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Operation of the Apollo Sextant Beam Splitter

The Apollo optical Sextant is used to update the vehicles State Vector. It is capable of sighting two celestial targets simultaneously and measuring the angle between them. One Line of Sight (LOS), called the Landmark Line of Sight (LLOS) is fixed along the Sextant shaft axis normal to the local conical surface of the Command Module. Therefore, the spacecraft attitude must be changed to align the Sextant Reticle with a selected star and the horizon of either the Earth, or the moon. The other line of sight, called the Star LOS (SLOS) uses an Indexing Mirror with two degrees of rotational freedom about its shaft and trunnion axes. Once the spacecraft LLOS has been aligned with the horizon and the star, the articulating SLOS Indexing Mirror is used to superimpose the star image onto the horizon. Light from the LLOS passes through a Beam Splitter reducing its full brightness to only 11% of the reflected light from the horizon. This is necessary due to the much dimmer light from the star which would otherwise be washed out by the brighter horizon at superimposition.
Light from the SLOS does NOT pass THROUGH the Beam Splitter. Instead, star light from the Indexing Mirror is reflected by two 90-degree mirrors onto the rear surface of the Beam Splitter, combined with the light from the LLOS, and directed into the Objective Lens Assembly.

A typical Beam Splitter is composed of two prisms fastened together at their bases using a synthetic resin which results in a box shape. These joined prisms separate a beam of light into two or more components. However, this "Beam Splitter" is depicted as a thin flat rectangular plate. Presumably, this particular "Beam Splitter" is not prism-shaped because it does not actually "split" a single beam. I'm unable to find a good description of how this Beam Splitter is constructed (e.g. is the entire rear side of the Splitter highly reflective or only a section?), the precise path of the LLOS through the Splitter, and exactly how the two Lines of Sight are combined. Any additional information regarding this "Beam Splitter" would be greatly appreciated.

Sextant line drawing

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    $\begingroup$ Per SE rules you should credit the image you've used. Have you already read the Apollo Optical Subsystem and LM Alignment Optical Telescope reports? I'm only skimming but at times (e.g. Vol 2, Part 2, pg 5-8) it's not even called a beam splitter, just a "fixed semi-transparent plate." Haven't found a spec for it yet. $\endgroup$
    – Erin Anne
    Commented Oct 13, 2022 at 0:49
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    $\begingroup$ I might've found a part number though, OCLI and Photronics (Beamsplitter) PN 2012222. Same report, Vol2 Part 2, Table 6-20 (page 6-59) $\endgroup$
    – Erin Anne
    Commented Oct 13, 2022 at 0:53
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    $\begingroup$ Most simple beamsplitters are also beam-combiners. The most familiar of which is that thing sitting in the middle of a classical Michelson interferometer where it is doing both. $\endgroup$
    – uhoh
    Commented Oct 13, 2022 at 1:18
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    $\begingroup$ is there still any confusion about how the SLOS and LLOS are combined? I feel like the diagram in the question (which again, where is that from?) makes it totally clear $\endgroup$
    – Erin Anne
    Commented Oct 25, 2022 at 5:05

2 Answers 2

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This is only partial and might put you on the right path:

https://www.edmundoptics.com/knowledge-center/application-notes/optics/what-are-beamsplitters/

enter image description here

So, you are right in the construction of a typical beam splitter, but you are probably looking for a plate beam splitter as depicted here:

a thin, flat glass plate that has been coated on the first surface of the substrate. Most plate beam splitters feature an anti-reflection coating on the second surface to remove unwanted Fresnel reflections.

Additionally, https://ntrs.nasa.gov/api/citations/19700015844/downloads/19700015844.pdf states that:

Overall light losses in the sextant are such that approximately 2 percent of the impinging light is transmitted along path A (fixed LOS landmark target) while approximately 38.5 percent of the light is transmitted along path R (movable mirror star target) . The significant difference is due to the 80 percent reflectivity of the beam splitter surface. Further variation in landmark intensity is provided by use of a polaroid filter attached to the beam splitter.

Below shows an octagonal plate (with polarizer)

enter image description here

Polarizing beam splitters are designed to split light into reflected S-polarized and transmitted P-polarized beams. They can be used to split unpolarized light at a 50/50 ratio, or for polarization separation applications such as optical isolation

enter image description here

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The above comes from : Apollo On-board Guidance History (Part 16, Apollo Control Systems)

https://dodlithr.blogspot.com/2018/04/apollo-on-board-guidance-history-part.html

A very interesting read.

Additional reading sources:

How did the automatic sextant on the late-1950's MIT Mars Probe work?

Could Apollo astronauts see other planets with the CM Scanning Telescope?

Was the Apollo sextant used (or tested) to determine position while in Earth orbit?

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    $\begingroup$ Hope my comment is not too polarizing! "Polarizing beam splitters are designed to split light into reflected S-polarized and transmitted P-polarized beams." That quote is not wrong as far as it goes, but actually it's extremely hard to make a beamsplitter that is not polarizing! You can find "non-polarizing beamsplitters" in Edmund Optics, Thorlabs and other catalogs, but if you actually look at their specifications they are of course still mildly polarizing. Once you break the symmetry, "all the king's horses and all the king's men" can't easily put the two polarizations back together again. $\endgroup$
    – uhoh
    Commented Oct 13, 2022 at 1:18
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    $\begingroup$ Also, see my comment under the question. $\endgroup$
    – uhoh
    Commented Oct 13, 2022 at 1:18
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    $\begingroup$ Thanks so much for your thorough answer! I've referenced it here. $\endgroup$
    – uhoh
    Commented Oct 24, 2022 at 23:47
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If this answer has value it's because it's standing on the shoulders of blobbymcblobby's giant answer! :-)

The question asks:

What was the function of the Apollo Sextant Beam Splitter?

and it seems to be inescapable based on all the diagrams in the question and blobby's answer that it combines two beams. Per my comment under the question:

Most simple beamsplitters are also beam-combiners. The most familiar of which is that thing sitting in the middle of a classical Michelson interferometer where it is doing both.


This image demonstrates a simple but typical Michelson interferometer.The bright yellow line indicates the path of the light.Photo was taken at the laboratory of physics of the Anhui Normal University

This image demonstrates a simple but typical Michelson interferometer. The bright yellow line indicates the path of the light. Photo was taken at the laboratory of physics of the Anhui Normal University.

A Michelson interferometer (image cropped from Wikimedia) uses a beam splitter to both split, and recombine beams of light. In this case it's a thick plate (like the one in the question) where -- according to the wiggly yellow lines drawn to indicate the light path -- the final exiting surface is where the splitting and recombining are happening. This recombination at the front surface seems similar to what's shown in the drawing in the question as well.

There are many ways to make beam splitters, and they all polarize to some extent (even the ones called "non-polarizing" which should really be called "not-so-polarizing")1 but per blobby's answer this polarization is critical to operation here, and so they've intentionally used a polarizing beam splitter to combine the two beams.

This way, a single, rotating linear polarizer (a bit like the little ones that used to come with Polaroid sunglasses to show that they polarize by rotating them against the glasses) located near the eyepiece can be a variable attenuator for the very bright landmark (Earth's limb?) so that light from a start can still be seen at the same time in the same eyepiece.


1So much so that it turns out if you search for "polarizing beam splitter" you get beam-splitting polarizer instead!

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