I'll probably write another article on the star tracker itself. But I can give you a quick summary of the spiral search mechanism. It was electromechanical: a motor turned a resolver, a device with coils to generate sine and cosine from the shaft angle. This gives the X and Y deflections for a circle. These signals went through potentiometers that were also turned by the motor to produce constantly growing magnitudes, so you get a spiral. But you need to slow down the motor as you spiral outwards since you're covering a much larger linear region. So the motor also turns a stepping switch that progressively reduces its speed.

Once the system finds a star, a complicated feedback mechanism keeps it locked onto the star. There is a spinning slotted disk in front of the photomultiplier tube. If the star is off center, the output will peak when the slot lines up with the star. Thus there is an error signal with phase that indicates the direction to the star. This signal is demodulated to produce X and Y signals that change the aim to move towards the star.

I would absolutely love to read something about that - thanks for putting in the work and sharing it.

I have a buddy working on restoring a set of binoculars that were attached to the Target Bearing Transmitter system for a US sub from the 50s. Last I heard he was able to find someone that actually had parts of the original schematics for it so that he’s able to machine some new pieces.

These things are definitely a labor of love.

Am I right in thinking it didn't matter which star it locked onto, and it didn't need to know which star it was? Would it be a problem if it locked onto another celestial body (e.g. Venus)?

No, it needed to lock onto the right star, the one that matched the coordinates. Otherwise, it would be pointing in a random direction. The navigator would check against three different stars to detect an error.

The system could also use planets or even the sun for navigation. A special filter was used with the sun to avoid burning out the photomultiplier tube.

Ah, so it could be used in the daytime. I read the whole article assuming it was only useful at night. (When else would you be flying a bomber and need high accuracy?)

The SR-71’s star tracker was so sensitive it could track stars during the day.

https://theaviationgeekclub.com/the-sr-71-blackbird-astro-na...

being halfway to space probably helped

I would love to read a more detailed article on this device. I came back from reading the article with this exact question.