How Charged Coupled Devices Changed Astronomy

January 18, 2024

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How did astronomers observe the sky before cameras?

When telescopes were first used astronomers had to look through them and then draw and write down what they could see.

crab nebula m1rosse
A drawing of the Crab Nebula from 1844.
Credit: Earl of Rosse/Wikipedia https://www.astronomy.com/science/unlocking-the-secrets-of-the-crab-nebula/

Then in the 1880s photographic plates were developed. This revolutionised astronomy as it removed the need for astronomers to draw what they could see which was not as accurate as a photo, but more importantly these images could be exposed for longer. Thus, we were able to see much fainter objects in the sky. The importance of this is that fainter in astronomy either means objects which are actually fainter or possibly more interestingly further away – so we are looking further back in time in the Universe as the further away an object is the longer the light has travelled to us. 

IMAGE 2. Crab nebula photographic plate image: https://adsabs.harvard.edu/pdf/1942ApJ….96..188B Credit: American Astronomy Society.

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The CCD revolution

the detectors of omegacam
The OmegaCAM camera lies at the heart of the VST. This view shows its 32 CCD detectors that together create 268-megapixel images. OmegaCAM was designed and built by a consortium including institutes in the Netherlands, Germany and Italy with major contributions from ESO.

The next major revolution came in 1969 when Charge Coupled Devices (CCDs) were first developed by two physicists at Bell Labs, Willard Boyle and George Smith. They received the Nobel Prize in physics, in 2009, for this work. These are the same devices as in the camera phones. CCDs are photon detectors. They comprise a silicon surface, which has an integrated circuit etched onto it, and divided into thousands or millions of light sensitive elements called pixels. When photons hit the surface they are converted into electric charge. The higher the intensity of the light the more electric charge that is created. Thus, a digital image of the light patterns hitting the CCDs is created. Each pixel is very small ~ 10 micrometre (µm) square, about the same thickness as tissue paper.

One of the first times CCDs were used by astronomers for ground-based observing was in 1976 at the University of Arizona. CCDs were improved over the next few decades, including increasing the size and pixel count, and reducing the noise (so images are clearer), and improved cosmetics. Much of the development and improvements to CCDs were driven by the Cold War. In fact, the CCD technology that was used in the Hubble Space Telescope was a result of the United States reconnaissance programme in the Cold War.

Why were CCDs so important to Astronomy?

crab nebula
Crab nebula CCD image with Hubble. This CCD images shows so much more detail compared to the sketch shown in image 1.
Credit: https://hubblesite.org/contents/media/images/2005/37/1823-Image.html

CCDs revolutionised astronomy. Their first major advantage was that they were able to take very long images. The motion of a telescope could be synchronised with the Earth’s rotation. This enabled the CCD camera on the telescope to view the same patch of sky for minutes or even hours at a time. Thus, as we mentioned with photographic plates the longer image you can take the more photons of light that will be collected on the CCD and so the fainter objects that can be imaged. Either looking at more intrinsically faint objects or those further off in the Universe.

Another major advantage of CCDs for astronomical images is that these devices have much higher spatial resolution, meaning that objects which appear close together in the night sky can be resolved (separated into separate sources) much better. CCDs are also better at imaging bright objects, are more robust, and consume less power.

Since astronomical images can be very long (minutes or even hours) the CCD cameras have to be kept very cold (−50°C to -100°C). This is to minimise thermal noise on the images, created by heat in the sensor.

CCDs can observe the universe in a much broader range of wavelengths than astronomers were previously able to using just the wavelengths visible to the human eye (the visible spectrum). Silicon used in CCDs is sensitive from 200-1200 nanometres, thus they are useful for observing wavelengths of light from x-rays right through to infrared. 

CCD’s are more time efficient than their precursor photographic plates. Photographic plates had to be developed and then digitised before they could be investigated. Whereas, CCDs create digital images ready for analysing straight away.

Conclusion 

CCDs transformed astronomical images. They allowed us to look at fainter and more distant objects in the Universe, meaning we are able to look further back in time. They also allowed astronomers to record images in a wide range of wavelengths and separate objects which appear close together in the night sky.

For more detail the night sky and observation, sign up for one of our courses today. Sky observation features in Mission Astro ‘The Escape from the Blue Marble’ course and our Certificate in Space Science.

This post was written by Dr Heather Campbell for Mission Astro.

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