NavList:

   

Reply

I thought I would chime in one last time on the subject of  
astigmatizers. I appears that there has been some question about  
whether astigmatizers were invented for bubble sextants or marine  
sextants. It is interesting that Bill Morris and I had this very same  
discussion as we were collaborating on the manuscript of his book The  
Nautical Sextant (used to be Naked Sextant). Bill thought the  
astigmatizer was primarily for bubble sextants, and I, that it was  
for marine sextants. As it turns out both of us were partly correct  
( I think).

On my behalf, the following history pertains to astigmatizers used on  
marine sextants. Between 1987 and 1993, C. plath offered an  
astigmatizer on their sextants.  Then apparently they ran out of  
them, and began offering an Opti-Level scope in 4x40 and 6x30  
versions which continued until 1999. Then they stopped making  
sextants altogether in 2002. The Opti-Level scope had a way of  
elongating a point of light from a star or the sun, but did not  
affect other viewing. These scopes were sold at about 25% more in  
price than the non-astigmatising scopes. Meanwhile, Cassens and Plath  
always offered an astigmatizer, even to this day.

As for bubble sextants, as far as I know, astigmatizers are only  
found on bubble sextants that also have prisms that allow them to be  
used as marine sextants as an option. But coincidentally, these same  
bubble sextants also had a bubble that was considered opaque. This  
meant that the bubble only contacted the upper part of the chamber,  
and not the bottom of it.  This meant that even though you could see  
through the bubble, the center part of it was not necessarily  
accurate, and it was recommended that the body be put alongside of  
the bubble instead of in its center. A strong case could be made that  
an astigmatizer would help cue the observer in bisecting the bubble  
symmetrically.

So in conclusion, it seems that astigmatizers have been used on both  
marine and some bubble sextants effectively in the past, although  
they have fallen into disuse lately.

I might add that Bill�s book (as published by Celestaire and Paradise  
Cay) came out beautifully, and can be found on those websites and  
Amazon.  All parties involved felt that regardless of financial  
returns, the book was so good that it simply HAD to be published.  I  
would note too, that with endorsements from George Huxtable (who also  
reviewed the manuscript), Frank Reed and John Karl, the book was a  
product essentially enabled by Nav List members all.

Ken Gebhart


On Sep 22, 2010, at 2:19 AM, George Huxtable wrote:

> 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
> ----------------------------------------------------------------
> NavList message boards and member settings: www.fer3.com/NavList
> Members may optionally receive posts by email.
> To cancel email delivery, send a message to NoMail[at]fer3.com
> ----------------------------------------------------------------
>
>
>
>

   

Reply