New Concepts in Astrocomb Technologies and Calibration

High-resolution spectroscopy is key to addressing STFC science challenges, like studying exoplanets via radial-velocity and atmospheric measurements, understanding star and galaxy formation, and refining values for fundamental constants, but is critically dependent on access to precise, traceable wavelength calibration.

For decades, astronomers have used hollow-cathode lamps as calibration sources for the spectrographs in optical telescopes, but their limitations mean that the community is now turning to astrocombs, which provide a sequence of ultra-narrow, drift-free, regularly spaced optical frequencies on a selectable multi-GHz grid. Uniquely, astrocombs exhibit broad spectral coverage and wide inter-mode spacings, a technically demanding requirement that has limited on-sky installations to only a few examples, including our own on the SALT telescope in 2016.

Our vision in this project is to develop simpler astrocomb light sources and associated high-versatility calibration tools, that anticipate the future needs of the observational astronomy community. For example, the search for Earth analogues -- planets whose mass, orbital period and "habitability" are similar to Earth -- requires instruments enabling extreme precision radial velocity (EPRV) measurements. The astronomy community has identified the need for a suite of such instruments with sufficient observing capacity to study long-period planets and to follow up candidates from the PLATO space telescope. Not only will many such instruments be needed -- each equipped with an astrocomb calibrator -- but it is likely that extreme adaptive optics (AO) in the visible, combined with fibre injection of single-mode fibre-fed diffraction limited spectrographs, will be necessary to reduce the size and cost of the spectrographs needed for these observations.

These needs motivate our proposal, which targets three world-firsts:

(a) To realize a diode-pumped Ti:sapphire astrocomb, achieving high performance in a compact, robust and simple format, and demonstrate diode-pumped astrocombs in the blue, green and red spectral regions.

(b) To apply comb-mode metrology for direct spectrograph calibration, dispensing with the need for a pre-calibration using hollow-cathode lamps or iodine cells, and instead based on isolating and directly measuring the wavelength of single comb lines in a totally new paradigm for spectrograph calibration.

(c) To introduce comb-calibrated photonic-lantern-fed spectrometry, applying astrocomb calibration to map the complex focal-plane spectra to achieve high-resolution, multi-mode performance in a compact, low-cost spectrograph.

Accompanying our research programme is a broad range of impact generation activities, targeting:

- Societal Impact, through public outreach events, schools' lectures, media engagement and online engagement;
- Academic Impact, through conventional scientific dissemination activities, complemented by dialogue and engagement with the astronomy user community;
- Economic Impact, by identifying, collating and protecting IP arising from the research; using proof-of-concept demonstrations to inform the potential for commercial exploitation (particularly in novel light source development); and by critically examining opportunities in the course of the project for the application of the technology in other fields e.g. life sciences imaging.