Slit function measurements of an imaging spectrograph using Fourier transform techniques

Knowledge of a spectrograph slit function is necessary to interpret unresolved lines and spectral features in an observed spectrum. In a scanning spectrometer with a single exit slit, the slit function is easily measured by illuminating the entrance slit with a broadband source and scanning the dispersive element. In a fixed grating/or disperser spectrograph, the slit functions have been measured by illuminating the entrance slit with a monochromatic light using a premonochromator or a tunable laser and by varying the wavelength of the incident light. Generally these techniques are very expensive, complex or subject to a poor signal-to-noise ratio so that an accurate measurement is often not possible. Also it would be very laborious and prohibitive to an imaging spectrograph or a multi-object spectrograph that has many sets of entrance and exit slit equivalents. We explore an alternative technique that is manageable for the measurements and where the measurement is not limited by the available signal. In the proposed technique, a Fourier Transform Spectrometer (FTS) is used instead of a pre-monochromator with variable wavelengths in the conventional techniques. This approach can be extended to the visible and ultraviolet (UV) wavelength range and to imaging spectrographs and multi-object spectrograph where multiple entrance slits and multiple exit slit equivalents (detectors) produce numerous different slit functions. In this approach, the advantages of FTS are fully utilized for available signals and the computerassisted nature of FTS makes the data processing of the measurements manageable.