Light, or optical, detectors are used in Optical Astronomy. They allow us to explore the universe using electro-magnetic waves emitted by distant objects. Optical Astronomy is only one branch of Astronomy, other branches being Radio, Microwave, Infrared, Ultra-Violet, X-Ray, Gamma Ray, Neutrino, Cosmic Ray and Gravitation Wave Astronomy. This article describes how light is detected by several devices.

Human Eyes (Biological Technology)

The human eye was the first instrument used to observe the heavens, at least by man. It consists of a lens to focus the light and a retina to record the light image. The retina of the human eye contains two types of photoreceptor cells. There are the cone cells that are used to detect colour. Some cone cells detect blue light, some detect green light and others detect red light. Together they allow us to detect all the colours of the rainbow. The cone cells work best in high light level conditions.

Normalised human photoreceptor absorbances for different wavelengths of light.

However, in low light level conditions, rod cells are used, as they are more sensitive. Rod cells are used to produce a black and white image by our brain. The rod and cone cells contain a pigment that changes shape when struck by light. Incoming light causes 11-cis-retinol to be converted to 11-trans-retinol. The 11-trans-retinol causes nerve signals to be sent to our visual cortex where we form an image. Once 11-trans-retinol has been formed, it defuses out of the rod and cone cells into our blood stream and when it gets to our liver, it is converted back to 11-cis-retinol. It is then sent back via our blood stream to our rod and cone cells. In normal light levels, the amount of 11-cis-retinol in our eyes is quite low. This is due to it being destroyed, that is being converted to 11-trans-retinol, as fast as it accumulates.

The absorption of light leads to an isomeric change in the retinal molecule.

If we are in a dark situation, then the levels of 11-cis-retinol in our rod and cone cells slowly increases, which gives us our night vision. This can take up to 20 minutes. It is also worth noting that the centre of our field of view contains the most cone cells and the least rod cells. Therefore, your peripheral vision is better at night.

Why are we using red lights for illumination during night observations?
The red light will destroy the red pigment in our red cone cells. However, it will leave intact the pigment in our blue and green cone cells and more critically, it will leave intact the pigment in our rod cells.

Human eyes are not optimised to work in low-light conditions.
Eyes of nocturnal animals have adaptations to allow them to work best at night, these include having light-receptive cells at the front of the retina and have special reflective cells behind the retina so light that was not captured the first time through will have a second pass.

Film (Chemical Technology)

Film is basically a layer of light-sensitive crystals, such as silver nitrate, suspended in an emulsion. The emulsion is to keep the crystals from touching each other. This is important when the film is developed.
When light hits a light sensitive crystal, it undergoes a chemical change: one molecule of the silver nitrate crystal is changed to silver oxide. At this stage, the exposed film looks the same as the unexposed film as the amount of black silver oxide in it is minutely small.

Once a film has been exposed, it needs to be developed. The developer is a chemical that converts the silver nitrate crystals to pure silver oxide crystals. However, it has the interesting property of first converting silver nitrate crystals that have been exposed to light that is that have some silver oxide in them already, to silver oxide. So, by carefully timing how long you leave the exposed film in the developer, all exposed silver nitrate crystals will be converted to silver oxide and none of the silver nitrate crystals that have not been exposed will be converted to silver oxide.

The final step is to “fix” the film where the un-exposed silver nitrate is removed. This just leaves the exposed silver nitrate crystals that have now been converted to silver oxide. If the film is not fixed, then it will eventually all turn black when exposed to light.

The result is a negative image. This can be converted to a positive image by basically taking a photograph of the negative image. The sensitivity of the film can be increased by increasing the size of the crystals. This gives the incoming light a bigger target and a higher chance of hitting a crystal. Unfortunately, larger crystal size results in a lower resolution, grainier image. For colour film, there are three layers of light-sensitive emulsion. The Yellow, Magenta and Cyan layers give us the familiar Red, Green and Blue colours.

Typical layers of a colour film.

In astronomy, when imaging started, we normally used the negative image as it contains finer detail than the positive image. The light-sensitive emulsion was placed on a glass plate, not a celluloid backing, so that larger images could be made. For colour images, rather than using colour film, we used different coloured filters to take three images on black-and-white film. The three images were then combined into a colour image. Film has been used in near-earth orbit spy satellites. In these, the film was returned to Earth for processing.

It was also used in the Russian Luna 3 probe, where the film was developed on-board and the resulting images scanned and transmitted back to Earth.

Film was also used to capture one of the most famous photographs of earth taken from space by the Apollo astronauts. The main advantage of film over human eyes is that it preserves the image. Fainter light sources such as stars can be recorded by increasing the exposure time, as well as sticking a ruddy great telescope in front of it.

December 24, 1968 – from the crew of Apollo 8. It’s not really an Earthrise, of course. As seen from any one spot on the moon’s near side, Earth doesn’t rise or set, but simply hangs in one spot in the lunar sky. The astronauts saw Earth rise because they were moving, in a spacecraft, above the moon’s surface.

TV Tubes (Valve Technology)

To understand TV tubes, you first need to understand the photo-electric effect. The photo-electric effect is that when light falls on a material, an electron can be emitted. As you know, all materials are composed of atoms. Each atom consists of a positively charged nucleus surrounded by negatively charged electrons. The inner electrons, that is those closest to the nucleus, are the most tightly held, whereas those furthest out are less tightly held. If an atom is hit by a sufficiently energetic photon, then it can be absorbed by the atom and an electron is ejected.

For atoms that hold their electrons tightly, say an insulator, it takes a high energy photon, an ultra-violet one for example, to eject an electron. For metal atoms, which hold their electrons loosely, a lower energy photon from visible light can eject an electron. So, detecting the ejected electron enables you to infer that a photon has been absorbed. Unfortunately, detecting a single electron is not easy but, by using a photo-multiplier tube, a single electron can be turned into several million ones. This is done by accelerating the electron using a voltage differential and have it collided with a secondary electrode. If the electron has sufficient energy, then several electrons can be emitted by the collision. This can be done several times to amplify the signal.

Schematic of a photomultiplier tube.

In a TV tube, rather than getting a single light reading, we want to get light reading over an entire image. To do this, the incoming light is used to form areas of different charge on a target screen. This implies that the target screen cannot be conductive as it would not be able to maintain different charges in different areas.

So, an insulating material with metallic cells that capture the charge is used. We can use an electron beam to read the charged image. To do this, the electron beam is scanned across the charged image using electro-magnets. The amount of returned current, and hence the charge in each part of the image recorded, is measured using an electron multiplier. Scanning the image also has the effect of removing the charge therefore of erasing the image.

Schematic of image orthicon (TV) tube.

TV images were used to transmit the Apollo moon landings to Earth in real-time.

Apollo 11 Moon Landing

The TV tube detects a black-and-white image. Therefore, to produce a colour TV image, we need three black-and-white TV tubes and filters, one set for each primary colour. Slow-scan TV tubes have been used in various space probes such as Voyager. Here the images of the 3 tubes are scanned after a suitable exposure time, recorded electronically and then later transmitted back to Earth.

Charge-Coupled Devices (Semi-Conductor Technology)

Charge Couple Devices are made from semi-conductor material. Semi-conductors have electrical resistance somewhere between insulators and conductors. If we take an insulating material such as silicon, there are no free electrons in the atom’s outer shell to conduct electricity but if we change some of the silicon atoms to be phosphorus atoms, which have one extra outer electron than silicon, then we have free electrons in the outer shell to conduct electricity. This is called a “n-type” semiconductor. Alternatively, if we change some of the silicon atoms to boron ones, which have one less outer electron than silicon, then we have electron “holes” in the outer shell to conduct electricity. This is called an “p-type” semiconductor. Only a small number of atoms need to be changed, say 1 in 100,000,000. So, a little dope goes a long way!

By having “n-type” and “p-type” regions close together, we can create a p-n junction. This allows the current through the semi-conductor and its resistance to be controlled. This is called a Transistor. CCDs are MOSFET, Metal Oxide Semiconductor Field Effect Transistor, devices. This term not only explains what they are, but how they work. “Metal” refers to the aluminium contacts, the control gates, on the surface of the chip. “Oxide” refers to the silicon oxide insulating layer that goes between the aluminium contact and the underlying semi-conductor.

It is the electrical field of the aluminium contact that controls the resistance of the underlying semi-conductor.
In a charge-coupled device, incoming light causes an electron in the semi-conductor to jump to a higher energy state. This results in a free electron and a “hole”. The electron is drawn towards the positive control gate, whereas the hole is pushed away from the control gate. So the charge at the control gate gives an indication as to the amount of light falling on the area of the CCD.

A MOS capacitor used as a light sensitive device.

By changing the voltages on the control gates, the charges can be shifted towards a sensor built on the chip. So a single sensor can read the charge on multiple control gates. As explained above, the opto-electric effect relies on a high energy photon to eject an electron from an atom. However, in a CCD, the incoming photon only needs to boost the electron into a higher energy state, not to eject it.

As this takes less energy, CCD devices are sensitive to less energetic photons such as infra-red light. To capture a colour image, we could use three CCD devices or a single CCD with a Bayer filter.

Colour image through RGB filter.

With a Bayer filter, half the pixels are covered by a green filter, one quarter by a red filter and one quarter by a blue filter. It is possible to infer the intensity of red light in a “green” pixel say be averaging the adjacent “red” pixels. However, this reduces the image quality.

In amateur astronomy, we tend to use a single CCD with three exposures using different filters. This gives a better resolution. In professional astrophotography devices with multiple CCDs are used to get a wider field of view. This can be seen in the Hubble image below.

The Pillars of Creation in the Eagle Nebula taken by the Hubble telescope in 1995.

Aside:
During a field trip to the OPAL (Open Pool Australian Lightwater) Nuclear Reactor at Lucas Heights (in Sydney), I found out that they are involved with semiconductor manufacture. To dope a silicon ingot, you just need to put it into a nuclear reactor. Some of the silicon atoms absorb neutrons and are converted to phosphorus atoms. This is called Neutron Transmutation Doping. Obviously, this is just one step of the entire manufacturing process.

David Wallace

References:
https://en.wikipedia.org/wiki/Photoreceptor_cell
https://en.wikipedia.org/wiki/Photographic_film
https://en.wikipedia.org/wiki/Video_camera_tube
https://en.wikipedia.org/wiki/Charge-coupled_device
http://digitalslrphotography.50webs.com/technology.html
http://particle.astro.ru.nl/goto.html?astropract1-1112