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2. DEVELOPMENT OF THE FABRY-PER.OT DOPPLER IMAGING SYSTEM 35
In the optical path, a pair of concave and convex lenses (10) and (12) and the interference
filter turret wheel (11) are placed. Three interference filters are mounted on the filter turret
wheel for the selection of wavelengths 01557.7 nm, 01630.0 nm and Na-D589.0 nm as shown
in Figure 2.10. The output image on the fluorescent screen of the PFPCI is reduced and
finally focused on the CCD camera (15) with the focusing lens (14). The composite video
output signal from the CCD camera is further processed by an image data processor which
is described in the following section.
2.4.2 Fabry-Perot etalon
The Fabry-Perot etalon used in the FPDIS is a piezoelectrically turned and servo-
stabilized etalon (Model ET116, Queensgate Instruments, Ltd). A view of the etalon and
its controller is shown in Figure 2.11. The clear aperture of the etalon is 116 mm with a
surface quality of A/100 at A 546.1 nm. The wedge angle of the etalon plate is set to 10-15
arcmin. The inner surface of the etalon is coated to provide a reflectivity of 0.88 + 0.03 in
the wavelength region between 557.7 nm and 630.0 nm. Figure 2.12 shows the dependence
of the reflectivity on the wavelength of incident light. This reflectivity corresponds to a
theoretical reflective finesse .T of 24.6. The normal optical spacing of the etalon is selected
to be 12.0 mm, which gives a normal free spectral range of 16.5 pm for a wavelength of 630.0
nm to meet the requirements for 1) obtaining the maximum resolution of the interference
fringe and 2) obtaining at least two interference fringes within the field of view of 165° for
each of 630.0 nm and 557.7 nm wavelengths. The optical spacing of the etalon and the
parallelism of the etalon plates are maintained with a stability of better than A/2000 by the
piezoelectric etalon-stabilization system (Model CS100, Queensgate Instruments, Ltd). The
electronic drift of the etalon spacing resulting from temperature changes is less than 0 + 50
pm 9C-1.
2.4.3 Interference filters
Three interference filters are prepared to select the wavelength of the aurora emissions
of 01630.0 nm and 01557.7 nm and the airglow emission of Na-D589.0nm. The center
wavelengths of these filters are 630.11 nm, 557.90 nm, and 589.16 nrn, while the full widths
at half-maximum (FWHM) of these filters are 0.96, 0.71, and 0.91 nm, respectively. The
auroral emissions of 01630.0 urn and 01557.7 rim were mainly observed at Syowa station.
Filter transmission functions of the two filters are presented in Figure 2.13.