Extended object spectroscopy

The definition of extended object spectroscopy follows from the fact that we wish to obtain spectra, not from an unresolved point source, but from a region of the sky for which we desire simultaneous wavelength and spatial information. Examples might include galaxies, nebulae, and planets within our solar system. While there is no fundamental difference between this type of spectroscopy and point source observations such as those described above, there are differences in the instruments used and the reduction techniques involved. We will present here a very basic introduction to the subject and refer the reader to the more detailed review given by Pogge (1992).

One method of obtaining spectra of an extended object is by using long-slit spectroscopy. While sounding like an entirely new method of observing, long-slit spectroscopy is very basic. When we discussed point source observations,

- Collimator

Fig. 6.10. Schematic view of a typical long-slit CCD spectrograph. Positions along the slit are mapped in a one-to-one manner onto the CCD detector. A number of optical elements in the camera, used to re-image and focus the spectrum, have been omitted from this drawing. From Pogge (1992).

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Fig. 6.10. Schematic view of a typical long-slit CCD spectrograph. Positions along the slit are mapped in a one-to-one manner onto the CCD detector. A number of optical elements in the camera, used to re-image and focus the spectrum, have been omitted from this drawing. From Pogge (1992).

we concerned ourselves with the details of the spectrograph and final spectrum as they related to the incident light from a point source focused on the spectrograph slit and imaged on the CCD array. We mentioned in that discussion that we desired to use the slit to keep out as much background light as possible. However, all objects that lie along the slit, say in the x direction as presented in Figure 6.10, will produce a spectrum and be imaged on the 2-D CCD array. This last statement makes some assumptions about the ability of the spectrograph optics and optical path to support such an endeavor.

One can imagine a case in which a number of point sources are lined up in an exact east-west manner and a spectroscopic observation is performed of these stars using an east-west slit alignment. The output image will contain a number of parallel spectra, one for each point source, lined up on the CCD. Obvious extensions of this simple example to real cases include placing the long slit of a CCD spectrograph along the axis of a spiral galaxy or alternately across and alongside of bright knots of ejecta within a recently erupted classical nova. The uses of such spectrographic techniques are many and yield information on both the spectral properties of an object (radial velocity, line widths, etc.) and the spatial properties (spiral arms versus the bulge, etc.). This type of observational program is indeed another that benefits from large-format CCDs.

A more complex and upcoming type of two-dimensional CCD spectrograph is the imaging Fabry-Perot interferometer. Within this type of a spectrograph, 2-D spectral imaging over a narrow band-pass can be collected on the CCD, and with a change in the Fabry-Perot etalon spacing, a new narrow band-pass can be isolated. These types of instruments are often quite complex to master and setup, produce complex data cubes of resultant imagery, and tend to be less efficient than conventional spectrographs due to their many optical surfaces. However, to its benefit, Fabry-Perot interferometry has a unique capability to provide tremendous amounts of scientific data per observational data set and to obtain fantastic results for the objects observed. The details of this type of spectrograph and its use can be found in Roesler et al. (1982), Bland & Tully (1989), and Pogge (1992).

A final method of obtaining spatial as well as spectral information is that of narrow-band imaging. The spatial aspect comes about through the use of a CCD as a wide-field imager and the spectroscopic information is provided at selected wavelengths by the use of narrow-band filters. This is no different from observing a given area of the sky in B, V, and R filters, except that here one usually is imaging one or more extended objects and selects carefully two or more restrictive filters in order to obtain specific results. Simultaneous imaging in all colors is lost as one has only limited spectral coverage with this technique, but one obtains full spatial sampling over the entire field of view and the instrumental setup and data reduction are quite simple. In addition, this type of 2-D spectrophotometry can be performed with any CCD imager with the addition of the appropriate filters.

There are numerous other complexities associated with extended object spectroscopy, some in the observational setup procedures needed and some in the data handling and reduction procedures. However, the wealth of information available from such observations far outweighs the difficulties of these more complex types of CCD spectroscopy.

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