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Optical path lengths

The total optical path through the NPOI can be found by adding various contributions listed in the files npoi/stations.config, npoi/fdl.config, and npoi/6way.config. For the purpose of calculating delay constant term and optical path lengths, we break up the paths into segments of four types, associated with siderostat station, delay line, beam combiner number (ID), and beam combiner input number. It is best to make the break between station- and delay-line associated lengths at the intersection of the N and E arms. However, the west arm never reaches that point. We treat the feed pipes associated with delay lines 4, 5, and 6 as if they run to the N- and E-arm intersection. If they feed the W arm, the path length is reduced by 1.4455m, which is why D for the west arm stations is reduced by that amount.

In the file stations.config, D contains the path from the siderostat pivot point to the mirror at the intersection of the N and E arms, but excluding the vertical path in the elevator can which belongs to the delay line based path.

In file fdl.config, the paths do not include about 86m of path from the mirror at the intersection of the N and E arms through the feed system to the FDL entrance.

In file 6way.config, about 4m of path through the beam combiner are not included. That is to say that the delays in this file and the previous one are relative paths. This is a result from their having been created using the npoiconfig procedure from measurements of the white light fringe positions in different configurations.