E. Order-sorting filters. The filters for the spectrograph is placed before or after the slit. In order to avoid a shift in focus when changing filters, the optical thickness of each filter was specified to be fixed. Therefore if the fused silica filter is 5 mm, then the Si substrate is 2.1 mm etc. Free of pinhole defects.
Single substrate is greatly preferred.
If in two pieces, no cement should be used and combined filter must meet tolerances. The adopted optical thickness can be adjusted if it helps in manufacture or in meeting specs, provided all filters are not thicker than specified and all filters can be handled with reasonable precautions. F. Vendor information.