To access the final published version, download the pdf file . The primary goal of a tunable filter is to provide a monochromatic field over as large a detector area as possible. With the present TTF, however, the field of view is not strictly monochromatic. The effect is most acute at high orders of interference. In this particular case we see rings at different wavelength appearing within the one order.
Wavelengths are longest at the centre and get bluer the further one moves off-axis.
It follows from Eqn. Since for the current Tektronix CCD, then is always . It also follows that remains fixed over a given radius, irrespective of order m . We define our monochromatic field by the size of the Jacquinot spot, the central region of the ring pattern. By definition, the Jacquinot spot is the region over which the wavelength changes by no more than of the etalon bandpass, .