Study of exoplanets, searches for the Earth analogues and investigation of the nature of the Universe are the main subjects of the high-fidelity astronomical spectroscopy. Since the discovery of the first exoplanet orbiting a solar-type star by Michel Mayor and Didier Queloz using the radial velocity technique, the field has been thriving, with new, often unexpected, discoveries emerging one after the other. High-fidelity spectroscopy has become the main technique to detect planetary atmospheres and bio-signatures, to characterise host stars, to measure the expansion of the Universe, or even to test fundamental physics by measuring the possible variability of fundamental constants. Presently, there are several tens of high-resolution astronomical spectrographs scattered all around the world. Their scope ranges over various domains of astrophysics, from the search and characterization of habitable exoplanets, through fundamental physics, e.g., the study of the variability of physical constants, to the direct measurement of the expansion rate of the Universe. These fundamental questions of astrophysics can all be addressed by high-fidelity spectroscopy provided that extreme accuracy is achieved.
The accuracy of the mentioned instruments, representing the state-of-the-art science and technology development, depends on the access to extremely precise and stable wavelength calibration sources. There are several available calibration sources (e.g., emission lamps, laser frequency combs, reference cavities) that can be used to calibrate an astronomical spectrograph. However, the calibration as it is currently performed is always local with the following consequences:
- Wavelength calibration of most of astronomical spectrographs is ‘precise’. While for some of the exoplanet study aspects wavelength calibration in relative terms is sufficient, other investigations, such as the detection of Earth analogues requires a long-term repeatability that can only be assured by ‘accuracy’, i.e., if the spectrograph is calibrated in absolute terms. Other example of a scientific measurement requiring the accuracy is a direct measurement of the expansion rate of the Universe.
- Vast majority of astronomical spectrographs lacks a common wavelength reference. This signifies that the data taken by one spectrograph cannot directly be compared nor directly combined with data taken by another spectrograph. This impedes possibilities for performing long-term measurements simultaneously by several spectrographs in order to increase the measurement sensitivity, time sampling or time span.
The usual scheme for spectrograph calibration requires that somewhere along the optical path from the telescope to the spectrograph a light from the calibrator is injected to calibrate the latter. Consequently, only a part of the optical path. Any error introduced in the path before the injection of the calibrator (atmosphere and telescope front-end) is not monitored, which may lead to additional unwanted effects in the scientific data that cannot be identified nor corrected for. νANCESTOR will address the above-mentioned issues with the wavelength calibration of astronomical spectrographs by embarking an optical frequency comb on-board a satellite equipped with an actively pointing telescope and precision orbitography. Its principal goals are:
- Provide an optical frequency anchor in form of an absolute laser source operating in several wavelength bands.
- Astronomical spectrographs around the world will have the access to a light source for cross-comparison of the local calibration source.
- Astronomical spectrographs will become calibrated with the same reference frequency finally allowing comparison and merging of data from different instruments.
- Provide a means for calibrating the full telescope–spectrograph optical path, such that any errors introduced in the instrumentation chain are identified and corrected.
The provided wavelength bands are chosen such that as many astronomical spectrographs as possible in all major observatories would be able to use the calibration source and compare their local calibration with the provided optical frequency anchor. The νANCESTOR project builds upon the expertise in the field of astronomical spectroscopy and instrumentation, technological development in the optics, electro-optics, and photonics fields as well as the experience of building a successful space mission. The concept of an absolute, common to many spectrographs wavelength calibration source, such as proposed by νANCESTOR, will advance the field of high-fidelity astronomical spectroscopy with its unique characteristics, and pave the way towards new discoveries that would not be possible to achieve otherwise. We propose to build a prototype of an absolute calibration source for high-fidelity astronomical spectrographs that is compatible with space environment, and study in-depth all the relevant aspects of the mission, such as spacecraft, orbit determination, precise spacecraft tracking, payload specifications, launcher, etc. The early phase of νANCESTOR project will furthermore investigate other possible applications of the proposed concept, e.g., its use as an artificial star for the static calibration of adaptive optics systems or as spectrometer for atmospheric sciences. The project aims thus at identifying the possible synergies with other areas of astrophysics and Earth sciences.