The figure below shows the span in free spectral range (FSR) available at different wavelengths. The TTF is periodic in wavelength over a baseline of FSR. Therefore, in regions of bright sky emission or multiple spectral lines, we need to block the neighbouring orders with an intermediate band filter which is comparable to or narrower than the FSR at a selected order of interference. The TTF plates are scanned through one FSR, i.e. one order of interference, every lambda/2 where lambda is the operating wavelength. Since the total range in plate spacings is 2-12 um, this sets the upper and lower envelopes in the figure below. The FSR in Angstroms is given by lambda/(m+1), where m is the order of interference. At the smallest plate spacings, i.e. largest FSRs, this needs to be modified to account for phase reflectance effects (Jones & Bland-Hawthorn 1998, PASP).