Unkown KM "optical" item

[sturmann]

Hallo,

Can you help me ID this Kriegsmarine piece that I have been offered please ?












The big base is made of frosted glass.
When looking though the occular one can see a cross graduation grid.
It is quite bulky : 24 cm long with a 25 cm diameter base weighting 6 Kg.

Many thanks for your help,

[sturmann]

[globbitz]

Hello sturmann,

Thank you for sending a link to this curious instrument to the Optics forum.

I'm unable to identify the instrument I'm afraid, but I'd suggest that the Tessar lens isn't an ocular but the objective, and that the purpose of the instrument is to display an image onto the ground glass screen for viewing or calibration purposes - much the same as an early plate camera will use a ground glass screen for forming the image for the photographer to view (although upside-down).

If you put the instrument into a darkened room with the Objective lens pointing outside (or set the instrument outside and cover yourself and the back of the instrument with a dark cover or blanket much the same as an early photographer), I'd imagine that a perfect image will be formed on the glass screen with a superimposed graticule? Is the image upside-down? Is the distance from objective lens to the screen the same as the focal length marked on the lens (21cm)? Does the lens have it's own focus adjustment via the knurled ring, or is that the purpose of the micrometer axial adjuster?

By the use of the bubble levels and perhaps the micrometer axial adjustment (if that's not the mechanism to achieve perfect focus) then I'd suggest that the purpose of the instrument is to ensure that the image displayed exactly matches the graticule's lines, and hence aid with the setting of the optical instrument being calibrated by matching or aligning the graticules / crosshairs. As to what that other optical instrument is, I can only guess that it would be a telescope (or half a pair of binoculars), or an optical sight for a weapon or rangefinder?

Through a bit of practical experimentation you might find that the operation of the instrument is exactly the reverse to that as suggested above - whereby the ground glass screen collects light which is then condensed and passd through the Tessar lens into the objective of the optical instrument being calibrated so that you would use that instrument, rangefinder, or telescopic sight in the normal manner through it's own oculars?

The use of bubble-levels does suggest to me that this intrument isn't for shipboard use, but for use in a static environment (a laboratory or optical lab) on land? I'm sure that an optical repair technician would recognise the instrument's function.

Any other ideas or observations?

Best regards,
Paul

[JAndrew]

Karl Zeiss is the manufacturer, Jena is the city where manufactured, Tessar refers to a 4 lens system that was patented by Zeiss. I believe this was used to align the
reticles of binocular range finders so that when to left and right images converged one side was not rotated relative to the other.
JA

[sturmann]

Paul / JA,

Many thanks for both of you for those explainations !!
I am begining to understand a little more about this object

[Altes_Glas]

The spirit levels all around it and the micrometer head suggest to me part of an optical comparator, but one in which this viewing screen was mounted absolutely vertical. Why this alignment should have been so critical I don't know, often comparators have this sort of screen mounted at about a 20° to 30° angle for the convenience of the user sitting in front of the machine.

[sturmann]

re-

Many thanks for your vieews . Silly question but what is a optical comparator exactly ?