2.4 Other spectrographs
Although the simple, single-slit spectrograph described above is the type you are most likely to find on a small telescope, there are other more complex designs available. Each of these has its own role to play in astronomical observations.
An echelle spectrograph has a second dispersing element, either a second grating or a prism, which disperses the light at right angles to the direction of dispersion produced by the main grating. Without going into details, the effect is to produce a spectral image that consists of a stacked series of spectra (see Figure 23). Each of the stacked spectra represents a part of the spectrum of the object, spanning only a very narrow range of wavelengths. You can imagine joining these individual spectra end to end in order to assemble the complete spectrum of the object.
What do you suppose are the advantages of an echelle spectrograph? What are its disadvantages?
An echelle spectrograph enables us to cover a large wavelength range at a high spectral resolution. However, since the light is dispersed over a large part of the image plane, the intensity at any point in the spectrum is very low, so these instruments can only be used successfully on very large telescopes or with bright stars.
Integral field unit spectrographs and multi-object fibre-fed spectrographs use optical fibres to feed light from various parts of the focal plane of the telescope through gratings to produce many individual spectra on the same detector. In an integral field unit, the fibres are closely packed together so that a spectrum from every point on a two-dimensional image of an extended object may be produced. Such instruments are useful for mapping the velocity field across a spiral galaxy, for instance. In multi-object fibre-fed spectrographs, the individual fibres may be automatically positioned at any location in the field-of-view, to feed the light of many hundreds of individual objects onto the spectrograph (see Figure 24). These instruments are useful for obtaining the redshifts of hundreds of galaxies within a single image, for instance. In both types of instrument, the resulting image consists of a series of individual spectra stacked one above another, essentially covering the whole of the detector.
An alternative to using a fibre-fed spectrograph is to use a multi-slit spectrograph. This technique is identical to single-slit spectroscopy as described earlier, except that, as implied by its name, there are multiple slits in the field-of-view. Each of these slits allows light from a different object to pass into the spectrograph and form a spectrum on the detector. In order to align the slits in their correct positions, a separate mask is usually prepared for each field to be observed with the slits simply drilled in the appropriate positions. Multi-slit spectroscopy has none of the throughput losses that are associated with passing light through optical fibres. However, a disadvantage of the method is that the field-of-view is usually smaller than for fibre spectroscopy. A modern multi-slit spectrograph may have a field-of-view that is only around 10’ in diameter. This may be compared with the 2° diameter field of the 2dF shown in Figure 24. Also, in order to prevent spectra from overlapping on the frame, the number of spectra which can be recorded simultaneously is usually less than 50 rather than the 400 that are possible with a device like the 2dF.