The History Of Astronomical Detectors

Our Eyes

The human eye was the first astronomical detector. The eye lens bring light to a focus on the retina. While being a relatively crude lens, the eye is attached to the most fantastic image processing system on the planet - the brain - which corrects for all kinds of stuff. The eye is adaptive and flexible. As an astronomical detector, the eye has many deficiencies. For every 1,000 photons that go into the eyeball, only about one of them is captured. In other words, 99.9% of the light that goes into the eye is wasted. The eyes also have a logarithmic sensitivity, which allows us to see in very dim light and in very bright light. But it’s not good for astronomy, where we want a linear detector.

Photographic Film

Astronomy's second detector was the photographic film. This technological revolution came in the 1840s. The film has the advantage of giving a permanent record. It is also an integrating device. The pixel sizes in the photographic film are fantastic. They're on the order of the size of molecules. Even today, the resolution of the photographic film is the best. With photographic film, 1 in every 50 photons is detected. Better than the human eye but it still wastes 98% of the light. The film has a non-linear sensitivity. It's not as bad as the logarithmic human eye but it's still non-linear.

Photomultiplier Tube

Astronomy's third detector was the photomultiplier tube, which came in circa the 1940s. It's a better detector in that for every five photons that go in, one of them is captured. So about 80% of the light is wasted. A photomultiplier gives a permanent record, and it's an integrating device. It has a linear sensitivity. Like the photographic film, it can be overexposed. One of the disadvantages of a photomultiplier tube is that it is a point detector. It is essentially taking a very thin tube and getting one pixel. So, If we want an image of something, we have to sweep the photomultiplier tube across the object. 

The CCD

Astronomy's fourth detector came in the 1980s. That's the Charge-Coupled Device or CCD. Each pixel in a Charge-Coupled Device acts like a miniature photomultiplier tube. The principle of operation is also similar to that of a photomultiplier tube. CCDs are fantastically efficient devices. They only waste about ten percent of the light going in. So for every 1.1 photons coming in, 1 photon in captured. They are also linear detectors; doubling the light doubles the electrons collected. They are, of course, digital. However, these features come with a high price tag which makes these largely inaccessible to the general public. And the pixels still aren’t quite as small as those of the photographic film. 

(Originally published in Telescopia for a design project from Arizona State University)