NavList:

Dear Bill!

Here you discussed the mirror labelled “3” in Figure 1...

I suspect is not a mirror at all but a pentaprism.
...<snip>...
... inaccessible silvered surfaces of the pentaprism, or mirror, if it is a mirror, have deteriorated.

As an owner of the ИМС-3 I can assure that there is the pentaprism indeed, not a mirror. It is very unlikely the mirror was used in the SOLD KM2 because all the Soviet intermediate clones (the ИАС-1 and the ИАС-1М) have pentaprisms there.

While cloning the SOLD KM2 our engineers attempted several alterations they saw as improvements over the original:

• the capability to view the natural horizon through the bubble illumination path - an extra lens there with the same focal length as the lens #4
• lots of covers intended to protect most movable parts of the octant
• some of those covers had to be transparent (as they shield the main - observing - mirror)
• a jump-mirror to switch between the natural and artificial illumination of the bubble (no need to detouch/insert the diagonal mirror), a set of three shades (the milky-white, the red and the dark) rotates infront of the opening, with the ability to sidestep then all
• the rheostat is moved into the left-hand-side handle, near the main ON/OFF switch
• the surface (where the SOLD's rheostat was) has a plastic board for pencil notes, the metal wall beneath it has an opening, the integrator's lamp lights through the plastic making writing/reading notes more convenient

Not all of those alterations proved to be improvements though. The extra layer of glass creates unwanted reflections of the Sun, often getting into view of the proper Sun's reflection.

The ability to see the natural horizon is intended to check for the index error. If one uses the factory-supplied bubble level - both the horizon images perfectly coinside with each other AND with the center of the bubble - when the drum is set to zero. Because of the odd number of mirroring surfaces - the horizons cross if the octant is not perfectly vertical, and one may check if the bubble is in the center too then.

As I have all three of those clones - I have four bubble levels between them (my ИМС-3 came with the spare one). I tried to check if I may swap them around.

Nope!

For example the level from the ИАС-1 - when installed into the ИМС-3 - shows a VERY huge offset: 1°22' too low and ~6' left banking. In its' original octant this level works fine.

By the way, the #6 (Fig.1) in the ИМС-3 is not the mirror too but a 90°-prism (1cm-by-1cm). It has no means for tilt/skew adjustments.

My general impression of the octant is very positive. I can get fixes well within a mile off my GNSS position - taking the Sun and the Moon sights with it. I'm yet to provide the LED illumination into it, so I did not try stars so far. The main problem with LEDs - they are excessively bright, the built-in reostat is too low on resistance to make a LED shine dimly enough.

Thank you for your excellent site!

Warm regards,
Tony
60°N 30°E