Funded by the Air Force Office of Scientific Research, the goal of this project was to simultaneously track and identify objects in orbit. The conventional system used to achieve this goal is an imaging system composed of a large diameter telescope with adaptive optics, but this method is very expensive. Adaptive optics are required to resolve small objects in orbit due to atmospheric aberrations. We constructed an alternative system based on coherence imaging for tracking and identification of objects in orbit. The system is composed of a set of rotational shear interferometers coupled to small telescopes. Rather than imaging the structure of the object in orbit, our system measures the object's spectral signature while simultaneously providing location information for the object.
The rotational shear interferometer (RSI) is similar to a Michelson interferometer except that the planar mirrors on the arms are replaced by right-angle folding mirrors. The RSI allows us to measure the mutual coherence of the incident wavefront by interfering the wavefront with a rotated and time-delayed version of itself. One of the arms of the RSI can be rotated about the optical path, producing the shearing angle. A wavefront incident upon the RSI is initially split by the beam splitter and then traverses both of the arms. One of the arms folds the wavefront while the other both folds and rotates the wavefront. The arm with the rotation mechanism is also translatable, allowing for a path length difference between the two arms, just like a Michelson interferometer. After traversing the arms, the wavefront is recombined and imaged on a digital focal plane.
Picture of RSI
The response of an RSI to a far field monochromatic point source is a two-dimensional sinusoidal pattern on the focal plane. The amplitude of the sinusoid is determined by the intensity of the source. For a constant shear angle, the frequency of the sinusoid as well as the rotation angle of the sinusoid with respect to the axes of the focal plane are dependent on the angle the source makes with the optical axis. The frequency of the sinusoid is also dependent on the wavelength of the source. For a point source with a non-monochromatic spectrum like a star or light reflected from a satellite, the sinusoid is modulated by the spectrum of the source. To recover both position and spectral information, one must make multiple measurements with a single RSI or an array of RSIs working in parallel.
RSI / Telescope System
Attaching the RSI to a telescope allows us increase the amount of light input to the RSI as well as increase the physical aperture of the system. The optical system is designed so that the input aperture of the telescope is imaged onto the focal plane, allowing us to use the full extent of the CCD to measure the fringe pattern created by the RSI. This type of system has been used by various astronomers over the past few decades, though generally with a very narrow spectral filter to take advantage of the van Cittert-Zernike theorem. Students in the DISP group have worked with RSIs coupled to both 10-inch Newtonian and 32-inch Ritchey-Chretien telescopes ( the Three College Observatory.)
RSI mounted at the Three College Observatory
The Three College Observatory
Interference Fringes from RSI while observing Pollux.
The picture of fringes shown above has a large dark circle near the center with several radial lines running to the edge. This is the telescope's secondary mirror and necessary support structure. Since this occluded area blocks part of the fringes that we are trying to record, we decided to use a refracting telescope for our initial design. Version one of our RSI+telescope system is shown below.
While the clear aperture makes recording fringe patterns easier, experiments in the field have demonstrated that a large aperture is necessary for light collection. Refracting telescopes larger than six inches are prohibitively expensive, so version two of the RSI+telescope platform uses the reigning champ of the aperture / price tradeoff: a Schmidt-Cassegrain telescope.