Astronomical Instrumentation

118-122

A popular misconception is that astronomers spend long hours "peering intently" through their telescopes. Astronomers do spend long hours "peering intently" but most likely at computer or video monitors. People rarely look through large telescopes. The beam from a large telescope is destined to be "seen" by a host of at times exotic pieces of instrumentation that can measure and tease out the information encoded in the precious stream of photons collected by the primary mirror. In this section you will learn about a few of these instruments.

The Spectroscope

In his work Opticks Isaac Newton described experiments with a prism to illustrate the "phenomena of colours". Although crude - this was the first recorded experiment in spectroscopy - the science devoted to the analysis of light and other forms of electromagnetic waves. We more properly associate the birth of spectroscopy with Joseph von Fraunhofer, a German optician who, in the early 19th century observed the spectrum of the sun. Fraunhofer's spectrograph used a prism to disperse light emerging from a thin slit into the familiar rainbow of colours. Figure 5.17a and c show a prism splitting visible light into its spectrum as well as an antique spectrograph similar to the one used by Fraunhofer. Figure 5.18 provides a schematic of the Fraunhofer spectroscope.

Figure 5.17a A prism is one of the most common devices that can disperse white light into its spectrum	Figure 5.17b A common CD creates a spectrum - it acts like a diffraction grating	Figure 5.17c A "Fraunhofer-style" spectrograph

Fraunhofer was one of the first to make an astronomical observation with the spectroscope. This occurred while he was testing a newly polished prism.

Figure 5.18 How a Fraunhofer spectrograph works.

Fraunhofer's usual practice was to use a candle as a light source and to pass light through the prism looking for any defects in the glass. When he used the sun as a substitute light source he observed an intriguing difference in what he saw - but a difference that was due to the light and not a defect. What he saw is captured in Figure 5.19.

Figure 5.19 Small portion of the solar spectrum showing numerous "Fraunhofer" lines.

The solar spectrum has a multitude of thin, dark lines of various intensities. Figure 5.19 shows a small part of the solar spectrum in the green region of the spectrum, the thin dark lines are called "Fraunhofer" lines. When these lines were first discovered they were inexplicable. With the birth of modern atomic theory we now use spectral lines to "unlock" the secrets of the stars. Stellar spectra tell us many things about a star or celestial object including: temperature, density, pressure, rotation, motion and more! Spectroscopy is the fundamental tool of modern astronomy.

Example 5.6 Estimate the wavelength region (express in nm) shown in Figure 5.19.

Solution: Use the electromagnetic spectrum applet in Chapter 5.2. The green part of the visible spectrum is from about 500 nm - 550 nm.

Modern, astronomical spectrographs can be large and massive. Figure 5.20 shows the spectrograph attached to the Plaskett telescope at the Dominion Astrophysical Observatory, in Victoria. Seen next to the telescope operator, the spectrograph is the size of a baby-grand piano. In many cases the spectrograph is attached to a telescope but there are also common configurations in which a series of mirrors directs the light beam from the telescope into a room which contains a large, fixed spectrograph. Even though prisms are still occasionally used to disperse light it is more common to use diffraction gratings. If you have ever noticed the rainbow pattern produced by light reflecting off a CD (see Figure 5.17b) then you have seen a crude version of a diffraction grating. One significant advantage that reflective diffraction gratings have over prisms is a much higher efficiency. Figure 5.21 illustrates the way in which a reflection grating works. A diffraction grating can work either by reflecting light or by transmitting light.

Figure 5.20 Roll-over image of the spectrograph attached to the 1.8 m Plaskett telescope at the Dominion Astrophysical Observatory, Victoria.

	The surface of the grating is scribed by thousands of thin lines. As white light interacts with the scribed surface of a diffraction grating it scatters in directions which depend on the wavelength (hence colour) of the light.
Figure 5.21 The reflecting diffraction grating

The CCD (Charge Coupled Device)

At the heart of your digital camera is a tiny microelectronic chip that has a Canadian connection and has revolutionized astronomy! The chip is a Charge Coupled Device or CCD for short. It has replaced photographic film for both photo-enthusiast and most astronomers.

Figure 5.22 shows the co-inventor, Canadian physicist Willard Boyle with an example of the CCD chip in the foreground and images from the Canada-France-Hawaii telescope in the background. The latter images are only possible because of the tremendous sensitivity of the CCD. Boyle and colleague George Smith developed the CCD, initially as a possible memory device in the fall of 1969 while working at Bell Labs. Boyle and Smith were awarded the 2009 Nobel Prize in Physics in recognition of the profound impact that the CCD has had on us all. A CCD works in many ways like photographic film but also with some fundamental differences. Similar to photographic film, a CCD works because of the particle nature of light. The surface of the CCD consists of millions of tiny rectangular regions called pixels. When photons hit the pixels electrons are released and the pixel acquires an electric charge. As more photons hit the CCD different pixels acquire different amounts of charge according to how much light they have been exposed to. Eventually the pixels are "read" in sequence by a computer and an image is reconstructed. CCD devices are now everywhere - from your camera, to the bar code scanners in supermarkets to the digital cameras on board the Hubble and other orbiting telescopes.

Figure 5.22 Willard Boyle is a Canadian physicist credited with the development of the Charge Couple Device in 1969 while working at the Bell Labs.

Photometry

Light (or other forms of electromagnetic radiation) are the "raw materials" of astronomy. The spectrograph allows astronomers to "dissect" light into its colours. The photometer allows astronomers to "weigh" light - that is to measure its brightness and compare how much light we get from one star or another. Photometry, along with spectroscopy are the most fundamental techniques of modern astronomy. In the case of both photometry and spectroscopy the CCD has had a profound impact. Figure 5.23 shows an animation of a photometric measurement made using a CCD camera attached to a small telescope. Since a CCD creates an image by storing electrical charge in direct proportion to the amount light hitting individual pixels it can also be used as a very precise "light meter" or photometer. As well, because of the sensitivity of CCD's even small telescopes can now be used to make precise measurements of star intensities.

The variable star DY Pegasi is shown in the top inset in Figure 5.23. This image was collected using a CCD camera attached to a 0.3 m telescope. In photometry it is common to compare star brightness and usually several stars which you believe to be constant (designated C1 and C2 in this figure) are used to measure by how much the light from the variable star has changed (V-C1 and V-C2) as well as how constant the comparators really are (C1-C2). The moving red dot shows how the light from DY Pegasi is changing on the graph V-C1.

Figure 5.23 CCD image of DY Pegasi as it varies over a 5.5 hour time span. Image courtesy of The King's University College Observatory.

The variation in light from DY Pegasi amounts to about 0.7 magnitudes. If we convert this to intensity units it means that at its brightest, DY Pegasi emits 1.9 times as much light as it does at its dimmest.

Example 5.7 A CCD is used to collect data on a variable star. During the course of a night the star varies by 2 magnitudes in brightness. How much more light energy does the CCD receive when the star is its brightest compared to when it is dimmest?

Solution: Recall from Unit 1 that for every 1 magnitude difference in brightness there is an intensity factor of about 2.5 times. So, if the star varies in total by 2 magnitudes then its intensity (and therefore amount of light energy received) varies by 2.5 X 2.5 = 6.3 times. The CCD is receiving 6.3 times as much energy when the star is its brightest.