A photon of wavelength \lambda \sim 1 \mu { m } interacting with a dust grain of radius a _ { p } \sim 1 { mm } ( a “ pebble ” ) undergoes scattering in the forward direction , largely within a small characteristic diffraction angle \theta _ { s } \sim \lambda / a _ { p } \sim 100 ^ { \prime \prime } .
Though mm-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 allows 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 require a careful understanding of the instrument PSF .