F S R
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).