Doppler Exoplanet Detection
Of the more than two hundred extra-solar planets that have been found to date, the vast majority have been found using the radial velocity (RV) technique. Current approaches to making RV measurements rely on using very high resolution echelle spectrographs, employing cross-correlation or fits to line profiles in stellar spectra to determine Doppler shifts in the centroids of the lines. In order to reach high Doppler sensitivity, the spectral resolution must be very high (such as R>60,000) since the Doppler sensitivity is proportional to the 3/2 power of the spectral resolution. Because of this, while the RV technique has been the most successful technique for locating extra-solar planets, traditional echelles have relatively low light throughput, large instrument volume, and tend to be very expensive. In addition, they can cover only a single object in each observation. The low light throughput limits survey sensitivity to relatively bright stars and single object operation leads to slow survey speeds.
A New Approach
The Exoplanet Tracker is a new type of relative Doppler RV instrument. ET 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 in traditional techniques, and consequently the overall instrument can have much higher throughput while have 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.
Besides the high throughput and multi-object advantage over the echelle technique, an DFDI can be fed into a high throughput and moderate high cross-dispersed echelle spectrograph (such as R~20,000) to significantly gain its Doppler sensitivity (such as sub m/s precision). Due to the use of the moderate high cross-dispersed echelle spectrograph, a few objects can still be recorded on a single detector to increase the DFDI survey capability over the echelle method while producing competitive Doppler sensitivity.