2009ApJ...702L..77S

A photon of wavelength λ ∼ 1 µm interacting with a dust grain of radius a_p∼ 1 mm (a "pebble") undergoes scattering in the forward direction, largely within a small characteristic diffraction angle θ_s∼ λ/a_p ∼ 100''. Though millimeter-size dust grains contribute negligibly to the interstellar medium's visual extinction, the signal they produce in scattered light may be detectable, especially for variable sources. Observations of light scattered at small angles allow for the direct measurement of the large grain population; variable sources can also yield tomographic information of the interstellar medium's mass distribution. The ability to detect brilliant pebble halos requires a careful understanding of the instrument point-spread function.