Recent technological advances could make interstellar travel possible, using ultra-lightweight sails pushed by lasers or solar photon pressure, at speeds of a few per cent the speed of light. Obtaining remote observational data from such probes is not trivial because of their minimal instrumentation (gram scale) and large distances (pc). We derive the optimal communication scheme to maximize the data rate between a remote probe and home-base. The framework requires coronagraphic suppression of the stellar background at the level of 10−9 within a few tenths of an arcsecond of the bright star. Our work includes models for the loss of photons from diffraction, technological limitations, interstellar extinction and atmospheric transmission. Major noise sources are atmospheric, zodiacal, stellar and instrumental. We examine the maximum capacity using the ‘Holevo bound’ which gives an upper limit to the amount of information (bits) that can be encoded through a quantum state (photons), which is a few bits per photon for optimistic signal and noise levels. This allows for data rates of the order of bits per second per Watt from a transmitter of size 1 m at a distance of α Centauri (1.3 pc) to an earth-based large receiving telescope (E-ELT, 39 m). The optimal wavelength for this distance is 300 nm (space-based receiver) to 400 nm (earth-based) and increases with distance, due to extinction, to a maximum of ≈ 3 μm to the centre of the Galaxy at 8 kpc.

We show that anomalous extinction (deviations from the traditionally adopted RV = AV/E(B − V) = 3.1 introduces large uncertainties in the distances of stars, for distances larger than 1-2 kpc. We argue that for such distances and for directions close to the galactic plane, the use of extinction models based on the gas distribution in the Galaxy is safer, for the moment, than the use of extinction maps.

Results based on the deep imaging survey of the inner region (~300pc of the bulge within |ℓ| ~ 1.5°, |b| ~ 0.5°) of the Milky Way are reported in this communication. This survey is about 2.5 magnitude deeper than DENIS and 2MASS and is able to detect stars of the red clump at a distance of the Galactic center. Toward some directions we find extinction reaching AV = 50 mag. A catalog of the sources is in preparation.