Diverse Field Spectroscopy
The fields of view of Extremely Large Telescopes will contain vast numbers of spatial sampling elements (spaxels) as their Adaptive Optics systems approach the diffraction limit over wide fields. Since this will exceed the detection capabilities of any realistic instrument, the field must be dilutely sampled to extract spectroscopic data from selected regions of interest. The scientific return will be maximised if the sampling pattern provides an adaptable combination of separated independent spaxels and larger contiguous sub-fields, seamlessly combining integral-field and multiple-object spectroscopy. This is called Diverse Field Spectroscopy
As an example of the kind of field that will need to be addressed by highly-multiplexed spectographs, consider Lyman-alpha blobs such as LAB-1 targetted by Anne-Marie Weijmanns et al. (MNRAS 402, 2245 (2009)). This image (obtained from a datacube obtained with a 24-hour SAURON Observation) may be representative of the data obtainable with Extremely Large Telescopes as they target galaxy assembly at the earliest epochs. The image includes nucleated and diffuse structures that will be difficult to target efficiently with traditional multiobject or integral field spectroscopy. Diverse Field Spectroscopy allows the observer to target only the interesting bits so that the spectrograph's capabilities are used most effectively.
The instrumentation needed for DFS is illustrated by the Celestial Selector concept which uses massive monolothic fibre bundles and networks of optical switches to select the desired parts of the field and feed the light to the slit of the spectrographs.
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