We describe a possible new technique for precise wavelength calibration of high‐resolution astronomical spectrographs using femtosecond‐pulsed mode‐locked lasers controlled by stable oscillators such as atomic clocks. Such ‘frequency combs’ provide a series of narrow modes which are uniformly spaced according to the laser's pulse repetition rate and whose absolute frequencies are known a priori with relative precision better than 10−12. Simulations of frequency comb spectra show that the photon‐limited wavelength calibration precision achievable with existing echelle spectrographs should be ∼1 cm s−1 when integrated over a 4000 Å range. Moreover, comb spectra may be used to accurately characterize distortions of the wavelength scale introduced by the spectrograph and detector system. The simulations show that frequency combs with pulse repetition rates of 5–30 GHz are required, given the typical resolving power of existing and possible future echelle spectrographs. Achieving such high repetition rates, together with the desire to produce all comb modes with uniform intensity over the entire optical range, represents the only significant challenges in the design of a practical system. Frequency comb systems may remove wavelength calibration uncertainties from all practical spectroscopic experiments, even those combining data from different telescopes over many decades.

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The European consortium SOLARJET has experimentally demonstrated the first ever production of jet fuel via a thermochemical H 2 O/CO 2 -splitting cycle using simulated concentrated solar radiation. The key component of the production process of sustainable "solar kerosene" is a high-temperature solar reactor containing a reticulated porous ceramic (RPC) foam structure made of pure CeO 2 undergoing a 2-step redox cyclic process. During the first endothermic reduction step at 1450− 1600 °C, the RPC was directly exposed to concentrated thermal radiation with power inputs ranging from 2.8 to 3.8 kW and mean solar flux concentration ratios of up to 3000 suns. In the subsequent exothermic oxidation step at 700−1200 °C, the reduced ceria was stoichiometrically reoxidized with CO 2 and/or H 2 O to generate CO and/or H 2 . The RPC featured dual-scale porosity: millimeter-size pores for volumetric radiation absorption during reduction and micrometer-size pores within its struts for enhanced oxidation rates. For a cycle duration of 25 min, mean reduction rates were 0.17 mL O2 min −1 g −1 CeO2 and mean oxidation rates were 0.60 mL CO min −1 g −1CeO2 . The solar-to-fuel energy conversion efficiency was 1.72%, without sensible heat recovery. A total of 291 stable redox cycles were performed, yielding 700 standard liters of syngas of composition 33.7% H 2 , 19.2% CO, 30.5% CO 2 , 0.06% O 2 , 0.09% CH 4 , and 16.5% Ar, which was compressed to 150 bar and further processed via Fischer−Tropsch synthesis to a mixture of naphtha, gasoil, and kerosene.

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