on admiring their own photographs of planets, comets and stars; more advanced observers will find it even more rewarding to record the variability of stars, discovering new comets and following planetoids in a systematic way. Our observations will be of much greater value if we manage to register them. The easiest way of registration is taking photographs. This method, however, is not without its drawbacks. In amateur conditions it is not possible to take many photographs in a short time – this usually requires a long time of exposure to achieve good quality. Also their further processing and comparison is rather difficult. The properties of photographic film make it easy to under- or overexpose the photo, since in a certain range of brightness the degree of optical density is not proportional to the light intensity. Most of these obstacles can be overcome by using the CCD webcam as a tool for registering the images.

 

CCD Observatory – What are we going to need?

In order to carry out the observations we are going to need: a webcam with a CCD sensor, a photographic lens or a telescope, and a computer with appropriate software. Let us look at these elements. In order to be able to judge properly the advantages and disadvantages of particular kinds of webcams we are going to need some basic knowledge about the structure of CCDs. The principal part of a CCD (Charge Coupled Device) consists of a board of photosensitive elements which form a matrix. The number of these elements – pixels on the board – determines the resolution of the system. The matrices used in amateur webcams vary from 320 x 240 pixels to 640 x 480 pixels.  

 

Webcam: how to choose it?

The most important parameter in a webcam is the type of sensor used in it. Although we can choose between cameras with CCD and CMOS sensors, the former are definitely better (more sensitive). It is better to choose a camera with a resolution of 640 x 480 pixels. The possibility of setting a very long time of exposure (up to half a minute) is a huge advantage of a webcam. Even though most webcams available on the market do not fulfil this standard, it is possible to modify some of them so that they permit longer times of exposure than those preset by the producer. 

 

Webcam: how it works?

When light falls on a CCD board, its intensity for each photosensitive element  is measured. In this way we are given  information about the brightness of the  registered image.   Fig. 1. Webcam's sensor is covered with a  filter in order to provide information about the  light's colour. To each of the pixels this filter  transmits light in one of three colours: red (R),  green (G) or blue (B). During the image  processing the intensity of light in these three  basic colours is measured. The true colour of a pixel is achieved by interpolation of  the neighbouring pixels. The components of the colour for each pixel are calculated  on the basis of the colour components in the neighbouring elements.

 

The lens of the camera

Replacing the original tiny lens of the webcam with a photographic lens from a 35 mm reflex camera can give very good results at a moderate cost. Because the CCD sensor is much smaller than a film frame, the view field of a webcam with a photographic lens is much smaller than the view field of a camera equipped with the same lens. To describe this effect in numbers, we introduce the notion of an equivalent focal length. It is as many times greater than the real focal length of the lens, as many times the diagonal of the CCD sensor is smaller than a 35 mm film frame. For Philips webcams with a sensor of the type ¼’’, whose photosensitive field has the diagonal of 4,8 mm, this ratio equals 9. This means that a webcam equipped with a standard lens with a focal length of 50 mm, has the same view field as a camera with a large telephoto lens with a focal length of 450 mm! Thanks to this fact, we can use the most popular lens, whose focal lengths are between 35 and 200 mm, to take the same photos as with photographic cameras equipped with telephoto lens whose focal lengths would be between 300 and 1800 mm! The lens produced for reflex cameras have many types of fastenings but for the purposes of astrophotography the most suitable are those with a M42x1 thread.


Interested, ready to try, want to learn more?

A manual with a full description of the experiment, including software use, can be found here (PDF).