  The spectral resolution of a dispersive spectrograph is dependent on the
width of the entrance slit. This means that astronomical spectrographs
trade-off throughput with spectral resolving power. Recently, optical
guided-wave transitions known as photonic lanterns have been proposed to
circumvent this trade-off, by enabling the efficient reformatting of multimode
light into a pseudo-slit which is highly multimode in one axis, but
diffraction-limited in the other. Here, we demonstrate the successful
reformatting of a telescope point spread function into such a slit using a
three-dimensional integrated optical waveguide device, which we name the
photonic dicer. Using the CANARY adaptive optics demonstrator on the William
Herschel Telescope, and light centred at 1530 nm with a 160 nm FWHM, the device
shows a transmission of between 10 and 20\% depending upon the type of AO
correction applied. Most of the loss is due to the overfilling of the input
aperture in poor and moderate seeing. Taking this into account, the photonic
device itself has a transmission of 57 $\pm$ 4\%.We show how a fully-optimised
device can be used with AO to provide efficient spectroscopy at high spectral
resolution.
