Infrared Optic f/0.7 with Aperture Iris Focusing Lens.
This optical assembly is complete. The only nomenclature present are a couple of hand-scribed numbers on the rear mounting flange. One number is the FSCM code 91766 (although the second digit looks more like a capital ‘Y’). The other number is probably a serial number – R1783-2. If you have access to a list of FSCM manufacturers, then you should be able to discover the manufacturer using the five-digit code..

The lenses appear to be made of germanium – they are entirely opaque to visible light. The mount is focusable through a range of about 4mm. The mount length is 69.7mm at its shortest. There is a graduated aperture ring with markings at f / 0.7, f / 1.0, f / 1.5, f / 2.2, f / 3.7, and C, which I assume means ‘closed’. The aperture setting ring would not turn past the 3.7 position so perhaps there is a lockout that is engaged. The lens mount has a slight taper with the largest diameter being 3.214 inches. The lens includes a metal thread-in protective cover for the front element.

Front element diameter is 66mm, rear element diameter is 47mm. Both external surfaces appear to be free of blemishes.

Photo set 1521.

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***MPN R1783-2
***OEM Cushcraft Corp
***SKU*** 21548