The Michelson stellar interferometer is one of the earliest astronomical interferometers built and used. Astronomical interferometry is principally conducted using Michelson (and sometimes other type) interferometers. The first of these interferometers was the 20 foot Stellar Interferometer. They are also used in optical parametric oscillators and some interferometers. Atom interferometers work on the principle of diffraction. Phase detectors are also known in optics as interferometers. Early radio telescope interferometers used a single baseline for measurement. Ring laser interferometers are self-calibrating. The latest research developments include the use of fibre optics, lasers and interferometers. This makes flexures a critical design feature used in optical instrumentation such as interferometers. Further, they can be observed with radio interferometers which can measure very small angular motions. Meanwhile, dynamically controlled loop coupling ratios allow the construction of arbitrary linear interferometers. The use of interferometers has become a standard method of testing optical surfaces. Also for accuracy laser interferometers need to be used. Large radio interferometers have been used to map the active jets emitted from active nuclei.