MARVELS Spectrograph

A schematic illustration of the MARVELS spectrograph. Figure taken from Ge, Erskine, and Rushford, 2002, PASP.
A schematic illustration of the MARVELS spectrograph. Figure taken from Ge, Erskine, and Rushford, 2002, PASP.

MARVELS is using a specially built spectrograph to obtain high precision radial velocity measurements of stars looking for exoplanet candidates. In contrast to the more traditional high-resolution echelle spectrographs, MARVELS uses a new approach to measuring radial velocities. The spectrograph is a fiber-fed dispersed fixed-delay interferometer (DFDI), a combination of Michelson interferometer and medium resolution (R~6,000-10,000) spectrograph, which overlays interferometer fringes on a long-slit stellar spectrum. The Doppler sensitivity for this approach is proportional to the 1/2 power of the spectrograph resolution. Because of this, the post-dispersing spectrograph can be of much lower resolution than those in more traditional techniques. Consequently, the overall instrument can have a much higher throughput while allowing for a much smaller size than the echelle instruments. The cost of the instrument is comparatively low, and most importantly, it operates in a single-order mode: a single spectrum only takes up one strip along the CCD detector. Hence, spectra from multiple stars can be lined up at once on a single detector to increase survey speed. In combination with a wide field multi-fiber telescope, multi-object surveying can be achieved (indeed, 60 stars are observed in each exposure).

MARVELS’s dispersed fixed-delay interferometer consists of a wide angle Michelson interferometer followed by a medium resolution spectrograph. The spectrograph can be thought of as dispersing the light output from the interferometer into a large number of narrow wavelength channels. The interferometer creates interference fringes within each of these channels. The interferometer has a fixed path difference between its two arms, and has one mirror tilted by a few wavelengths in the slit direction. With a wide beam entering the interferometer, this produces a spectrum that is spread out so that a very narrow range of delays is effectively scanned across the length of the slit. Over this range, the interference fringes show a sinusoidal response in the slit direction, with an approximately constant phase offset and visibility. Such fringing spectra can be thought of as an overlap between the interferometer comb and the stellar spectrum. When there is a small Doppler shift in the spectral lines, there is a correspondingly large shift in the phase of the sinusoidal fringes. Hence by fitting sine functions to each detector column and measuring how they shift in phase, one is able to determine a relative Doppler shift.