Introduction

The development of state-of-art spectrographs for the VLT and La Silla observatories over the last decade has demonstrated the scientific interests of precision spectroscopy in astrophysics (HARPS, UVES, Xshooter, ESPRESSO). The detection of earth-like planets and the long-term goal of directly probing the evolution of the cosmic redshift (Sandage test) require cm/s velocity measurement accuracy. However, the current instruments are limited by the long-term wavelength accuracy and stability of order several m/s.

The commonly used cathode line lamps cover the astronomical wavelength range only insufficiently and suffer from wavelength shifts due to ageing effects. The HARPS and ESPRESSO spectrographs were the first to start the development and implementation of Laser Frequency Combs (LFC) as calibration sources (in this context called Astrocombs). In principle, LFCs provide a unique frequency stability, which is only limited by the time reference (e.g. an atomic clock). 

In reality however, a number of effects can degrade the calibration stability on the spectrographs.

Slow variations of the point-spread function, scrambling of the modes in the feeding fibers and subtle mechanical effects can impact the calibration on the level of 0.1 - 1m/s velocity precision.

While standard LFCs have reached technical maturity, the conversion from a LFC to an Astrocomb, i.e. a calibration source for astronomical spectrographs is still research and development. Just to name a few areas of active development are, the spectral flattening, the mode scrambling and the feeding into the spectrograph. Those pose challenges and are sources for calibration uncertainty.

PhD project objectives

• Analyze the long term behavior of the existing Astrocombs
• Ensure a long term stability compatible with the most challenging requirements (a few cm/s in the long term).

The project will include the following research and development activities

• Data reduction and data analysis of Astrocomb calibration data from the spectrographs HARPS, ESPRESSO and NIRPS. This includes:
  • Development of an optimum data extraction, taking into account known issues such as inter-line background and variable photometry
  • Development of reliable performance metrics
  • Characterization of systematic error sources and mitigation strategies

• Characterization and optimization of the in-house developed mode scrambling and feeding (fiber) optics.
  • Development of a test bench to measure the mode scrambling properties of different fibers and different scrambling techniques (active and passive)
  • Development of a deployable measurement setup to monitor the mode scrambling efficiency live at the observatory
   
• Development of a new mode scrambling fiber feed (using passive and active components), which homogenizes and stabilizes the fiber point spread function to the equivalent of cm/s velocity shift.

Required skillset:

The PhD candidate should have a master's degree in physics, optics, photonics, astronomy, optical engineering or equivalent. Experience with similar projects on fiber optics and or high resolution spectroscopy will be advantageous. Experience with data analysis and high level programming languages such as Python would be an asset.

The candidate will be part of a multidisciplinary and -national team, working in Europe and Chile. The work will likely include trips to ESO’s observatories in Paranal and La Silla, Chile.