Fabry-Perot Spectroscopy

This image was taken with the WHAM spectrometer collecting light from a thorium laboratory calibration source. The yellow rings are two atomic lines in the thorium spectrum. In these spectral images, larger rings have shorter wavelengths. When observing the sky, the primary operating mode of WHAM collects all the light from a one-degree and only images the spectrum onto the detector.

To convert the ring image to a more traditional two-dimensional spectrum, we average image pixels in an annulus at a particular radius (or wavelength) to create one data point in the figure above. Fabry-Perot spectrometers map equal spectral intervals to equal area intervals, not equal radial widths. Each of these spectral data points is calculated from equal area annuli—bins with the same number of image pixels.