We live in a world full of electronic gadgets. Smartphones, big-screen television sets, GPS receivers. Every year, they seem to get bigger and better.
Most astronomers today, both professional and amateur, use electronic detectors to take astronomical images. Is there any doubt that our current cameras are bigger and better than any used by scientists decades ago?
Let me try to convince you that, if we choose one logical manner of measuring the quality of a detector, it is only now -- or, perhaps, in the past five years -- that modern electronic detectors have finally caught up to the good old photographic plate.
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For many tens of thousands of years, the only detector used by humans was the good old Mark I eyeball.
Image courtesy of
Jezebel.com
The eye is more than just a detector -- it includes all the optics you need, too!
Image courtesy of
Online-Utility.org
The eye focuses light rays onto light-sensitive cells in the retina, along the back surface of the eyeball. When enough photons strike a set of these cells, a signal is sent through the optic nerves to the brain.
| quantum efficiency | permanent? | linear? | ease of use | size | |
| Eye | 6 % | no | no | simple | 36 sq. mm |
In the middle of the nineteenth century, chemists and artists figured out that it was possible to focus light onto a specially prepared film of material which could record the image permanently. The early photographic techniques had very low sensitivity to light -- requiring long exposure times to build up a good picture (that's why the people in early photographs are posed so stiffly). The first photograph of the Moon, a daguerreotype, required Henry Draper to expose the plate for 20 minutes!
Image courtesy of Greenwich Village History
As the decades passed, chemists developed materials which were much more sensitive to light, enabling photographers to take images with exposure times of one second or less. Special emulsions were developed for astrophotography which were especially sensitive to low light levels. Do you recognize any of these names?
Taken from "Kodak Scientific Imaging Products",
Kodak Publication L-10, 1987.
Photography offered two big advantages over the human eye:
However, even the best photographic emulsions record only a tiny fraction of all the light that strikes them. Pay attention to the second of these graphs.
As you can see from the films which we are passing out to the audience now, another big advantage film offered to astronomers was its size: one could (with very careful fabrication techniques) spread photographic emulsion over very large areas, over a backing of glass (plates) or plastics (film). Opticians like Bernhard Schmidt designed special telescopes which could project sharp images covering wide areas of the sky onto large photographic plates.
Image courtesy of
Wikipedia and Krzysztof Zajączkowski
You may recognize this particular Schmidt Telescope: it was used to create the Palomar Observatory Sky Survey in the 1950s and the Second Palomar Observatory Sky Survey in the 1980s.
This drawing by Russell Porter is copyright
California Institute of Technology's Palomar Observatory.
| quantum efficiency | permanent? | linear? | ease of use | size | |
| Eye | 6 % | no | no | simple | 36 sq. mm |
| Photograph | 1 - 5 % | yes | no | needs developing | 122,000 sq. mm |
In 1970, scientists at Bell Labs were thinking about ways to store information in solid-state memory. They came up with the idea of a silicon chip divided into an array of small regions, then moving electric charges into and out of the array. By varying voltages applied to small sections of the chip, the user could couple the charge to specific regions. These chips became known as charge coupled devices, or CCDs.
It turns out that silicon has many interesting properties: one is its ability to convert individual photons of light into individual electrons. People soon realized that with a little preparation, CCDs could be used to convert light into an image, and then to transfer that image digitally to a computer. Because the devices were novel and expensive, the first applications were in space: astronomers suggested using CCDs for the cameras aboard the Galileo mission to Jupiter, and on the orbiting telescope we now call the Hubble Space Telescope. As years passed, many companies started to fabricate CCDs for industrial applications and the prices dropped. Even ground-based astronomers -- at first professional, but then amateur -- were able to afford these new imaging devices.
Q: When did CCDs really take over in the
community of professional astronomers?
A. around 1970
B. around 1980
C. around 1990
Watch as the fraction of papers with the word photographic in the abstract slowly decreases ....
... while the fraction with CCD in the abstract increases.
These silicon-based detectors had two big advantages photographic film
The "sensitivity" part is easy to understand,
but what does the "linear" part mean? It means that if one exposes the detector for twice as many seconds, one OUGHT to record a signal twice as large.
| quantum efficiency | permanent? | linear? | ease of use | size | |
| Eye | 6 % | no | no | simple | 36 sq. mm |
| Photograph | 1 - 5 % | yes | no | needs developing | 122,000 sq. mm |
| CCD | 70% | mostly | yes | needs reduction | 1,800 sq. mm |