Video Observation Techniques

Video systems consist of three main parts. You need a fast lens, a powerful image intensifier, and a video camera. The lens projects the sky on the sensitive photocatode, and the camera records the amplified image from the intensifier's fluorescent screen.

Most important for the power of the whole video system is the image intensifier. Preferably it should have a gain >10.000, a photo cathode diameter >18 mm, and as little noise and image distortion as possible. There are different generations of intensifiers available.

• First generation intensifiers often do not have the power required for meteor observation unless they contain three stages of amplification. They produce more noise, but are cheaper than intensifiers of later generations. Unfortunately, many of the intensifiers sold second hand at low prices are single stage first gen intensifiers and therefore unsuitable.
• Second generation intensifiers contain a micro channel plate (MCP) which improve the image quality significantly. They often have a high gain combined with a small distortion of the field of view and only little noise. These devices are well suited for meteor observation.
• Image intensifiers of generation second and a half are of similar design, but have an extented sensitivity in the infrared range, which is not required for meteor observations.
• Third generation intensifer use Gallium Arsenide as photoelectric material which gives them higher sensitivity and a much longer lifetime than second gen tubes. Of course, the better quality has it's price...

To get an image intensifier at a reasonable price you chould check for military surplus devices. We made good experiences with intensifiers sold by Stano components, Dedal, Delft Photonics, Hamamatsu, and Proxitronic.

The type of observation you intend to carry out determines the lens you should use. Most important for good system performance is the f-number of the objective. The lens should be as fast as possible. The longer the focal length of the lens, the smaller the field of view and the fainter the average meteor recorded by the system. The plain number of meteors recorded per hour is often independent from the focal length, only the percentage of sporadic meteors is increasing with the limiting magnitude.

We can distinguish between three types of video systems.

• For wide angle video systems, a wide angle photo lens is used. Such a system has a field of view of more than 40° diameter. The limiting magnitude varies usually between 5 mag and 7 mag for stars.
• Standard video systems have objectives with longer focal lengths. Their field of view ranges from 40 to 20° diameter, the limiting magnitude varies between 7 and 9 mag.
• If you attach a tele lens to the image intensifier, you end up with a telescopic video system. In this case, the field of view is smaller than 15° in diameter and the limiting magnitude is usually better than 9 mag.

Even though standard video systems are currently most common, all three types do have advantages for certain types of observation and meteor showers.

There are only a few constraints for the video camera. If you do not use an integrated system with a CCD chip attached to the image intensifier, you should choose an CCD video module. Every Camcorder can be used, but also ordinary CCD video surveillance cameras do a good job. The camera does not necessarily have to record the time, but a superimposed clock in the video image makes the following analysis easier. Manual gain and brightness control may be of help under certain circumstances but is not a must either.