That looks like an older Geodimeter style prism. The edm is actually above the scope, not in the scope like the newer ones. For HR, you would measure to the target and not to the center of the prism.
But wouldn't the measured distance change when the prism is tilted?
It shouldn't and I don't think it does. I think the instrument software takes that into consideration. It's been over 15 years since I used one, so I don't remember everything about it.
If the target and prism are tilted up (say) to look directly at the gun that is higher, then the increased distance of the prism relative to the target axis is exactly what is needed to offset the decreased distance of the EDM (tilted down) relative to the instrument axis.
I agree with Bill. That looks like one for the Leica and/or Wild Heerbrugg for the edm that mounted on the top of the t2 telescope. I found a site with one for sale and a picture here:
http://www.amazon.com/CST-Berger-63-3010-Compatible-Assembly/dp/B000KAXOJO
I see said the blind man...
Yes, it's Leica/Wild, as stated elsewhere here.(Although the one in Adam's link is NOT Leica, as one may be able to see, the offset is different.)
They were used for the top-mount EDM's that would be fitted onto the instrument.
The vertical offset would compensate for the vertical difference between the optical line iof sight and the EDM line of sight.
And you can still use them, just have to shoot to the center of the prism and ignore all the yellow.
We've got a few dozen in the back room.....took out the glass and put them in the newer ones...saved a ton of $$$$.
I don't think you would want to use them without the top mounted EDM. Right?
> I don't think you would want to use them without the top mounted EDM. Right?
Not a great idea, but if necessary it can be done by aiming at the center of the prism. 😐