Introducing NIFITS, a standard for Nulling interferometry

Our latest paper is coming out in A&A and introduces a new standard for nulling interferometry. We think this could be a milestone for the discipline, and here I’ll give you a bit of context to explain why.

The context

Since the two-telescope fringes in the summer 1974, astronomical interferometry has bocome decisive in the architecture of the Paranal observatory, and has become a key asset of observational astronomy at high angular resolution. Nowadays, the instruments of CHARA and VLTI are available to the community and used daily by scientists of many different institutes.

While the improvements in technology of the instruments and of the telescopes have played an important role, the biggest factor that made possible this state of affairs may be the broad adoption of OIFITS as a common data exchange format. This format has constituted a template for the output of any modern interferometric instrument, which helps the users prepare observations, and exploit the data.

But OIFITS is built around the initial concept of interferometry as a measurement of the complex visibility for the different baselines of the array (i.e. for each pair of telescope).

Arguably, nulling interferometry has remained a niche discipline because it does not fit into this neat package, and this is what we meant to fix with our work.

The approach

Indeed, modern nulling architectures such as the or other produce at their outputs the recombination of four or more telescopes, conditions in which data cannot be directly associated to a baseline.

We put together a team researchers involved in most of the nulling interferometers on the planet (from KU Leuven, ETH Zürich, ESO, Tucson and Sydney) Our goal was to put together a framework that would fill the need of the discipline and serve the same role as OIFITS. The result was [NIFITS] which is available on this github repository.

While the NIFITS files share some metadata with their OIFITS cousins to facilitate their insertion in existing observation databases, the bulk of the content had to be made from scratch. The biggest challenge was to make sure it had the flexibility to handle the extreme diversity of nulling interferometry architectures. This is done by including essentially a forward model in a kit that allows the creator of the file to describe the optical behavior of the instrument. The user can then use a standard backend to generate mock data for any statistical inference they want, while remaining fully agnostic of the instrument details.

The hope

We believe this framework will:

• promote the exploitation of current and future nullers by a broader community of observer,
• promote the development of more powerful data reduction tools,
• harmonize the way we describe instruments in their design phase,
• fill an important need for the archival of observation data and reproducibility of results,

and through this allow nulling to become more mainstream.

Finally, we believe that the coronagraphic community may benefit from NIFITS. By enabling the simulation of complex instruments in a standard way, it could enable the sharing of instrument models, including the dark hole state, and its effect on the off-axis PSF; and therefore provide a turnkey forwad model system that is also in high demand in the small IWA coronagraph community.

Indeed, what is coronagraph if not a continuous aperture nuller ?

Acknowledgements:

This work has received funding from the Research Foundation - Flanders (FWO) under the grant number 1234224N. D. D. received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement CoG - 866070). Part of this work has been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation under grants 51NF40_18290 and 51NF40_205606; JTH acknowledges the financial support of the SNSF. JPS is supported by NASA through grant 80NSSC23K1473 (PI: Ertel).