Emu: A near-infrared wide-field photometer for space

'Emu' is a compact wide-field photometer destined for a 6-month mission on the exterior of the International Space Station (ISS), commencing in 2021. Emu will undertake a sky survey in the 1.4 mu m 'water band', as a method of estimating oxygen abundance in the atmospheres of cool stars down to a magnitude of m(AB)approximate to 13 (H-band). The targeted band, which lies between the astronomical 'J' and 'H' bands, cannot be observed from Earth due to the effects of water in our own atmosphere, so a space-borne instrument is necessary. The ISS presents a novel platform for such an instrument, due to its precessing orbit that repeats every similar to 70 days. It follows that a high-speed imager with a similar to 0.9 degrees transverse field of view (FoV) toward zenith can map the entire sky without active pointing, while providing a generous (similar to 65%) overlap to allow for orbital drift and pointing disturbances. This scheme is a form of time-delay integration (TDI) imaging, with successive frames that are registered and stacked after readout. This demands that stars do not traverse more than one pixel per readout, to avoid smearing. These frames are then shifted, aligned and co-added to produce a continuous strip with an effective exposure of the time taken for a target to cross the entire array. Emu's speed and sensitivity requirements make conventional near-infrared detectors unsuitable due to their inherent readout noise. Hence, Emu will use a Leonardo 'SAPHIRA' linear-mode avalanche photodiode (LmAPD) array, providing a signal gain of up to 100x before readout, thus reducing the effective readout noise by the same factor.