J. Oberling
At Betsy's request, I measured the pitch and the vertical position of the ETMy as a check of where the ETM is sitting with respect to the ideal vertical position (1742mm from the ISI optics table). Pitch was measured to ensure a gross pitch error was not causing the appearance of the ETMy sitting in the wrong vertical position.
To measure the vertical position I used the total station to measure the elevation angles required to sight the top and bottom edges of the ETMy. The tangent of the angle is multiplied by the distance from the total station to the optic to give the vertical distance from the ideal center, as established by the total station, to the respective edge of the ETMy. To get the distance to the ETMy I used the total station's Electronc Distance Measurement (EDM) feature, which has the ability to measure a distance without using a corner cube prism (the reflectorless EDM has a 3mm error bar). I sighted the right side of the quad cage, in the recessed portion and measured the distance; this distance was then multiplied by the cosine of the azimuth angle to sight the quad cage to give the distance from the total station to the ETMy. This is not ideal and does introduce error into the vertical position measurement, not only due to the 3mm error bar of the reflectorless EDM but also due to the fact that the HR surface of the ETMy does not align to the recessed surface of the quad cage but is actually several mm behind it (I assume the ETMy HR surface is within 10mm of the recessed surface), but will give a general idea of where the ETMy is with respect to the ideal vertical position (when measuring a distance of ~7m, a change of 10mm in either direction only results in a change of the vertical distance from ideal center to ETMy edge of ~0.3mm). The distance from the ideal center to the top and bottom edges should be 170mm (the radius of the ETMy).
- Pitch: 2.02 mrad up
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EDM distance: 6921mm
- T.S. to ETMy distance: 6918.3mm
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Vertical position
- Ideal center to top edge: 172.9mm
- Ideal center to bottom edge: 167.1mm
- ETM vertical position: 2.9mm high
The result here is that the measured pitch of 2.02 mrad is not enough to cause the ETM to appear to be sitting ~2.9mm too high in the quad assembly, therefore the ETMy is actually sitting ~2.9mm too high in the quad assembly.