NavList:

I have used both types of mirrors and much depends on the telescope I'm using.  When using a lower powered scope (say 4x and less) I much prefer the whole horizon mirror.  I am generally annoyed by the body disappearing as I am rocking it and find it much easier to find the nadir with a whole-horizon mirror.  I also find it easier to find the stars on the whole horizon mirror.  As mentioned, there is trouble with dimmer horizons, as there is a loss of light and contrast, but I've never had a problem getting a round of sights in with the whole-horizon mirror.

 

When I use my 7x scope, the effect is that the half-horizon mirror acts like a whole horizon mirror with none of the light loss.  I find this an idea solution.  This is the combination I use on the ship.  I have tried with the lower powered scope on the same sextant and have not been as happy.

 

As usual, choosing a sextant configuration is mostly a matter of the circumstances you are going use it as well as preference.  Since I am using it on a large ship that is relatively stable, I can get away with high powered scopes and lower light transmission of whole horizon mirrors.  The needs for small boat or land based sextant use are far different.

 

Jeremy

 

John reminds me of my first experience with the whole horizon sextant
mirror, which I called the half silvered mirror, was that finding the
horizon in dim light was difficult. I found no advantages, in that the
sun and moon images can be seen in the clear part of the mirror, aiding
in bringing them down.

Now, if someone would make a horizon mirror that was half silvered on
only the left side, that might be worth something. But I might still have
the same problem at twilight, where I need all the light available to see
the horizon.

I'm sure that the article mentioned noted that half silvered mirrors (one
way glass) allow only a portion of the light to pass. When we remodeled
our bath, we had one installed in our shower for privacy. The room was
significantly darkened, and plants wouldn't grow. The amount of light
reduction is significant.

Jim Wilson
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