Detectors

Once the electromagnetic radiation has been collected by a telescope, we still have to detect it. In the old days, hundreds of years ago, this was all done by looking through the telescope - human eyes were the detectors of choice because we did not know how to make any other types. Obviously, human eyes do not work for detecting the large variety of radiation we use now to study the sky - they work only in the narrow visible range of the spectrum. Even there, they have been replaced with electronic detectors.

Here is an example (from Teledyne Imaging Sensors). It works in the visible and near infrared, and has 4 million detector elements. Thus, it is a bit like the sensor in a 4 megapixel digital camera. Both devices absorb the photons and each photon frees an electron in the detector material when it is absorbed. These electrons can move around as an electric current, and this current is finally amplified and sent to a computer to display an image. Signals of just a few photons can be detected in this way, allowing the beautiful pictures that you will find in these notes.
Detectors for the X-Ray, ultraviolet, visible, and much of the infrared all look a lot like the one above, but different approaches are needed for photons of much shorter or longer wavelengths. For example, here is one for the very far infrared. Little gold coated cones collect the light and funnel it into reflective cavities, where is is eventually absorbed by a small cube of material, resulting in a change in the electric current passing through this material. This detector is now in space, on the Spitzer infrared telescope.
Detectors change even more when we are trying to study particles other than electromagnetic radiation. Here is the super Kamiokande neutrino detector in Japan. A huge chamber is filled with water (not in place yet in this picture). When a neutrino interacts in the water, it can produce a tiny pulse of light. The "fly's eye" appearance of the walls is due to many sensitive light detectors that will pick up this pulse.