The optical system

To fully exploit the detector performance, a very fast custom designed optical system would need to be designed for ALIS. Ideally this would include a set of replaceable front-lenses, preferably mounted on a rotating disc in front of the filter-wheel, resulting in a remotely selectable field-of-view. As narrow-band interference filters are to be used, a telecentric lens system is required to have nearly parallel rays through the filters (see Section 3.5 for details). It was realised early that the requirement of changeable field-of-view had to be dropped due to reliability as well as budgetary constraints. Therefore, given the station separation, a field-of-view of approximately $50^{\circ}$- $90^{\circ}$ would be the best compromise (See also Figure 2.3 in Chapter 2). Following quotations from various optical manufacturers in the field, it was also quickly realised that the cost of one custom designed optical system would quickly exceed the entire project budget. Therefore a commercial telecentric optical system, frequently used in this field, was selected.

This system consists of a replaceable front-objective (A), two plano-convex telecentric lens-elements (B), a $3''$ filter holder (C, replaced by the IRF-designed filter-wheel Section 3.5.1), a field-lens (D), two close-up lenses (E,F) and a camera objective(G) (Figure 3.7).

Figure 3.7: Schematic diagram of the optical system for an ALIS imager. The two front objectives give a diagonal field-of-view of about $90^{\circ}$ and $70^{\circ}$ respectively. Information about the optical elements and their orientation after Keo Consultants [1997]; Keo Consultants [1994]

Item	ccdcam	Description
A	1,6	Front objective: Mamiya Sekor C 35 mm $f_{\char93 }=3.5$
A	2-5	Front objective: Mamiya Sekor C 45 mm $f_{\char93 }=2.8$
B	1-6	Two plano-convex telecentric lenses (plano side to rear)
C	1-6	$3''$ interference filter.
D	1-6	Field lens: Achromat, (lowest curvature side to rear)
E	1-6	Close-up lens 1: Meniscus, reverse configuration (convex side to rear)
F	1-6	Close-up lens 2: Achromat, reverse configuration (lowest curvature side to rear)
G	1-6	CCD camera lens: Canon FD 85 mm $f_{\char93 }=1.2$
H	1-6	Window of the hermetically sealed CCD enclosure (inside CCD camera head)
I	1-6	CCD (inside CCD camera head)

As discussed in Section 2.1.1, the station separation baseline was selected for a moderate field of view of about $50^{\circ}$-90$^\circ$. For the first four imagers, the 45 mm front-objective resulted in a diagonal field-of-view of about $50^{\circ}$. Experiences gained from tomographic reconstruction using images from these four systems resulted in a decision to slightly widen the field-of-view for the next two imagers. Consequently these were equipped with a 35 mm front-objective, giving a diagonal field-of-view of about $90^{\circ}$.

According to vendor specifications [Keo Consultants, 1997; Keo Consultants, 1994], the f-number setting on the front lens, $f_{\char93 fl}$, will determine image brightness on the CCD, as well as the maximum ray-angle through the filter, $\theta_{max}$, given by the following approximate formula:

$$\theta_{\mathit{max}}=\arctan \left( \frac{1}{2 f_{\char93 fl}}\right)$$ (3.44)

A photograph of the six ALIS imagers appears in Figure 3.6.