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How a CCD readout works. In this example, we just look at 16 pixels, with rows numbered and columns lettered. The readout device is on the right. (a) An exposure is finished. Each pixel has a value indicated by the label for that pixel. The readout device has levels zero. (b) The numbers are all shifted one pixel to the right. The first row is now zeroed, and the readout device has the contents of the last row. (c) The contents of the readout row are shifted down one, as the first number (bottom right value) is read. (d) The process continues until that whole row is read. (e) Everything shifts one more to the right, and the readout process of the last row repeats. This then continues until all values have been read and pixels have been set to zero.

exposures and add them together (using a computer) than one long exposure. There is also an error introduced in the readout process, called readout noise. This can put a limit on the faintest signals CCDs can see.

Having the image in computer readable form is actually very convenient, because many new techniques are being used to computer enhance very faint images. This provides a large dynamic range, meaning that we can see faint objects in the presence of bright ones.

When photometric observations are being made, we generally compare the brightness of the star under study with the brightnesses of stars whose properties have already been studied. By changing filters we can measure, for example, the U, B and V magnitudes of a star, one after another. Some method of recording the data is still needed. One option is photographic. The brighter the star is, the larger its image on a photographic plate. (This is an artifact of the photographic process and atmospheric seeing.) We can measure the brightness of a star by measuring the size of its image. (Remember the actual extent of the star is too small to detect in our images.) Photoelectric devices are well suited for photometry. Almost all photometry is now done using photomultipliers or CCDs. Some of the standard colors even account for the wavelength responses of various commercially available photomultipliers.

4.5.2 Spectroscopy

In spectroscopy we need a means of bringing the image in different wavelengths to different physical locations on our detector. We have already seen that this can be done with a prism. Since a prism does not spread the light out very much, we say that the prism is a low dispersion instrument. Dispersion is a measure of the degree to which the spectrum is spread out. Low dispersion spectra are sometimes adequate for determining the spectral type of a star. Sometimes a thin prism is placed over the objective of the telescope and a photograph is taken of the whole field. Instead of seeing the individual stars, the spectrum of each star appears in its place. These objective prism spectra are quite useful for classifying large numbers of stars very quickly.

When better resolution is needed, we generally use a diffraction grating, illustrated in Fig. 4.19. For any wavelength A, the grating produces a maximum at an angle given by d sin 9 = mX

where d is the separation between the slits and m is an integer, called the order of the maximum. The higher the order is, the more spread out the

Incoming Light

Incoming Light

Grating

Diffraction grating. Light comes in from the upper left.The beam reflected off each step spreads out due to diffraction. However, interference effects result in maxima in the indicated directions.The angles of the steps can be adjusted (blazed) to throw most of the light into the desired order.

Grating

Diffraction grating. Light comes in from the upper left.The beam reflected off each step spreads out due to diffraction. However, interference effects result in maxima in the indicated directions.The angles of the steps can be adjusted (blazed) to throw most of the light into the desired order.

Incoming Light

Destructive Interference

Incoming Light

Destructive Interference

] Plates

Constructive Interference

] Plates

Constructive Interference

Operation of an interference filter.

spectrum. Suppose our grating just lets us separate (resolve) two spectral lines that are AA apart in wavelength. The resolving power of the grating is then defined as

If the grating has N lines in it, then in the order m the resolving power is given by

Some gratings have over 10 000 lines per centimeter over a length of several centimeters. This means that resolving powers of 105 can be achieved. In general, light will go out into several orders. It is possible to cut the lines of a grating so that most of the light goes into a particular order. This process is called blazing.

It is possible to use interference filters such as that shown in Fig. 4.20. There are two flat parallel reflecting surfaces placed close to each other. There is a maximum in the transmitted radiation when twice the spacing between the surfaces, d, is equal to an integral number of wavelengths. That is

One problem with this approach is that we can only measure a small wavelength range at a time, and must keep changing the spacing, d, to obtain a complete spectrum. Another problem is that different orders (m) of different wavelengths can get through at the same time. You can solve this problem by adding a second filter with a different spacing, set to pass the desired wavelength and remove the unwanted orders. A device with multiple interference filters is called a Fabry-Perot interferometer.

A major recent improvement has been the development of devices that produce a Fourier transform of the spectrum. These devices provide astronomers with a great deal of flexibility and sensitivity. Fig. 4.21 shows the operation of one such device, called a Michelson interferometer. The incoming radiation is split into two beams, which are reflected off mirrors so that they come back to the same location and interfere with each other. The path length of one of the beams can be altered by moving a mirror. This changes the phase of the incoming beams. By seeing how the intensity changes as we move the mirror, we form an idea of the relative importance of longer and shorter wavelength radiation.

According to Fig. 4.21, the total path length difference is x. We look at the electric field for each wave. In this case it is convenient to write d

Mirror
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