NavList:

Thanks Bill, figure 12 does show the pentaprism in two orientations; up and down.  I missed that.

Greg will want to use the down orientation, such that his range of measured angles is extended.  This is what Greg was after and the pentaprism in this orientation does that.   Nicolas agrees with the that orientation and purpose, explicitly.  In this orientation, the maximum angle is extend to 215°… with a minimum measured angle being 90°. 

Nicolas points out that he uses the up orientation to verify his index error at 90° (??).  He sets the index to 90° and with the pentaprism in the up orientation, he can see the same thing as a normal index check.   He is at a different position along the arc.  Was he attempting to determine arc error ala the Heath patent using prisms? That's not stated nor is any other purpose.  I'm not sure what to make of this but I never claimed to know everything!   Perhaps someone knows a purpose for this orientation and is willing to explain it.   In the meantime, I continue to see little or no utility in this orientation.  Reducing the maximum angle measured from 125° to 35° doesn't seem very useful.   Nor is an index correction at 90° as arc error will not be linear from 0° to 90° and what to extrapolate when over 90°?  {My actual thought is that he did it simply because he could.  There is no real purpose!}

I do wish Nicolas would rejoin us here on NavList.  It would be better if he would speak for himself.

Brad