Modelling the application of integrated photonic spectrographs to astronomy

Abstract

One of the well-known problems of producing instruments for Extremely Large Telescopes is that their size (and hence cost) scales rapidly with telescope aperture. To try to break this relation alternative new technologies have been proposed, such as the use of the Integrated Photonic Spectrograph (IPS). Due to their diffraction limited nature the IPS is claimed to defeat the harsh scaling law applying to conventional instruments. The problem with astronomical applications is that unlike conventional photonics, they are not usually fed by diffraction limited sources. This means in order to retain throughput and spatial information the IPS will require multiple Arrayed Waveguide Gratings (AWGs) and a photonic lantern. We investigate the implications of these extra components on the size of the instrument. We also investigate the potential size advantage of using an IPS as opposed to conventional monolithic optics. To do this, we have constructed toy models of IPS and conventional image sliced spectrographs to calculate the relative instrument sizes and their requirements in terms of numbers of detector pixels. Using these models we can quantify the relative size/cost advantage for different types of instrument, by varying different parameters e.g. multiplex gain and spectral resolution. This is accompanied by an assessment of the uncertainties in these predictions, which may prove crucial for the planning of future instrumentation for highly-multiplexed spectroscopy.