Chopping secondary mirror specifications for SOFIA

摘要

An infrared optimized telescope uses a chopping secondary to switch the instrument beam back and forth between positions in the sky. This allows synchronous detection of signal from an astronomical source in parts of the spectrum where these observations are strongly background-limited by thermal emission from the telescope and the atmosphere. To most efficiently reject this fluctuating background, as well as detector noise, which has a l/f character, the highest possible chop frequency is desired. To maximize the spatial dynamic range of the observations, the largest possible chop amplitude is desired. These parameters must be traded off against image degradation by the chopper system, which will limit our ability to measure the shape and extent of astronomical sources and, for background-limited measurements, will reduce signal strength in an otherwise optimally small beam. In order to make full use of the image quality of the telescope, the chopping secondary system should be capable of producing secondary motion that does not contribute substantially to the image size at every wavelength at which the telescope will be used. This is the primary goal of the specifications for the SOFIA secondary mirror system.