I have seen these referred to several times, but I have never seen one. I have no doubt they exist, but possible fell out of favor? What manufacturer originated them?
I have seen other surveyors misquote the diameter of the prism glass (as often quoted on SECO's website) as the constant. I have seem 4omm and 25mm (mini) diamter prisms, both with a constant of -30mm. Not saying that's what I thought was going on, though.
My K&E MicroRanger had -40's back in the late 70's.
I have always associated them with Sokkia and my understanding is that -40 and -17.5 are the 2 offsets that give the least pointing errors due to the prism not pointing directly at the gun. I'm not sure what is magical about those numbers and would like to know. I also read that -30 became popular mainly because you could reverse it and get 0.0, so you sacrificed some pointing forgiveness for more versatility. Again, I would like to know if that is actually true
Because they are "nodal" meaning the center of the prism is not relative to pointing.
Apparently nodality is a function of offset and diameter, because a -17.5mm mini is nodal:
And a 60mm diameter is nodal with a -40mm constant (see Second link above).
I seem to remember seeing some -35mm nodal prisms once, too.
Mike Copeland of SECO gives a very good explanation in the video from the link above
Thanks, I always forget they have these videos. For a solo operation, a set of nodal mini prisms in a traverse kit would be the berries!
Plumb Bill
Omni makes a 38mm Super Jr. prism
It's not nodal, either -30mm or 0mm offsets.
I wish someone would make a prism holder that
would make it nodal.
Hard to find but pretty nice prism for medium distance
work. Sometimes 25.4 mm is too small and 62mm is a little
to much. Kind of a good all around prism for small sites.
:good:
Thanks, I've never seen one before.
Back in 1993 or so, Pacific Lasers, Inc., of Tualatin, OR and Redmond, WA, was visited by an optical engineer who worked nearby in the same industrial park in Tualatin. One of the guys, I think it was Randy Stuart, showed this guy around, and when shown the prisms and after asking a bunch of questions, he was heard to say, "well that's not right!". They did some tests that consisted of shining a visible beam laser in to a standard -30mm prism, turning the prism from side to side and watching the beam move from side to side on its return beam. That led to this engineer, in conjunction with PLINC, to develop the Eclipse Prism system. It was a really nice prism, didn't break like the cheap Topcon and Seco stuff, and it was a true Nodal Point prism. The problem was, it was over $200, and it meant you'd have to change all your prisms over to the -40mm units. It didn't sell all that well, but for some reason Seco bought the whole operation, and still sells it under the Eclipse name ( http://surveying.com/Products/Prisms-and-Assemblies/Front-Locking-62-mm-Prisms-and-Assemblies/6400-00). Maybe Randy or one of the other former PLINC guys (Jeff Whitaker?) could shed some more light on it, but that's how I remember it!...
DUH moment
Through reading this discussion, and doing a bit of experimentation, I've just come to the realization that unless a prism is nodal, one must NOT use the nodal point for aiming at the prism, but should instead use the target placard "V"'s around the prism! Even a slight mis-aiming of the prism towards the observer will have a significant effect on the observation (more so at closer distances).
DUH moment
rfc
Exactly
Many people make the mistake of aiming at the "cross hairs" in the glass
when they should be aiming at the target and forgetting the center of the prism.
Add Robotics and in close staking of concrete and steel and you had better be
using a nodal prism. Best to use a 17.5mm nodal mini prism for in close stakeout.
I started out with an top-mounted Autoranger in 1983. We had three mirrors. a -30mm single, a -30mm triple (triangle stack) and a -40mm triple vertical stack. the prisms were square and it was a HEAVY hunk to haul around....
