Direct laser frequency comb calibration of an astronomical spectrograph

High-resolution echelle spectrographs are critical for modern astronomy. Determining their wavelength solution is a prerequisite calibration, conventionally performed by illuminating the instrument with a broadband hollow-cathode lamp and cross-referencing the resulting two-dimensional spectrum to an emission-line atlas. Challenging requirements for next-generation spectrograph calibration are driving the adoption of laser frequency combs in place of lamps. Laser frequency combs offer an exceptional relative calibration scale, but the task of wavelength-tagging individual comb modes currently makes a spectral-line source essential for absolute calibration. Here, we present a new approach that utilizes a laser frequency comb as the sole calibration light source. An ancillary “spectral shaping” spectrograph can excise an individual comb mode and measure its wavelength, which can then be refined to provide sub-fm accuracy wavelength tagging on a proxy astronomical spectrograph echellogram. In a secondary procedure, a comb mode is isolated for each of the echellogram orders in a single measurement, revealing the free spectral range and allowing the order-to-order relationship to be established. These complementary techniques allow the complete calibration of a spectrograph to be achieved using only a laser frequency comb, eliminating reliance on auxiliary light sources and providing direct access to GPS-referenced accuracy.