Dekkers

Bowen Mirror Astronomy

Most spectroscopes can change the available length of the slit by the use of an external stop. This will also usually have separate apertures to allow the comparison spectrum (see below) to be placed either side of the main spectrum without overlapping it. Such a stop is called a Dekker, and will usually have a range of lengths to allow for different objects and different observing conditions (figure 9.5). Exit field matched to the shape of the spectroscope entrance slit Figure 9.3 Image...

Rotational Transitions

The rotational energy levels can have transitions between themselves, resulting in lines in the microwave and radio parts of the spectrum, but they also produce fine structure within lines arising from the vibrational levels. This latter effect is considered in the next section. Vibrations within molecules occur so much faster than rotations, as we may see from the respective frequencies of their lines (figure 5.2), that we may consider rotation as that of a molecule whose internuclear...

Spectroscopic binary stars

Unless the plane of the orbit is close to perpendicular to the line of sight, the Doppler shift due to the orbital motions about their common centre of mass for the two stars in a binary system leads to changing wavelengths for the lines in the spectrum. When the two stars are within one or two stellar magnitudes of each other in brightness, the lines from each will be detectable in the combined spectrum. If they differ by more than about two magnitudes, then generally only one set of lines...

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Normal Zeeman Effect

In the optical region, therefore, the splitting is by about 0.01 nm per tesla (10u mT-'). For the situation sketched in figure 6.1, the line splits into three components, each being triply degenerate. The radiation absorbed or emitted for each component is polarized. When the line of sight is perpendicular to the magnetic field direction, all the components are linearly polarized, the directions for the outer (cr) components being perpendicular to the field and that for the central (n)...

Obafgkm

The order can be remembered from the mnemonic Oh Be A Fine Girl Guy Kiss Me , but it would have been preferable for a more logical system to have been chosen in the first place. In another hangover from the early days of astronomical spectroscopy, in which the sequence was thought to be an age sequence as well, stars of classes O, B and A are often referred to as Early Stars, and those of classes K and M as Late Stars. While a convenient usage, it should be stressed that this terminology does...

The Faraday Effect

Linearly polarized radiation has its plane of polarization rotated on passing through a magnetized plasma. It is only of significance in astrophysical Instantaneous electric vector of the linearly polarized Figure 6.16 Linear polarization as the sum of two circularly polarized components. Instantaneous electric vector of the linearly polarized electric vector of N. _____ electric vector of one of the circularly one of the circularly polari ed components polarized components Figure 6.16 Linear...

Space Quantization

Space Quantization

Two further quantum numbers are required to specify the energy of an electron within an atom completely. The first of these arises because the atom is three-dimensional. This third quantum number determines the orientation of the electron's orbit in space. In the absence of any reference direction, all the quantized orientations will have the same energy (i.e. the energy levels will be degenerate). However, when there is a reference direction, such as when a magnetic field is present, some or...

T

The first zeros occurring at angular positions of I I I I I t t ii t. t ) I I ( , , 1 < I > 4 4 II I II I Figure 8.6 Interference patterns for monochromatic sources of different wavelengths through ten apertures (Xj > X2 > X3). The angular width of a primary maximum is thus 2A0 and is given by The Rayleigh criterion for resolution is for one fringe maximum to be superimposed on the first zero of a second fringe pattern. Thus the angular Rayleigh resolution of a diffraction grating is...

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Partial Hydrogenation Diagram

(J _________________________10.198874 , _ ,,, Figure 43 Partial Grotrian diagram for hydrogen showing all the Ha transitions possible in the absence of selection rules. Figure 4.4 Partial Grotrian diagram for hydrogen showing Ha transitions allowed by the selection rules. Figure 4.4 Partial Grotrian diagram for hydrogen showing Ha transitions allowed by the selection rules. Bohr's Correspondence Principle), then the selection rules follow, for all transitions. For our purposes, it is sufficient...

Doppler Broadening

Line Doppler

After pressure, the broadening of lines due to Doppler shifts is the most commonly occurring effect. Such broadening occurs when atoms contributing to a spectral line have different line-of-sight velocities, so that their individual contributions are shifted by differing amounts from the normal wavelength. In all astronomical objects, thermal motions, turbulence and other fine scale Line profiles, with an increasing number of atoms producing them between cach succcssivc Line profiles, with an...

Astronomical Spectroscopy

Astronomical Spectroscopy A D Thackeray, Eyre & Spottiswood, 1961. Interpretation of Spectra and Atmospheric Structure in Cool Stars Y Fujita, University of Tokyo Press, 1970. Spectral Classification and Multicolour Photometry C Fehrenbach and B E Westerlund, D Reidel, 1973. The Impact of Very High Signal to Noise Spectroscopy on Stellar Physics ed G Cayrel de Strobel and M Spite, Kluwer, 1988. Index of Galaxy Spectra G Gisler and E Friel, Parchart, 1979. Stellar Spectroscopy Peculiar Stars....

Nebulae

Nebulae may be observed with conventional spectroscopes or with specially designed instruments such as slitless spectroscopes or TAURUS (chapter 10). Long-slit and multi-object spectroscopes are also useful because many nebulae are angularly resolved. Smaller and or more distant nebulae may not be angularly resolved, and for these spectroscopy is generally essential in order to distinguish them from stars. The nebulae divide into the hot nebulae such as H II regions, planetary nebulae,...

Selection rules

Particles and radiation interact only through the effects of the radiation's oscillating electric and magnetic fields on the particle. Generally it is the electric component that is important (though some forbidden line transitions arise through magnetic interactions, chapter 4). Thus either the electron configuration of the particle must change during the interaction, or the particle must be an electric dipole. The dipole arises when the centre of charge for the electrons is spatially...

X

Figure 6.4 Free-free transitions on an energy level diagram. Actual path of the electron Path of the electron Figure 6.4 Free-free transitions on an energy level diagram. Actual path of the electron Path of the electron ion, and b is the minimum separation of the ion and electron for the electron moving in a straight line in its original direction. This quantity is usually called Path of the electron in the absence of the ion Figure 6.6 The definition of the impact parameter, b. the Impact...

Infrared Spectroscopes

Full coverage of the infrared region requires that the spectroscope be lifted above most or all of the Earth's atmosphere, and balloon, rocket and spacecraft-borne spectroscopes have been so used. The atmosphere does, however, have some gaps in its near-infrared absorption spectrum, allowing radiation to reach the surface. Thus ground-based instruments, especially those working from dry sites at high altitude, can still be of use. The main difference between such an infrared spectroscope and...

Conventional Cassegrain Spectroscope

The workhorse of spectroscopy is a conventional grating-based slit spectroscope attached to the Cassegrain focus of a large telescope. Its size is limited by the space available at the back of the main mirror mounting of the telescope, and its weight is limited by the strength of the telescope's tube and mounting. There are numerous examples of this type of instrument, only one of which, ISIS, is discussed here. ISIS (Intermediate dispersion Spectroscopic and Imaging System) is one of the most...

Element Abundances

Clearly, all other things being equal, the more atoms of an element that are present in a star's atmosphere, the stronger will be its spectral lines. The variation of line strength with the number of contributing atoms is called the curve of growth (figure 13.2). Using the curve of growth to determine abundances is a process known as coarse analysis. Figure 14.2 Schematic element abundance determination from the curve of growth. Figure 14.2 Schematic element abundance determination from the...

D

Where D is the diameter of the lens or mirror (assumed circular) in the same units as the wavelength (X). The diffraction-limited resolutions of the collimator and focuser should not therefore degrade the spectral resolution of the disperser. However, their optical quality needs to be sufficiently adequate to ensure that their aberrations also do not significantly affect the image. Generally of more significance for the spectral resolution is the width of the entrance slit. Let us consider the...

Pressure Broadening

Pressure broadening has already been mentioned in connection with the luminosity classification of stars (chapter 11) and is often the dominant broadening mechanism in stellar spectra. It arises because an atom, ion or molecule will be influenced by the nearby presence of other atoms, ions, molecules, free electrons or free nuclei. In the limit of a solid, this results in the formation of the valence and conduction bands etc (chapter 7). For a gas sufficiently rarefied to be producing line or...

Polarization

After the form of its spectrum, the other major characteristic of synchrotron radiation which distinguishes it from thermal emission is its polarization. This is generally elliptical polarization, becoming closer to linear polarization as the electron's energy increases. Considering a single electron, its radiation is polarized with the electric vector parallel to the acceleration vector. Thus for electrons moving perpendicular to a magnetic field, the observed projection of the acceleration...

Types Of Spectroscopy

The fundamental spectroscopic techniques of interest to astronomers are atomic and molecular absorption and emission spectroscopy. Fluorescence spectroscopy can also crop up very occasionally, and the spectral properties of solids are of interest within the solar system and for the study of the interstellar'medium. The principles of these areas of spectroscopy are briefly summarized below, and dealt with in detail later in this chapter and elsewhere within this book. The other areas of...

Prism Spectroscopes

A slit-type prism spectroscope design can be produced for most of the diffraction grating instruments discussed earlier by replacing the grating with the prism. The most commonly encountered such designs are the basic spectroscope (figure 8.14) and the Littrow spectroscope (figure 8.15). For the Littrow design, a 30 prism would be used, with the rear surface aluminized. The light is therefore reflected and passes twice through the prism making it equivalent to a single 60 prism. The reflected...

Vibrational Transitions

As already mentioned, a transition involving a change in the vibration of a molecule requires so much more energy than that involved in changing the molecule's rotation that the latter usually also occurs. We do not normally therefore have pure vibrational transitions, but ones which involve changes in both vibration and rotation. Thus the reader will often find such changes elsewhere referred to as Vibrational-Rotational transitions. The rotational changes generally have just the effect of...

Multiobject Spectroscopes

The LDSS (see above) can obtain the spectra of up to 100 objects simultaneously and is one example of this class of instrument. It requires, however, a mask containing slits positioned to match the objects to be studied and this has to be made up for every observation. A more flexible system, which also does not require the large field required for the optics of LDSS, is to use fibre optic cables to bring the light from the stars in the field of view to the slit of the spectroscope. The fibre...

The Interstellar Medium

The material between the stars is little different from that in H I regions, save that it is more rarefied. A typical density would be around 106 atoms per cubic metre, a factor of a thousand down on that of the gaseous nebulae. About 1 of the material is in the form of dust particles thought to have a refractory core of graphite or of silicate compounds and a mantle of frozen gases. This gas is difficult to observe directly. Absorption lines produced by the interstellar gas along the line of...