Measure the Eye Relief of a Sextant Scope

The question came up today of the eye relief of the 7 x 35 monocular we sell for taking sun sights with more precision—it is also valuable for more accurate index correction measurements. Eye relief is the
distance from the eye to the front face of the lens in the sextant eye piece. With the eye at that distance from the lens, we see the full view of the telescope without distortion. Closer or farther offers some distortion around the fringe of the view. An adequate eye relief is valuable for mariners who must wear glasses when taking a sight. 

In principle the manufacturers of the instrument should provide this spec,  but we notice that most do not for sextant scopes. Therefore we looked into the procedure for measuring this and report it here.  If the published procedure is correct, then this is a fairly easy measurement that can be made to within ± a couple mm.

Procedure: 

1) Find the exit pupil diameter by dividing the objective lens diameter by the magnification. We have a 7 x 35 scope, so this is 35/7 = 5 mm exit pupil.

2) draw a circle of this diameter on a paper and shine a light into the objective lens to view it on the paper below, as shown.

There is some distortion in this image which looks like 5.5 mm but it was drawn with needle tip dividers to more exactly 5.

Here a light shines down through the scope making a bright pattern, then the scope is moved up and down until the image just matches the exit pupil.

Here is the view just before alignment. The scope has to go down slightly to make the light pattern match the exit pupil diameter ring.

Once the light pattern is aligned with the ring, we measure the distance from eye piece to paper

This turned out to be just under 10 mm. 

Next we measure the depth of the lens inside the eye piece using two crossed tongue depressors.

This is very close to 7 mm.

Thus the total distance from eye piece lens to eye is 10 = 7 = 17 mm, which is the eye relief of this instrument.

In short when using this monocular, you would want the surface of your eye to be about 1 cm away from the lip of the eye piece, which seems about what it is when using this scope, which typically calls for pressing the eye against a large eyecup placed on the eyepiece.  At sea we need that extra point of stability plus no light in from the side.

This might be as good a place as any to note that we have known mariners to make a custom set of glasses for taking sights. They remove the lens and frame on the sextant eye so it can press up against the cut and leave a lens on the other side for seeing around them.