Apollo Sextant Beam Splitter

MIT's Role in Project Apollo Volume 2 Optical Subsystem

Apollo Sextant Beam Splitter

For onboard navigation to and from the Moon, a sextant was used by Apollo astronauts to measure the angle between two stars, or between a star and a planet landmark or horizon. The sextant had two lines of sight (LOS) - one fixed, and the other movable via a tiltable mirror on a trunnion axis, and the rotating sextant shaft.

The fixed LOS, called the "LLOS" was pointed at the star and the target horizon or landmark by changing the attitude of the spacecraft. The star LOS, called the "SLOS" was captured by rotating the Shaft of the telescope to align the sextant reticle vertical Reference Line with the star and the center of the Earth or Moon. Next, the horizontal Measurement Line of the reticle was aligned tangent to the body's horizon. Finally, the mirror was rotated on its trunnion axis to move the star into superimposition of the body horizon. At this time a Mark button is pushed to record the shaft and trunnion angles, as well as the time. With this data, the computer was able to update the spacecraft State Vector.

The horizon of the selected body is significantly brighter than the star. Therefore, while moving it into superimposition over the horizon, the star was washed out and become extremely difficult to align precisely. To compensate for this effect, prior to reaching the objective lens, the LLOS passes through an octagonally-shaped Plate-Type Beam Splitter/Combiner, mounted at a 45°angle, which reduces the light to 11% of its original brightness. The SLOS, however, does not pass through the Beam Splitter. Instead, it is directed around the beam splitter by the movable mirror and two fixed mirrors. Next, the light is reflected off the rear side of the Beam Splitter and into the Objective Lens. Thus, only a slight reduction in star brightness occurs.

  1. What sort of magic "combines" the two LOS, and under the above conditions, what precisely does "combine" mean?

  2. Why plate-type, rather than the typical prism shape? (because it combines rather than splits?)

  3. Any reason its octagonal, rather than simply round?

  4. 45-deg mount to reflect SLOS off the rear side, any other reason?

  5. Is the entire rear side "highly reflective"? This would mean that the LLOS passes through the reflective side. Does this create any adverse issues?

Any answers to the above questions, or additional information that be useful will be greatly appreciated.

  • 1
    $\begingroup$ These are good questions, but they are not about the physics of the operation of the device - they are about the engineering of its design. For most (if not all) of the choices involved, the alternatives you mention are perfectly reasonable, but there will be other constraints in the equipment that are not documented in the passage (or the choice is arbitrary). You may find a better response on the Space Exploration site. $\endgroup$ Oct 22, 2022 at 9:21

1 Answer 1


A beam splitter (aka beam combiner) is a partially reflective mirror, typically mounted at $45^o$ as you show. Some of the beam is transmitted through.

In most cases, the mirror coating is designed to reflect $50\%$ and transmit $50\%$ of the beam. A single beam is split into two parts of equal intensity.

In this case, it is $89\%$ reflective and $11\%$ transmissive. The mirror coating is only on the back side. The front side is either just glass, or it may have an anti-reflection coating on it.

      1. The LLOS and SLOS beam approach the beamsplitter from opposite sides. Mirrors have been arranged so the beams form an angle of $90^o$. $11\%$ of the LLOS beam emerges at the same angle as it entered. $89\%$ of the SLOS beam is reflected $90^o$ and becomes parallel to the LLOS beam. The two beams now have been combined into a single beam.
  1. A prism is a different optical component. It deflects each color through a different angle, separating the colors. The wedge shape is needed for that. A beam splitter is just a special mirror.

  2. The octagonal shape is not important for the function of the beam splitter. Spacecraft pay a lot of attention to weight. It may be the minimum mirror that was needed to intercept the beam.

Additional information Beam Splitters


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.