Knowledge Bank

In spectroscopic studies of transient phenomena, one would often like to scan 3000--7000{\AA} or $1.5-5\mu$ in one millisecond, with a linear wavelength scan in real time, and to obtain absolute intensities or absorptances, while maintaining the resolving power, photometric accuracy, and flexibility of a good quality grating spectrometer. In trying to fulfill these requirements, we found it necessary to avoid moving the grating or any other critical optical parts. Our approach was to construct a high performance 0.5 meter Czerny-Turner grating monochromator, and transport a sequence of corner mirrors through the focal plane, where the dispersed radiant energy is focused simultaneously for all wavelengths. This results in a double-passed spectrum sweeping through the plane of the entrance slit, with the spectrum quality preserved. The wavelength range scanned can be selected to cover the entire first order of the spectrum from $2/3\lambda$ to $2\lambda$. Half the spectrum is displaced to the left of the entrance slit, the other half to the right. Each half is intercepted independently and focused on a separate exit slit and detector. In the present design---the Model 501 High Speed Wavelength Scanner---24 corner mirrors are mounted on a scanner wheel, resulting in 1000 scans per second for a wheel turning at 2500 RPM. In practice, the scan speed and the quality of spectra obtained---resolution, photometric accuracy, signal/noise---are limited only by the electrical band width and the detector used. The Model 501 Wavelength Scanner has been used to measure infrared spectra of transient solid propellant combustion. The $1.5-5\mu$ region was scanned repeatedly during a total burning time of 0.3 second. Design and performance of the instrument and representative spectra will be described.