An adapter may be used to couple an imaging device, such as a smartphone, to an optical device such as a telescope. The adapter may have an optical device mounting feature that mounts the adapter to the optical device, an imaging device mounting feature that mounts the imaging device to the adapter, and a positioning mechanism that adjustably positions the imaging device mounting feature relative to the optical device mounting feature. The optical device mounting feature may have a first arm, a second arm that is movable relative to the first arm to clamp a portion of the optical device between the first arm and the second arm, and a lock that can be actuated to restrict motion of the second arm away from the first arm. The positioning mechanism may be actuatable to move the imaging device, relative to the optical device, along three orthogonal axes.