Astronomy Principles and Practice Fourth Edition.pdf

324 Astronomical optical measurements
Figure 19.12. The essential elements of an astronomical spectrometer.
be recorded simultaneously. If a narrow exit slit is placed in the spectrum a small element can be made
available for detection by a photoelectric cell; a progressive movement of the slit through the spectrum
allows the spectrum to be scanned.
The basic elements of a conventional astronomical spectrometer comprise an entrance aperture,
usually in the form of a slit when the spectrum is recorded photographically, a collimator, the
dispersing element and an imaging device or camera to bring the spectrum to a focus. A simple
spectrometer is illustrated in figure 19.12. The collimator and spectrum imager may either be in the
forms of lenses or mirrors. In a photographic or CCD spectrometer, the spectrum imager is frequently
referred to as the camera optics. The instrument is usually provided with retractable viewers so that
any star field can be identified and checks can be made that the light from the star under investigation
is falling centrally on the entrance aperture or slit.
The dispersing element is normally one of two types, being either a glass prism or a diffraction
grating.
The glass prism causes the collimated beam to be deviated and, as its refractive index is
wavelength-dependent, the amount of deviation is also wavelength-dependent. For a collimated beam
of white light incident on the prism, a continuous range of collimator beams emerges from the prism
with a spread in the angle of emergence. By focusing these beams into a plane, a spectrum is produced.
Prism spectrometers are now very rarely used, however.
The diffraction grating consists of a series of accurately ruled slits or grooves. When a collimated
beam is incident on it, the light is diffracted by each of the rulings. In some special angular directions,
given by the spacing of the rulings and the wavelength of the light concerned, constructive interference
takes place. In these particular directions, waves of a given wavelength from the rulings are exactly
in phase. For this condition, the optical path length of the emerging light from one ruling to the next
differs by an integral number of wavelengths. The general grating equation states that
d(sin i + sin θ) = mλ (19.13)
where d is the space between adjacent slits or grooves, i the angle of incidence of the collimated beam,
θ the angle of emergence, λ the wavelength and m the order of interference, which for a conventional
spectrometer would have a value of either one or two.
Two of the important properties of any spectrometer are given by the size of the spectrum that is