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The Admiralty Manual of Navigation, 1938, vol. 1, has an appendix on the
sextant, containing these words, on page 390-

"There are, on the market, various sextant attachments of which the
following are examples-
Power 6 prism star monocle
Double star prism, for use with cloudy horizons.
Astigmatiser, foe elongating the image of a star."

I think that astigmatiser (or perhaps astigmatizer to Americans) is the
right word to use, and Greg might find it to work better when searching
than the stigmatizer that he referred to.

I have an English Vernier sextant, around 90 years old, which has in the
side of the box a slot containing a little "lens" that can fit over the
eyepiece. At a first glance, this looks like simple plain glass, but it has
a mark on the rim which is clearly there to show which way up it should go.
I have always taken it to be a star astigmitiser, though have never
bothered to try it out on a star. If it is, it has a very weak action,
because it has no discernable effect on viewing of ordinary objects.
Unfortunately, stars here are in short supply at present. Whether my old
eyes are good enough to tell the difference is a question.

I doubt whether spreading a star image into a short streak is intended to
avoid the need for rocking the sextant; but on the other hand, I'm unsure
what other benefit is intended.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Greg Rudzinski" 
To: 
Sent: Tuesday, September 21, 2010 8:42 PM
Subject: [NavList] Re: Benefits of Stigmatizing


Jeremy,

I have never seen a double prism on a sextant so it is good to hear your
first hand experience with this attachment. My original question though
pertains to an optical lens that stretches out a point of light into a line
which is perpendicular to the sextant frame. Modern sextants don't seem to
have this stigmatizing lens and I was wondering if this was to lower cost
or because there is no real benefit to the star observation. I too prefer
rocking the sextant to consistently determining the vertical.

Greg Rudzinski
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