BETTII - the Balloon Experimental Twin Telescope for Far Infrared Interferometry

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(as per submitted Joint proposal - MJG_BETTII_PRD)More than half of the energy generated in the Universe since the big bang now exists in the form of electromagneticradiation in the far infrared part of the spectrum - wavelengths a few hundred times longer than those of visible light. Muchof the energy emitted by developing stars and young stars is absorbed by the clouds of gas and dust in which they form,and re-radiated in the far infrared. So if we want to understand the birth processes of stars in our own galaxy today, or toinvestigate how galaxies grow and develop (through star-formation), we need to make observations in the far infrared. Inrecent years, huge advances have been made by telescopes in space and operating on high altitude balloons in ourendeavours to gain knowledge of the details of star and planetary system formation and the history of galaxies like our ownMilky Way. But astronomy at far infrared wavelengths has a long way to go before we will be able to take pictures withsimilar quality to that obtained by optical telescopes at shorter wavelengths and by arrays of radio telescopes at longerwavelengths.Observations have to be made from space or from high altitude because the atmosphere is strongly absorbing in this partof the spectrum. That makes it difficult to use large ground-based telescopes. In addition, the technique of "interferometry",used in the radio region of combining the beams from number of smaller telescopes to make an image as good as thatfrom a much larger one, has not yet been perfected in the far infrared. So at present, the image quality of observations inthe far infrared is not nearly as good as astronomers need to get a better view of stars and galaxies and their origins.In this project we will make a big advance by building the first scientific interferometer working in the far infrared, flying it ona high altitude balloon, and making observations of both of star forming regions in our galaxy and also of active galaxiesharbouring massive black holes at their centres, to learn more about how stars form in clusters and how black holesinteract with the galaxies they inhabit. These pioneering measurements will be the highest quality far infrared observationsever, in terms of our ability to discern fine details in the image. Besides giving us new scientific insights, they will allow usto perfect and demonstrate the technology needed to build a more ambitions and powerful instrument in the future, perhapsa space-borne system, to provide even better observations. The scientific importance of such an observatory has beenrecognised by both NASA and by the European Space Agency, who are considering space missions of this type. However,the necessary technology and expertise is not available yet, and this work will make a big contribution to establishing ourcapability to fly such a mission.