The tools of Astronomy
How many people have looked through a telescope? Historically this is how astronomers would investigate the universe – by looking through a telescope and drawing what they saw, their eye acting as a detector, measuring the amount of light and dark coming from the object to Earth.
Figure : Left - Lord Rosse's drawing of the question mark galaxy. Right - A more modern photograph.
Today we attach instruments to telescopes to measure and record the light from objects in space, this means that we can measure and record all kinds of light – not just visible light that our eye can see.
Making a colour picture of space objects.
Our eye is an imaging system just like a digital camera or a telescope. The detector in our eye is the retina, a layer contains two types of cells; rods and cones. Each of these cells has a different function. The rods detect the amount of light (intensity) where as the cones detect colour. Unlike the cones, rods are extremely sensitive to the amount of light and therefore if there is very little light to see we tend to see in black and white. We can make the analogy that the rods alone act like a black and white camera.
Experiment 1: Turn all the lights out in a room and make it as dark as possible, wait for a few seconds and you will be able to see objects around you but not their colour.
For a similar reason using black and white detectors on telescopes help us to maximise the contrast in our image – particularly if there isn’t much light emitted from the object. So if we are using “black and white” cameras how do we produce such amazing colour pictures of space objects? We use filters to separate out the different amounts of light of different colours and then colour and recombine these images using a computer.
What do filters do?
This is answered by a snappy experiment for the whole audience where they look at a series of slides with and without a red filter. I use slides of red, green and blue rectangles on a black background and then the summary slide shown here.
Experiment 2:For the red filters I was able to acquire some old photographer’s tubes which contained red filter paper that were used over fluorescent lights to make a dark room for developing photos – however, filter gels for stage lighting would work too (e.g. as long as you make sure it is purely red. Each person only needs a small piece of ~4cm x 2cm.
The slide as it appears normally The slide viewed through the red filter.
The light from the red, green and blue words is reflected off the screen, passes through the red filter and then enters the eye. The red filter only allows the red light to pass through and so the light from the words that are pure green and pure blue is blocked (reflected back or absorbed) by the filter.
The next step investigates secondary colours of light and asks students to use the red filter to discover which primary pairs of light produce each secondary colour.
The slide as it appears normally The slide viewed through the red filter.
The key observations are:
1)Yellow, magenta (purple) and cyan (light blue) are secondary colours of light.
2)The cyan words cannot be seen through the red filter even though they look “bright”. The filter only lets red light through so the cyan cannot contain any red light – as a secondary colour it must therefore be a mixture of blue and green.
3)You can see the yellow and magenta words so they both must have red as one of their two primary colours.
4)The final deduction has to be a guess but if you start with magenta, a relatively logical guess is that it is blue mixed with red. This means that the only other pair, green and red, must make yellow when mixed.
Experiment 3: Having developed a theory about how filters and mixing coloured light works we can build up a link to how filters combined with black and white detectors enable us to produce coloured images. I have taken three photographs of a coloured parrot with a black and white camera with each of a red, green and blue filter in front of the lens. Can we now work out what colour the different parts of the parrot’s body are? The white dots represent the 4 parts of the body that I ask students to think about.
For younger students I emphasise the path of the light. The light is reflected off the parrot, through the colour filter and into my camera’s lens and detector.
We begin with her chest. The picture taken with the green and blue filters show her chest as black therefore no light has passed through those filters. We conclude her chest is red. On to her wing tip –in the red and green image the wing tip appears black and therefore we conclude it is blue. The third body part is the middle of the wing and students should look carefully at all three images. The main brightness here is in the red and green filter images, which means that red and green light is present. When we mix both red and green light we get yellow. Finally to round off we look at the beak where light has passed through all three primary colour filters and therefore her beak must be white. Here she is.
Astronomers can however take the black and white images above and artificially colour them whatever colour they would like. They could make the image taken with the red filter blue, the green - red and the blue – green. The image we would produce would then be a false colour image – it would still give us information but it wouldn’t be the true colours of the parrot.
False colouring an image is necessary if we are detecting and measuring light that our eyes cannot see – what colour is infra-red light? We have developed instruments that can record all types of light from radio through to X-rays and gamma waves. We apply colour to the black and white images to show how much light is emitted in different parts of the object – each colour representing a different intensity.
For example, take thermal infrared cameras. The infrared galaxy zoo website ( has pictures of all kinds of animals as viewed through a thermal infrared camera. I use two examples, the first uses our everyday assumptions about colours to infer about the animal. The second image uses our knowledge of the animal to infer about the colour scheme.
The image of the dachshund has a colour scheme which uses our assumption that blue is cold and white is very hot (a healthy dog has a cold nose – yes?). The second image allows our knowledge that reptiles are cold blooded to deduce that black is very cold then purple through to yellow and then again white hot. In both cases the creatures are not the colour that they would appear!
To finish I show images of the Sun that we can create by detecting all the different sorts of light that it emits – where all the images were taken at approximately the same time on the same day. It is particularly interesting to look at the position of the sunspots in the visible (black and white) image and compare those positions in the other images.
(Top left to bottom right: Radio, Microwave, Infrared, Visible, Ultraviolet, Xray – NASA images).