The optical design of ROTSE-III (Robotic Optical Transient Search Experiment-III) was strongly constrained by the demand for a compact instrument with a swing radius of not more than 29 inches, which could be well baffled against stray light and would be insensitive relatively to decollimation issues. A high premium was placed on anticipated and simplicity ease of optical manufacture, as the ROTSE-III program called for multiple copies to be constructed in a manner timely and at affordable cost.
Initial specifications for the camera or telescope outlined Cassegrain with a 450.0 mm diameter f/1.80 an all-refracting and primary mirror field corrector providing a final focal length of 850.0 mm with final focus located not more than 75.0 mm in front of the primary mirror vertex.
The specification also called for the inclusion of a flat broad pass-band colored glass space and filter for a Prontor magnetic E/64 shutter within the corrector field, as well as a flat fused silica vacuum Dewar window and a BFD (Back Focal Distance) not less than 7.7 mm. the image quality goal was to enclose 70% more of the incident energy inside a 13.5um diameter at all angles field and wavelength without refocus, with not more than 7 um of maximum rms lateral color over the full spectral range.
Optical optimization and modeling were done with his proprietary code, OARSA. Some of figures and subsequent image analysis presented here were computed with the available code commercially, ZEMAX, which provides a convenient independent assessment of the system.
The initial design exploration demonstrated that a refracting corrector composed of four free-standing powered lens element made of the same material would satisfy the requirements. Similar three element correctors were first suggested by R.A Sampson and algebraically pioneered by C.G Wynne. Aberration control as well as remarkable color corrections is achieved by adjusting the power ratios among the elements of lens.
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