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Far Infrared Interferometry

Staff: Matt Griffin, Dr Enzo Pascale, Peter A Ade.
Postdocs: Dr Georgina Klemencic, Dr Locke Spencer.
Collaborators: Giorgio Savini, Bruce Swinyard, Roser Juanola-Parramon (University College London); Stephen Rinehart, Dave Leisawitz and the BETTII team (NASA Goddard)

Graph showing BETTII, Herschel, ALMA, HST and JWST angular resolution vs. wavelength.

BETTII will complement the high spatial resolution of other facilities, filling a critical need for higher angular resolution in the FIR (click to enlarge).

The far infrared (FIR) region of the electromagnetic spectrum (roughly 30 µm to 300 µm wavelength) is essential to study processes that are obscured at shorter wavelengths by cosmic dust. This includes the formation of stars and planetary systems, the active star-forming stages in the evolution of galaxies, and the environments of massive black holes as they grow at the centre of active galaxies. Space-borne telescopes, most recently the Herschel Space Observatory have made a huge contribution to knowledge of the Universe in the FIR. But even Herschel suffers from the fact that the angular resolution of any telescope – its ability to see fine details in an image – is determined by its size. In fact the smallest feature on the sky that a telescope can reliably discern is proportional to the wavelength of observation divided by the diameter of the telescope (λ/D).

This is now our biggest limitation in FIR astronomy. Comparing, for example, the Hubble Space telescope (diameter 2.4 m) operating in the visible region (wavelength 0.5 µm) to Herschel (diameter 3.5 m; operating at 100 µm wavelength) we see that the size of details that the Hubble telescope can measure is more than a hundred times smaller (better) than Herschel. To achieve equivalent resolution at Herschel's wavelength we would need a mirror of some 350 m diameter. It is clearly impossible to launch something of that size into space, but through a technique known as interferometry, two or more much smaller telescopes, separated by a large distance, can be operated together to produce an image equivalent to what a larger telescope would produce. Studies of possible designs for a far infrared interferometer have been carried out in Europe (FIRI; see Helmich & Ivison, 2007) and the US (SPIRIT; see Leisawitz et al., 2007).

Schematic of BETTII interferometer

Optical layout of the BETTI balloon-borne FIR interferometer (click to enlarge).

Ultimately, a space-borne FIR interferometer can give the necessary combination of high sensitivity, wide spectral coverage and good angular resolution. The scientific power of such an observatory would be immense, but there are great technical challenges. We are working with collaborators at the Optical Sciences Laboratory at University College London and the Astrophysics Science Division of the NASA Goddard Spaceflight Center at the NASA Goddard Space Flight Centre (NASA GSFC) in the USA, to develop and demonstrate some key technologies needed for a future FIR interferometry space mission. This includes setting up a prototype FIR interferometer in the laboratory at Cardiff to study its behaviour, test and perfect optical components (also developed at Cardiff), and learn how best to process the observational data (see Grainger et al., 2012)

We are participating in the Balloon Experimental Twin Telescope for Infrared Interferometry, BETTII Project, which is led by NASA Goddard and which is scheduled for a first balloon flight in 2015. See the Cardiff BETTII page and the NASA Goddard BETTII project page for more details.

several views of the laboratory prototype

Components of the far infrared interferometer laboratory prototype at Cardiff University (click to enlarge).

We are also participants in the EU Funded Far Infrared Space Interferometer Critical Assessment (FISICA) project, in collaboration with UCL and institutes in France, Ireland, Italy, and Canada. The FISICA team will review and update the science case for a European-led FIR interferometer based on the most recent results of the Herschel and Planck space missions, and the ground-based ALMA interferometer. The scientific goals will be used to develop a key mission design concept in terms of spectral resolution, sensitivity, polarimetry, mapping capabilities etc. The critical technology developments needed to make a space borne far infrared interferometer a realistic proposition will be identified. A comprehensive computer model of the chosen conceptual design will be created and used to predict the observing performance of FIRI in as many different scenarios. The technological readiness of a some key optical components required for the interferometer design will also be advanced in an experimental development programme.