Long-slit spectroscopy

Long-slit spectroscopy is a technique used to obtain both spatial and spectral information at the same time.

The development of infrared detector arrays in the early 1980s led to revolution in infrared astronomy. Before we had arrays, infrared astronomy depended on measurements with single detectors. To obtain an image, the telescope had to be raster-scanned across the sky, mapping the region of interest one point at a time. Spectrometers had to obtain spectral data one wavelength at a time - not the most efficient means of observing.

The most common use of infrared arrays is in cameras, which obtain two-dimensional images (left), usually through various filters to limit the light to a small range of wavelengths. Much of my research, however, utilizes arrays in instruments known as long-slit spectrometers. These instruments get their name from a long slit which blocks most of the field, allowing only a narrow strip to pass through (right). This radiation then passes through a dispersive device such as a prism or diffraction grating which breaks it into its component wavelengths.

In a long-slit spectral image (right), the horizontal axis no longer corresponds to a spatial direction in the sky - it now represents wavelength. The vertical axis is still spatial, so that for an extended source, a long-slit spectral image contains several spectra, each corresponding to a different position in the slit, or more precisely, a different part of the source along the slit.

The (clickable) image to the left is a narrow-band image of NGC 1333 SVS 3. The long white rectangle shows the position of the slit used to obtain the long-slit spectral image to the right. In the narrow-band image, the central source dominates the total emission; it shows up as a bright ribbon in the spectral image because it radiates at all wavelengths covered. In the narrow-band image, there is more emission in the slit south of the central source than to the north, and this shows up in the spectral image as well, with stronger spectral emission to the south than the north. Note the difference in the nature of the spectrum off of the central source and on it. We can use the spectral image to trace the variations in the spectral emission from NGC 1333 in this area. For more information on what we have learned about this source, check out our on-line poster paper on NGC 1333.

The power of this type of instrument lies in the ability to obtain both spatial and spectral information simultaneously, which allows the astronomer to search for changes in the spectral behavior of a source with position. Much of my research has capitalized on what long-slit spectroscopy can tell us about the structure of dust shells produced by dying stars and the complicated spatial and spectral behavior of organic materials like polycyclic aromatic hydrocarbons.

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Last modified 9 December, 2014. © Gregory C. Sloan.