Sensitive imaging systems with high dynamic range onboard spacecrafts are susceptible to ghost and stray-light effects.
During the design phase, the Dawn Framing Camera was laid out and optimized to minimize those unwanted, parasitic
effects. However, the requirement of low distortion to the optical design and use of a front-lit focal plane array induced
an additional stray light component. This paper presents the ground-based and in-flight procedures characterizing the
stray-light artifacts. The in-flight test used the Sun as the stray light source, at different angles of incidence. The
spacecraft was commanded to point predefined solar elongation positions, and long exposure images were recorded. The
PSNIT function was calculated by the known illumination and the ground based calibration information.
In the ground based calibration, several extended and point sources were used with long exposure times in dedicated
imaging setups. The tests revealed that the major contribution to the stray light is coming from the ghost reflections
between the focal plan array and the band pass interference filters. Various laboratory experiments and computer
modeling simulations were carried out to quantify the amount of this effect, including the analysis of the diffractive
reflection pattern generated by the imaging sensor. The accurate characterization of the detector reflection pattern is the
key to successfully predict the intensity distribution of the ghost image. Based on the results, and the properties of the
optical system, a novel correction method is applied in the image processing pipeline. The effect of this correction
procedure is also demonstrated with the first images of asteroid Vesta.
Based on the analysis of known, corrected, classical triplet type objectives and given the basic specification requirements, diagrams of the constructional length and image distance of their equivalent thin lens systems are prepared. By keeping the constructional length and image distance constant and applying simple formulas, the focal lengths and air gaps of the thin lens triplet variations can be generated. These variations can be used for selection based on any criteria, such as, for example, the Petzval sum. According to the original description of the Petzval sum, to our computational experiences and to the examples given, which were the results of precise corrections, we concluded that also in case of triplets, the designer should target the minimal Petzval sum, which is constrained only by the basic specification data.
The evolution of computer technology in the last decade produced a new computer category: the graphical workstation. These high performance computers, with multitasking graphical operating systems can increase the productivity of optical engineers significantly, however, the development of an optical design software which utilizes the extraordinary possibilities of these new environments requires special considerations. In this paper a new, highly interactive optical design software is introduced together with some useful methods which support the early stages of the design process.
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