White Light Correlation Holography Using a Random Lens for Astronomical Imaging Applications

Abstract

Holography technologies (HOLOTECH) are attractive than lens-based direct imagers due to the capabilities of HOLOTECH to record and reconstruct complete three-dimensional information of an object or an event with one or a few camera shots. In the past, HOLOTECH was mainly associated with coherent light and as a matter of fact, the development and application of HOLOTECH waited several years until the invention of laser even though the idea of holography was introduced much earlier. As the initial HOLOTECH concepts were based on two-beam interference, the lack of spatial and temporal coherence in incoherent light introduced numerous challenges and demanded stringent optical configuration requirements. For this reason, HOLOTECH was mostly applied with coherent light sources and for the “light in/from space” which is spatially and temporally incoherent, HOLOTECH could not be applied efficiently for three-dimensional imaging. In the recent years, there has been a transformation in the concepts of HOLOTECH which is rapidly reshaping the field of incoherent imaging. The invention of inter-ferenceless coded aperture correlation holography (I-COACH) has rekindled the area of spatially incoherent holography and the two-beam interference is no longer a requirement to record and reconstruct three-dimensional information. I-COACH concept was adapted into satellite telescope applications as partial aperture imaging technique and synthetic aperture imaging method. Both have proven to perform better than lens-based imagers under extreme imaging conditions. In this study, we extend the concept of I-COACH to a land-based telescope. This concept is called as 3D telescope with sparse coded apertures (3D-TELESCA). The 3D-TELESCA concept consists of quasi-random coded apertures with different sizes that are sparsely distributed within the aperture. Every coded aperture consists of two phase functions: quadratic and linear to generate carrier waves to deliver the intensity distribution to the sensor within the sensor area. The linear and quadratic phase depends upon the radius vector of the coded aperture to the center of the sensor. This aperture consists of circular zones capable of rotating about the center independently of one another. The above rotation enables multiple aperture configurations that can generate intensity distributions with cross-correlation significantly lower than autocorrelation which is desirable for statistical averaging. The imaging process consists of four steps: PSF training, PSF engineering, recording and reconstruction in which the first two are one-time offline procedure and the next two are online procedures and so only the final two steps impact the temporal resolution of the system. In the first step, a pinhole is scanned axially, and the corresponding PSF intensity distributions are recorded and saved as a library. The PSF library is engineered using phase-retrieval algorithms and computationally processed to reduce background noise. For imaging application, an object is mounted, and the object intensity (OI) distribution is recorded under identical conditions of optical configuration for recording PSF library. The OI is processed with the engineered and processed PSF library and the 3D image of the object is reconstructed. The developed method can be applied directly to “light in/from space” for three-dimensional imaging in regular as well as synthetic aperture based astronomical imaging systems. Preliminary experimental results are presented using a single element of the 3D-TELESCA.