In this chapter we review the challenges of, and opportunities for, 3D spectroscopy and how these have led to new and different approaches to sampling astronomical information. We describe and categorize existing instruments on 4 m and 10 m telescopes. Our primary focus is on grating-dispersed spectrographs. We discuss how to optimize dispersive elements, such as VPH gratings, to achieve adequate spectral resolution, high throughput, and efficient data packing to maximize spatial sampling for 3D spectroscopy. We review and compare the various coupling methods that make these spectrographs ‘3D’, including fibres, lenslets, slicers, and filtered multi slits. We also describe Fabry–Perot (FP) and spatial-heterodyne interferometers, pointing out their advantages as field-widened systems relative to conventional, grating-dispersed spectrographs. We explore the parameter space all these instruments sample, highlighting regimes open for exploitation. Present instruments provide a foil for future development. We give an overview of plans for such future instruments on today's large telescopes, in space and in the coming era of extremely large telescopes. Currently-planned instruments open new domains but also leave significant areas of parameter space vacant, beckoning further development.

Fundamental challenges and considerations

The detector limit I: six into two dimensions

Astronomical data exist within a six-dimensional hypercube sampling two spatial dimensions, one spectral dimension, one temporal dimension, and two polarizations. In contrast, high-efficiency, panoramic digital detectors today are only two-dimensional (with some limited exceptions).