Review of spectrographs

Spectroscopic observations can be thought of as a method by which one samples the emitted energy distribution from an astronomical source in wavelength bins of size AA. Broad-band filter photometry, for example, is a form of spectroscopy; it is merely one with extremely poor spectral resolution. To use spectral information to learn detailed physics for an astronomical object, one must be able to differentiate specific spectral features (lines) from the continuum within the observed spectrum and be able to make quantitative measurements of such features. Generally, this type of analysis requires a spectral resolution of at least 20-40 A or better. Keep in mind, however, that various scientific objectives can be accomplished with varying amounts of spectral resolution. Schmidt telescope observations using an objective prism and imaging each spectrum onto a CCD have fairly low spectral resolution, but the imaged spectra are indeed useful if the purpose is to identify objects that have blue color excesses (see Section 6.7).

Figure 6.1 illustrates a typical astronomical spectrograph with the common components identified. An entrance slit, onto which the telescope focuses the

Diagram Spectrometer Telescope
Fig. 6.1. Schematic diagram of a typical astronomical spectrograph. The major components are the CCD detector, the continuum and comparison line calibration sources, the TV slit viewer, and the grating. From Wagner (1992).

incoming light from the source of interest, is used both to set the spectral resolution and to eliminate unnecessary background light. An internal light source for the production of a flat field (called a projector flat in spectroscopy) and various wavelength calibration emission line sources are also included. These lamps usually consist of a quartz projector lamp for the flat fielding and a hollow cathode or arc lamp for the calibration sources. Both types of calibration lamp are included in the spectrograph in such a way as to attempt to make their light path through the slit and onto the CCD detector match as closely as possible that of the incoming telescope beam from an astronomical object. Some type of grating (commonly a concave reflection grating) is needed as the dispersive element, although a prism can also be used. Various camera optics, to re-image the slit onto the CCD detector and provide chromatic and field flatness corrections, finish the suite of standard components. Numerous variations on this standard theme have been and will continue to be used as cost, complexity, and purpose of the instrument are always issues.

Spectrographs with gratings (compared to prisms) and CCD detectors usually cover 1000-2000 A of optical spectrum at a time with typical resolution of 0.1-10 A/pixel. In order to cover more spectral range, a few observatories have built double spectrographs. These instruments consist of two separate spectrographs (each similar to that shown in Figure 6.1), which share the incoming light that is divided by a dichroic beam splitter into red and blue beams. Operating double spectrographs are discussed in Gillespie et al. (1995) and DePoy et al. (2004).

Two other astronomical spectrograph types are worth mentioning as they are increasingly used today. These are echelle type spectrographs and fiber fed spectrographs. Figure 6.2 shows an example of a cross-dispersed echelle spectrograph. This type of instrument provides high resolution spectroscopy (R = 50000 to 100000 or more) through the use of both an echelle grating to produce the high spectral resolution and a cross disperser to separate the orders and project them in two dimensions onto a CCD array. As an example of this type of observation, Figure 6.3 presents a cross-dispersed echelle spectrum of the Bpe star MWC162 obtained with the 6-m Bolshoi Teleskop Azimutalnyi (BTA) located in central Russia.

Astronomical spectrographs, of the types mentioned above, can also be fed by optical fibers that collect light at the telescope and bring it to the instrument. Many examples exist that use a single fiber to feed a table-mounted spectrograph, 10-30 fibers in a close bundle called an integral field unit (IFU), or cases of 100 or more fibers being placed in the focal plane at the telescope. The fibers collect light from individual objects at the focal

Direction of dispersion by the echelle grating

Fig. 6.2. Schematic diagram of a cross-dispersed echelle spectrograph showing the echelle grating and the cross disperser. The final 2-D spectral image is projected on to a CCD.

plane and carry the light as a fiber bundle to the awaiting spectrograph mounted on a table on the observatory floor or in an isolated room. Fiber fed spectrographs can provide ease for spectral observations (moving a single fiber into place is simpler than dismounting the current instrument and mounting a spectrograph), weight alleviation (a fiber is lighter than an instrument), or the ability to obtain multiple spectra at once (each fiber is positioned in the focal plane to observe one source). Figure 6.4 shows an example 2-D CCD image of nearly 70 spectra obtained simultaneously using the HYDRA multi-fiber spectrograph on the WIYN 3.5-m telescope at Kitt Peak National Observatory.

Let us define a few useful quantities in CCD spectroscopy. Table 6.1 lists the various definitions needed for use when discussing the optical properties of the telescope, collimator mirror (or lens), and the camera itself. Using the definitions in Table 6.1, we can define the magnification of the spectrograph as M = Fcam/Fcol, the projected width of the slit at the CCD will be wM, and the slit width will subtend an angle on the sky of © = w/F. The projected width of the slit at the CCD detector is then given by ii j ^T^n/r /cv rn Fcam ©fDFcam r = wM = ©FM = ©fD-=

Fig. 6.3. A cross-dispersed echelle CCD image of the Bpe star MWC162. The spectrogram covers 3900 A to 5700 A at high (echelle) dispersion with each order separated vertically by the cross disperser. Notice the presence of both emission and absorption lines as well as P Cygni profiles in the stronger Balmer lines. This image was obtained using the LYNX instrument on the BTA.

Fig. 6.3. A cross-dispersed echelle CCD image of the Bpe star MWC162. The spectrogram covers 3900 A to 5700 A at high (echelle) dispersion with each order separated vertically by the cross disperser. Notice the presence of both emission and absorption lines as well as P Cygni profiles in the stronger Balmer lines. This image was obtained using the LYNX instrument on the BTA.

To avoid loss of efficiency within the spectrograph, the collimator focal ratio should match that of the telescope. To make this clear, we can write the projected slit width at the CCD detector as ^ = ©D/cam. We assume here that the dispersing element does not change the collimated beam size.1 If the slit is opened wide enough to allow all the light from a point source to pass through, the projected image size of the point source at the CCD detector is simply

The ability to separate closely spaced spectral features is determined by the resolution of the spectrograph. Spectral resolution is defined as R = A/AA, in which AA is the difference in wavelength between two closely spaced spectral features, say two spectral lines of equal intensity, each with approximate wavelength A. Optical light spectral resolutions of a few hundred thousand to

1 While not always true for diffraction gratings, this condition is realized for prisms.

Fig. 6.4. A multi-fiber spectrograph image obtained with HYDRA of a cluster of galaxies. Each of the 70+ fibers was positioned at the focal plane to collect the light of a single cluster member. The fiber bundle was then formed into a linear array and each fiber's light passed through the spectrograph in a normal fashion. The formed 2-D image of each spectrum was collected by a CCD and presented to the user. Each spectrum will be extracted and examined separately. Note how the spectral lines are nearly coincident for each object (as they are at the same redshift and similar in type) but a few interlopers are present as well.

Fig. 6.4. A multi-fiber spectrograph image obtained with HYDRA of a cluster of galaxies. Each of the 70+ fibers was positioned at the focal plane to collect the light of a single cluster member. The fiber bundle was then formed into a linear array and each fiber's light passed through the spectrograph in a normal fashion. The formed 2-D image of each spectrum was collected by a CCD and presented to the user. Each spectrum will be extracted and examined separately. Note how the spectral lines are nearly coincident for each object (as they are at the same redshift and similar in type) but a few interlopers are present as well.

one million have been obtained for the Sun, whereas for typical astronomical spectra, R is much less, being near a few thousand down to a few hundred for very faint sources. For comparison, current R values in the infrared are typically less than about 10000.

Spectroscopy with CCDs Table 6.1. Spectrograph definitions

D

Diameter of Telescope

F

Focal Length of Telescope

f

Focal Ratio of Telescope

w

Width of Entrance Slit

dcol

Diameter of Collimating Mirror

Fcol

Focal Length of Collimator

fcol

Focal Ratio of Collimator

dcam

Diameter of Camera Mirror

cam

Focal Length of Camera

fcam

Effective Focal Ratio of Camera

For further details of the actual components of astronomical spectrographs, various types of spectroscopy for different applications, and the way in which these components are used to produce spectra at various wavelengths and resolutions, see Robinson (1988b), DeVeny (1990), Pogge (1992), Wagner (1992), Cochran (1995), Corbally (1995), Quetoz (1995), and Stover et al. (1995).

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