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From: Ken Muldrew (no email)
Date: Tue May 04 2004 - 12:28:27 EDT
On 2 May 2004 at 8:02, Jim Thompson wrote:
> The thread on the AstraIIIb's split mirror was very helpful, but did
> not thoroughly review the subject of sextant optics, and raised more
> questions in my mind. We have not yet thoroughly described issues of
> depth of field, field of view, magnification, and glass and lens
> manufacture with respect to modern sextants.
>
> I have been prowling around the Internet looking for online
> information regarding sextant optics to fill in the gaps. My guess is
> that these are closely guarded industrial secrets, or that there is
> information buried in specialized print publication that has not made
> its way to the Internet yet.
Since nobody has stepped forward with an informed opinion, I'll add
an uninformed one to perhaps goad the optics experts into posting.
I can't see sextant telescopes being much of a secret. The kind of
optics that are needed can be supplied with pretty old technology.
There's basically 3 types that one might choose. A Galilean scope
(otherwise known as "opera glasses") is the simplest design to give
an upright and left/right corrected image. This design uses a
positive objective lens (biconvex) and a negative eyepiece lens
(biconcave). I think these were used in old sextants but I don't know
if they're still used. The next type is an astronomical scope that
uses an achromatic objective and a great variety of different
eyepiece designs, but both are positive lenses overall. This type of
scope reverses left/right and up/down. The third type is an
astronomical scope with either a roof prism or a porro prism stuck in
the middle to bring the image back to normal left/right, up/down
orientation.
Magnification is given by the focal length of the objective divided
by the focal length of the eyepiece. In general, the greater the
magnification, the less the field of view, the lower the brightness
of the image, and the smaller the depth of field (the range of
distances that are in focus). Higher magnification must be
accompanied by a larger objective lens in order to allow the image to
be resolved, but this is unlikely to be an issue with the low
magnification used in a sextant. Also, since the focus is set at
infinity, one would think that depth of field wouldn't be an issue
either, but as we saw in the split mirror thread, having the mirror
well outside the depth of field actually makes the sextant easier to
use. The larger the objective lens diameter, the greater this effect,
as the lens can gather light from the edge of the scope and bring it
into the observer's eye. A larger lens also gives a brighter image;
good for stars but a nuisance for the sun.
The (well) used sextant that I bought didn't have a scope so I built
a simple one using inexpensive optics from Surplus Shed, an exhaust
pipe reducer, a right-angle bracket, and some 3/4" aluminum channel.
The optics that I used can be found here:
The objective lens:
http://www.surplusshed.com/pages/item/l2190d.html
The eyepiece:
http://www.surplusshed.com/pages/item/l2086.html
The roof prism:
http://www.surplusshed.com/pages/item/l2080.html
I was going to use a better eyepiece but this isn't a very good roof
prism and it limits the field of view too much for anything but a
Huygenian design. I get about a 3° field of view with this 5x scope
and it seems to work fine. A better prism would give brighter images
but this isn't an issue with any of the lunar stars I've used so far.
For use at sea, one would want a scope that's weatherproof. This
would be trickier for the hobbyist.
Ken Muldrew.
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