Ah. That clears a lot:
The ring is that of a green photographic filter, made of green or yellow glass by Schott (GG3 = "Grünglas3" or "Gelbglas3"). All of these filter glasses are long passes, meaning they block short wavelengths.
As for the serial number with that Zeiss logo:
This can have come from anywhere. But it looks quite old. This type of label was in use up to the 1950s at most. And you can be quite certain that no US military installation would have been able to buy anything from Zeiss/Jena after 1948.
Unfortunately, knowing the serial number by itself is not very useful: Zeiss always had some hundred specialist departments making anything from spectacles to whole observatories, and every single department had their own system of serial numbers. There are, for any given serial number literally dozens of parts bearing that same number.
(As a small historical side-note: The main problem, however, for pre-war and WWII documents by Zeiss is the treaty of Yalta:
Eisenhower and Patton retreated some hundred kilometers in April 1945 to allow the Soviets to enter Berlin and to occupy eastern Germany. This meant that Jena was first occupied by the Americans, then, after only a few weeks they left again and the Soviets captured the city. Before leaving, the Americans took with them whatever documents they could grab, and also some specialists from the Zeiss works. This means that the documents of the period are very much in disarray.)
But now for the important bit: this really is a catadioptric objective, no matter where it came from.
Now, you can play around with it: try finding the focus plane, first.
To start with the basics:
1) you cannot find an image through this with a camera when you have an objective lens attached: You would be looking through two camera objectives set behind each other.
2) We do assume at this point that the objective lens is complete, viz. there are no missing parts which would contribute to image formation in any crucial way. This may be incorrect, but it's the only thing we can do at this point.
-> Just start out like this: build a small translucent screen: Make a small frame of cardboard strips, say 3/4 inches wide strips and connect them to form a square with an internal size of about 3" x 3". Onto this frame you glue some tracing paper (or sandwich paper).
-> Place the objective lens on a table, so that it faces a bright scene like a street or a garden in day time. Most elements of this scene should best be at least some 30 ft away.
-> Now, move the screen backward and forward behind the objective untill you find an image of any kind.
-> Then measure the distance between the screen and the back of the lens.
-> Build a rig which will hold your camera at this distance from the lens.
To this day, camera housings have a mark to indicate where the sensor is in the housing: a circle with a long line bisecting it. This mark has to sit at the same distance from the back of the system you tested as your screen did. Then the chip will be in the vicinity of the focus.
However, as this will be a very fast objective, be prepared for the focus to be a very narrow region along the optical axis.
3) The easiest way to measure the focal length is to take an object that has about half a degree of angular size and measure how big its image is. The focal length would then be 109 times that value.
For example, the moon or the sun have this angular size. Say, the image of the full moon, as produced by your optics is 20 mm in diameter. Then the focal length of the instrument producing this image would be 109 x 20 mm = 2180 mm.
This will give you both an idea of how large the field is and of the scale of the images.
Edited by Eikonal, 31 October 2017 - 12:49 PM.