NavList:

What is the pentaprism doing as an add-on for a Freiberger surveying sextant? What it's most fundamental optical purpose? And if you wanted to experiment with the concept today, couldn't you make your own? You're not going to manufacture a pentaprism, of course, but wouldn't a fixed-angle sextant work just as well? Mount two mirrors in a frame inclined at an angle of 45° with respect to each other, and they will deflect light by 90° (doubling the angle between the mirrors, as sextants and other instruments of reflection always do!). Then put that frame in front of the horizon glass, maybe mounted on a separate tripod, and there you go. But possibly I'm missing something.

I made a fixed-angle sextant for demos some years ago. It's just two little mirrors mounted in a small piece of wood. I had lost my calibration of it. I tried a few nights ago and brought the reflected image of Vega over into contact with the direct view of Zubenelgenubi. Actually I just swung around for a minute until the Vega image was aligned with any reasonably bright star. That happened to be Zubenelgenubi. The angular distance between them is 76.0° so that's the calibration for this particular fixed-angle sextant.

Frank Reed

File: