NavList:

A Simple explanation of the Schick Mk5 Stadimeter
(To be read alongside the diagram in https://navlist.net/imgx/schick-stadimeter-3-instructions.jpg)

Based upon inputs from: Philippe Posth, Luc Van de Borre, Bill Lionheart, Martin Caminos, Alexandre Eremenko, Don Seltzer, Art Leung, Gary LaPook, Frank Reed, Adrian F, Murray Buckman, and David Pike.

1. In figure 1, point A is the centre of the telescope; point B is the centre of the horizon mirror; and point C is the centre of the index mirror.

2. If a light ray from C to B is to be reflected to A, the horizon mirror must be mounted such that its perpendicular bisector divides the angle ABC into two equal parts (fig 2).  I’ve used angle ABC = 30 degrees

3. Assuming we are looking a 50 ft high lighthouse (easier to draw than ships), If the view through the telescope is to show the base of the horizon mirror lighthouse and the base of the index mirror lighthouse level with each other, initially the index mirror must sit parallel to the horizon mirror (fig 3).

4. If we wish to bring the top of the index mirror lighthouse down to bottom of the horizon mirror lighthouse, we must tilt the index mirror a bit.  Suppose the lighthouse subtends an angle of 7 degrees at C, we must tilt the index mirror 3.5 degrees clockwise to ensure the ray from the top of the lighthouse ends up next to the bottom of the horizon mirror lighthouse when viewed through the telescope (fig 4).

5. We do this by turning the distance drum, which causes its axle to screw itself through the carrier and push up the Arcuate Arm. This causes the index mirror to swivel.  However, it should be noted that the drum should be marked such that as we screw in the drum raising the Arcuate Arm further, the distance values marked around the outside of the drum must reduce against the drum index, because the bigger the lighthouse appears the closer we must be to it.

6. If all lighthouses were the same height, we could set the drum against a single position of the Arcuate Arm.  Calibration would then be a matter of knowing the distance of the drum along the Arcuate Arm from C, and the pitch of the drum screw.

7. However, if we encounter a taller or smaller lighthouse, 100ft say, we must slide the drum around the Arcuate Arm so that the distance measured is not affected.  E.g. for a100ft lighthouse, the angle subtended by the top of the lighthouse would be doubled so we must tilt the index mirror twice as much.  We do this by halving the distance between the drum and C.  If you look at a photograph of the Mk5, the distance between C and height = 100ft is half the distance between C and height = 50ft, and the distance between C and height = 200ft is one quarter the distance between C and height = 50ft.  There must be a name in Mathematics for that.  Having decided on your initial height for calibrating the distance drum, 50ft say, calibrating the height scale on the quadrant is easy, just keep dividing by two.

8. The Arcuate Arm is curved so that the drum can swing on the Radius Arm enabling the drum position to be quickly set to the correct object height against the Height Scale. DaveP

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