How to build a 'Dark Energy Camera'
A new project hopes to reveal more about the mysteries of “dark energy” by studying the night sky with a giant camera. The Engineer contacted the UK team behind the camera to find out more about it.
Source: Dark Energy Survey Collaboration
All the matter we can see around us only accounts for less than 5 per cent of the universe. Scientists believe that almost 75 per cent of the universe is made up of something known as “dark energy” and that this is responsible for the fact that all the galaxies are flying away from each other at an increasing speed. The problem is we can’t see or otherwise detect this hypothetical dark energy and so we know very little about it – or even whether it really exists.
So to try and improve our knowledge of dark energy, more than 120 researchers from 23 institutions across the US, UK, Spain, Germany and Brazil are planning to study the night sky in unprecedented detail, a project known as the Dark Energy Survey (DES). To do this, they will use a special camera that will determine the shape and position of the galaxies by calculating their redshift – the way light from objects moving away from us increases in wavelength (shifting into the red end of the spectrum) as they accelerate – in order to work out how dark energy might be affecting them
The Engineer contacted two of the UK scientists from the Survey, Prof Will Percival of Portsmouth University, who co-ordinates one of the research strands, and Dr Peter Doel of University College London (UCL), who helped build the camera, and to find out more about it.
TE: For those who don’t know, what is “dark energy”?
WP: “Dark Energy” is simply a name that has been given to the process that is driving the observed acceleration of the Universe. We don’t actually know what that process is! On large-scales, the only force that we think should be important for the evolution of the universe is gravity. Gravity is an attractive force, pulling together matter, so we would expect the universe’s expansion to be decelerating. However observations have now conclusively shone that it is accelerating, and we do not know why. There are many possibilities that have been discussed, but none has yet stood out at the most likely solution.
If we can’t see dark energy, how will a camera help you look for it?
WP: The camera will observe what we can see - galaxies. The positions, shapes, brightnesses and clustering properties of these galaxies do trace the expansion of the universe and growth of structure within it, which in turn depend on dark energy. The specify probes are: looking for supernovae in the galaxies, and using these as standard candles; looking at the shapes of galaxies to see the distortions created by matter along the line-of-sight to the galaxies; looking at clusters of galaxies and how they evolve over the evolution of the universe; using the galaxy clustering pattern as a standard ruler to measure the expansion rate.
Source: Dark Energy Survey Collaboration
TE: How is this camera different from other astronomical cameras?
PD: The main difference is the size of the field of view and the size of the telescope mirror. The field of view is 2.2 degrees (giving an area of sky 20 times larger than the Moon). The camera is mounted on the Blanco telescope at Cerro Tololo in Chile that has a 3.8m diameter mirror. The camera also uses 62 newly developed 2x4K [pixel] CCD [charge-coupled device] chips that have an enhanced red/near infrared light sensitivity. All this combines to give a camera with exceptional sensitivity and sky coverage.
What are the scientific principles behind how it works?
PD: The camera will take images with five different colour filters over a large part of the sky. By analysing the intensities in each colour scientists will be able to calculate the approximate redshift of millions of galaxies to map out their distribution in space. The shapes of the galaxies will also be measured, as dark matter between us and the galaxy is expected to distort their images and from maps of this distortion we can investigate the distribution of dark matter. From the dark matter distribution with cosmic time we can infer the parameters of dark energy.
WP: Redshifts are cosmological Doppler shifts caused by the expansion of the Universe. Distant galaxies appear redder because they are moving away from us (because the universe is expanding) and the light is observed at redder wavelengths. The Doppler effect is most commonly encountered by people when a moving object such as an ambulance is heard at different frequencies either coming towards us or going away from us. Redshift is the cosmic version of this applied to light rather than sound.
What is the UK’s involvement in the project?
PD: The UK is playing a leading role in many of the DES science projects and has been involved in defining the science goals and in developing analysis techniques to extract the measurements and interpret the vast amount of data that will be produced. On the technical side, UCL was involved in the design, assembly, alignment and testing of the camera lens system. This lens system contains five lenses, the biggest being about 1m in diameter and about 180kg in weight.
What were the technical challenges in constructing the camera and how did you overcome them?
PD: One of the challenges was to design lens cells to hold the lens rigidly but with low stresses in the lens and to compensate for differing thermal expansion properties of the glass, cell and camera barrel. To do this we designed an essentially athermal cell constructed of a nickel/iron alloy that was produced with a carefully chosen coefficient of thermal expansion.
Another challenge was to align the lenses to an accuracy of ~50 micrometers (about half the thickness of an A4 sheet of paper) in tip, tilt, decentre and separation. This accuracy is required to produce the high image quality that is needed to achieve the science goals.
Source: Fermilab/DES collaboration
The alignment was performed by mounting the lenses on X-Y-tilt stages mounted on a large precision rotary table. The lenses were rotated and by measuring the surface variation with a digital dial gauge we could position the lenses to a high accuracy whilst placing them in their lens cells. The lens in their cells were then placed in the camera barrel and their positions checked with a pencil laser beam by measuring the deflections of the light reflected from and passed through each lens.
Will the equipment be developed any further?
PD: There are proposals that after the end of the survey the camera optics could be used in a wide field multi-object spectrograph instrument that could take approx 4000 astronomical spectra in one exposure.
How will the camera be used and what will the output be?
PD: All the images from the survey will be released to the general community after a period of time. As well as the stated science goals of the DES survey it is expected that a lot of other science will result from use of the data.