NavList:

Randall,

A sight tube is a good addition to your sextant box. There might be more room for stowing the sight tube under the boxed sextant by the sextant handle. One piece of bubble wrap will prevent clatter.

Pros:

1. Large field of view.
2. No lens fogging.
3. Quicker observation times.

Cons:

1. Ability to focus precisely.
2. Star/planet observations during early twilight or daytime very difficult.
3. Index error determination less accurate.
4. A distant horizon is harder to see.

A sight tube may also be preferred for observing horizontal sextant angles when piloting or checking position while at anchor. An old ruined scope can have a second life as a sight tube by removing the lenses.

Can you post an image of your sextant box arrangement ?

Greg Rudzinski

Open sight tubes
From: Randall Morrow
Date: 2013 May 6, 15:35 -0700
I hope this is not a repeat posting. I thought I had pushed “send” but since it has not appeared, here it is again.
Having only taken sights from a sailboat once, and on a clear calm day at that, I found the mirrors and lenses quickly fogged with spray and mist. Even my own glasses were fogged by mist. This does make sight taking on a good day more difficult and I can only imagine what it would be like on a stormy day. One writer suggested an open sight tube, both for the lack of glass to wipe off and the wide angle of view. I recently bought an open sight tube for my Astra IIIB to see how it performed, and have some comments to post below.
Theses sun sights were taken 5 May 2013, IC (-) 0.1, DST ZD (+)7, watch error zero, artificial horizon used so divide all Hs by 2.

1) The results were very good for sun sights, comparing favorably with a 3X telescope.
10-57-37 Hs 115* 45.0 a=0.8
11-00-57 Hs 116* 53.4 a=0.8
11-06-23 Hs 118* 47.6 a=0.9
11-13-56 Hs 121* 24.0 a=0.3

2) The lack of magnification makes the entire index mirror and frame visible. When I got past 120 degrees on the arc the very acute angle of the index mirror made viewing tangency very difficult. The last two in this group I had much trouble seeing tangency. This would not be a problem at seat where sights will likely be less than 90 degrees.
11-16-57 Hs 122* 17.4 a=4.0
11-17-50 Hs 122* 40.8 a=1.1

3) At night only planets and the brightest first magnitude stars were visible and usable for my 57 year old eyes and glasses.

4) The wide angle of view makes finding bodies in the sky a bit easier.

5) The plastic was shiny inside the tube and the internal reflections affected viewing. Gently sanding the inside of the tube with 300 grit emery paper eliminated these reflections

6) All three scopes fit nicely into the case for the Astra. (tube, 3X & 7X) But I had to use some conical black plastic plumbing washers to raise the 7X up from the bottom of the box. There are 2 scope attachment screws and one scope would be on the instrument of course. The tube sticks up quite high in the box but the lid still closes nicely.

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