Fabry-Perot Spectroscopy

A spectrometer is an essential tool that astronomers use to examine the intensity of light as a function of wavelength (color). The resulting curves can reveal a tremendous amount of information about the physical properties and motions of the object emitting the light as well as any material through which the light shines. Like the quality of digital imaging, the ability of a spectrometer to record finer pieces of wavelength space is known as its spectral resolution. Advances in physics and optics over the last century have made many types of spectrometers available for use in astronomy.

One of the most common in use today is the diffraction grating spectrometer. These devices achieve their best performance and highest resolution when fed light from tiny angular sources containing as much light as possible. Thus, telescopes with better angular resolution and larger collecting area are capable of supporting higher quality output from a grating spectrometer. But this performance scaling of a grating spectrometer is not as ideal for studying the spectrum of astronomical objects that are not point-like. In particular, for a source like the Warm Ionized Medium (WIM) that emits optical radiation which covers the sky, they are extremely inefficient. Fortunately we have other tools at our disposal!

[Fabry-Perot ring image]

A Fabry-Perot "ring-image" spectrum of a Th-Ar calibration lamp taken with WHAM.

One alternative instrument, the Fabry-Perot spectrometer, is particularly useful for studying sources that extend over large portions of the sky. Unlike the grating spectrometer, its resolution is not limited by the size of the light cone entering the instrument. Instead, it can accept a large cone of light and still achieve excellent spectral resolution. As a result, it is a great instrument for studying features that are distributed over the sky, including the earth's atmosphere, dust in the solar system, and the insterstellar medium of our Galaxy.

At the left is a (false-color) image of a thorium-argon laboratory calibration lamp that was taken with WHAM's Fabry-Perot spectrometer. The yellow rings are two atomic emission lines from the characteristic spectrum of a heated thorium gas. In these spectral images, larger rings have shorter wavelengths (i.e., are bluer).

[Thorium Spectrum]

A plot of the intensity versus wavelength of the two Th emission lines recorded in the image above.

We convert the ring image to a more traditional two-dimensional spectrum by averaging image pixels in an annulus at a particular radius (or wavelength) to create one data point in the figure to the right. Fabry-Perot spectrometers map equal spectral intervals to equal area intervals, so that each of these spectral data points is calculated from equal area annuli—bins with the same number of image pixels. These spectra form the primary data set for all of our WHAM science.

Fabry-Perot spectrometers can be used in several ways when coupled to telescopes. WHAM is configured so that all the light from the sky within a "beam" (or cone) having an angular diameter of 1° (about double the width of a full moon) is analyzed by the spectrometer. In this mode, we loose any information about the spatial distribution of the light on the sky within that beam, but we gain a tremendous amount of sensitivity to very faint emission. By stepping the beam across the sky, we can still construct a map of the WIM where each beam is like a pixel in the resulting image. Since the WIM covers the whole sky from our vantage point inside the Gaxlay, structures tend to have large angular extents and WHAM's 1°-resolution is adequte for tracing structures throughout the ionized medium. The tradeoff in spatial resolution nets us huge increases in sensitivity over traditional imaging projects as well as the ability to collect spectra, which are essential to uncover the dynamics of the gas in the Milky Way as well as some insight into its three-dimentional structure.