Interference filters

To make spectroscopic imaging, narrow-band interference filters are required. These filters are sometimes referred to as Fabry-Perot filters, as they are multiple cavity solid Fabry-Perot etalons [See for example Hernandez, 1986; Macleod, 1986, and references therein]. Rays passing through an interference filter must be nearly normal to the filter system. This dictates the use of a telecentric lens-system as discussed in Section 3.4.

The passband of an interference filter with effective index of refraction, $N_e$, ( $N_e\approx 2$ for most interference filters) and filter centre wavelength at normal incidence, $\lambda_{cw}$, shifts to $\lambda_{\theta}$ at angle of incidence $\theta$ according to the following equation^3.5:

$$\lambda_{\theta}=\lambda_{cw} \sqrt{1 - \frac{sin^{2}\ \theta}{N_e^2}}$$ (3.45)

As seen, the passband shifts to shorter wavelengths at increasing angle of incidence. This fact is often utilised to fine-tune filters, or to measure background intensity by tilting the filter out of the passband for the emission line under study. In the case of the ALIS optics, let $\theta$ define a cone between normal incidence and $\theta_{max}$ given by Equation 3.44. Optimum filters for this situation should be designed as insensitive to angle as possible, and the bandwidth should be wide enough to pass the wavelength(s) of interest at all angles within the cone. Thus $\theta_{max}$ also defines the minimum possible bandwidth of the filter. Given a wavelength region of interest, specification of the optical system, filter centre wavelength ( $\lambda_{cw}$) and desired filter-bandwidth, $\Delta\lambda$, an optimal filter is manufactured. As almost every batch of filters are tailored for a particular application, high-quality interference filters for imaging applications are rather expensive devices. Data on the filters used for ALIS appear in Tables 3.4-3.5. A discussion of the selected emission lines is found in the work by Gustavsson [2000, Chapter 2] and references therein.

The filter wheel

The filter wheel (Figure 3.8)

Figure 3.8: The six-position filter wheel with cover removed exposing the filter compartment. Starting at filter position zero (approximately five o'clock in the photo) filters for: $O(^1S)$ 5577 Å, $O(^1D)$ 6300 Å, 6230 Å (background), white light (empty), 5324 Å (a smaller filter in an adaptor ring intended for LIDAR studies), and finally in position five, the $N^+_{2}$ 1Neg. 4278 Å auroral filter. To the right the cover with the back end of the front lens is seen. A stepping motor attached to the rear top side of the filter wheel enclosure (not shown) actuates the filter wheel by a small cog-wheel driving cogs in the inner perimeter of the filter wheel. An angular encoder is attached to the filter wheel axis on the rear side of the enclosure (not shown).

has six positions for 76.2 mm ($3''$) interference filters numbered 0-5. Temperature stabilisation is achieved through a heating blanket and temperature sensors inside the filter compartment. Positions 0, 1, 3 and 5 have standardised filter assignments (Table 3.4), and the

Table 3.4: Filter placement standard for ALIS. The empty position is for white-light imaging (without filter). Two optional positions are available, see Table 3.5

	Emission		Filter
Pos.		$\lambda$ [Å]	$\lambda_{cw}$ [Å]	$\Delta\lambda$ [Å]	Notes
0	$O(^1S)$	5577	5590	40
1	$O(^1D)$	6300	6310	40
2					See Table 3.5
3					empty
4					See Table 3.5
5	$N^+_{2}$ 1Neg.	4278	4285	50

remaining two positions have been used for various other filters (Table 3.5). A stepping motor actuates cogs along the

Table 3.5: Optional filters and their usage. The last six columns display the filter position assignments for the six imagers. See Section 6.6.2 regarding the filters for meteor studies.

	Filter CW			Positions at ccdcam
$\lambda$ [Å]	$\lambda_{cw}$ [Å]	$\Delta\lambda$ [Å]	usage	1	2	3	4	5	6
4227	4225	280	meteor studies						2
	6230	40	background filter.	2	2	2	2	2
	5324	50	Lidar filter			4
	5100	40	background filter.						2
5893	5898	200	meteor studies (Na)	2
8446	8455	40	$O(3p^3P)$	4			4	4	4

perimeter of the filter wheel. An angular encoder on the filter wheel axis senses the filter position. The filter change time is 1-2 s. The filter wheel cover, which also supports the front lens, is easily removable for filter changes etc. However, the cover should only be removed in a clean room, to avoid dust in the filter compartment. The power-supply and stepping motor drive electronics are mounted in a $19''$ enclosure, the Filter Wheel Control unit (FWC) (Figure 3.9 and the block diagram in Figure 3.12).

Figure 3.9: Left: Power supply, stepping motor drive circuits, cables etc. for the filter wheel. Right: The same equipment, but doubled for the camera positioning system (2 axes). See also Figure 3.12.