Although some of the filters have shifts in X greater than 10 m m, it is always possible to select a filter of each type with a shift less than 5 m m. If the filters are installed with the wedge angles in the same orientation, then the shifts will always be in the same sense and an average position can be used. It should be noted, though, that if this solution is adopted, filters 4 and 5 will not have effective spares. The main effect of filter curvature is to shift the object focus. This leads to a blur of 2.7 and 1.7 m m, respectively.
Some improvements are possible by picking the better filter of each pair.
This suggests that focus shifts can be kept below 15 m m, which will help the error budget. The mechanical properties of the filters as delivered are acceptable, provided it is understood that this is achieved by selecting the better filter of the pair provided, and orienting it properly. In this case, it is not necessary to adjust the slit for each filter. It should be noted that it several cases the second filter substrate is of sufficiently poor quality that it does not constitute a comparable spare. The order was only for single filters, however, so the second filters are a bonus in any case.