From Venus to the Parking Lot: A Flexible Stellar Interferometry Experiment

We describe a simple extension of our earlier Venus interferometry demonstration that can be performed indoors or outdoors, independent of planetary visibility or atmospheric seeing. A small flashlight is placed 15-50 meters away from a telescope and covered with a circular aperture mask (hole diameter 2-6~mm). A small telescope fitted with an opaque mask bearing two 2~mm holes, spaced between 5 and 20~mm apart, is then used to observe interference fringes from this artificial extended source. By varying the hole separation and observing the fringes students can estimate the angular diameter of the source and verify their result directly by simple geometry, a check not possible with astronomical targets. The experiment requires no telescope tracking, no dark skies, and no special equipment beyond materials described in the original paper. It serves equally well as a pre-lab to build intuition before observing Venus, a standalone indoor daytime or outdoor nighttime activity, or a quantitative test of the interferometric method itself.