Another Sounding Sextant

17 11 2018

A little while ago I acquired yet another sounding or survey sextant for a relatively small sum. It is based on a Cassens and Plath nautical sextant. As with most sounding sextants, it has no shades, but where the index shades would normally be mounted is a leg and where the horizon shades would be mounted is a bracket for a pentagonal prism or “penta prism”.

Over two hundred C&P surveying sextants were obtained by the US Coast Guard Service from Weems and Plath around 1978, provided with a handle that would make holding the instrument horizontally easier and stripped of the lighting system, to save unnecessary weight. In my instrument, which bore a USGCS label, the lighting system is intact and there is no provision for a modified handle.

Figure 1 shows the state of the instrument as received and it had plainly not been well loved in the autumn of its life (by the way, the hand holding it is not mine).

As found labelled

Figure 1: C & P Sounding sextant as found.

My usual method is to strip the sextant down to the last screw and washer and then to clean and repaint everything, stripping all the old paint off if necessary. As I go, I fix electrical faults, renew wiring, replace mirrors , clean optics, and re-grease moving parts. As I have elsewhere in the blog described these activities, I will not go into them here, but instead focus on the main point of difference from other sounding sextants: the pentaprism. I have given a very brief account of the use of sounding sextants in the post for 26 April, 2009, and this should be read in conjunction with the comment kindly sent by Peter Catterall.

In a pentaprism, the emerging ray is at right angles to the incident ray, and the angle between the two rays (really two parts of one ray) is independent of any rotation of the prism about an axis parallel to any of its faces.  The image is not inverted or reverted. However, if the prism is rotated about another axis, the incident and emergent rays will not be at a right angle. Although there are two internal reflections in a pentaprism, they are not total internal reflections as, say, in a 90 degree Porro prism, and so the reflecting surfaces have to be silvered. If the paint film and underlying silvering gets damaged, the damage will be apparent in the view through the prism.


Figure 2: Position of the pentaprism.

Figure 2 shows the location of the pentaprism behind the clear glass of the horizon mirror. It is located in a spring-loaded bayonet socket by means of a peg, which allows it to be placed in two positions (Figure 3). Rotating it anticlockwise locates it in the position shown in Figure 4.

Pentaprism base

Figure 3: Pentaprism base

When located in this position, if the index arm is set at 90 degrees, the two light paths should be parallel, so that a distant vertical object should form a continuous vertical, straight line when the instrument is held with the frame horizontal. For this to happen the faces of the prism must be at a right angle to the frame of the instrument, so three adjustment screws are provided to bring this about. It is a great deal easier to do this if a 2 mm diameter torus of thin, soft copper wire is placed centrally under the face opposite the adjusting screws, so as to allow a little rocking  to take place. This is a little simpler than following the official advice promulgated in the US Coast Guard Service manual, available on line here:  The prism is held in place by two rectangular “springs” which offer quite a lot of resistance to the movement of the prism when adjusting it, so it is easier simply to leave them a little proud of the prism faces and rock the prism as I have suggested. The adjusting screws then do double duty of adjusting and retaining with the springs as back-up retainers.

Any index error of the sextant must of course be allowed for, in addition to any error found with the prism in place, and normal checks for perpendicularity of the index mirror and side error made and corrected.


Figure 4: Position of prism to check index error.

Figure 5 shows the prism in its orientation in normal use and you can see that with the index arm set at 30 degrees, the rays diverge by 90 + 30 = 120 degrees, the practical limit for a normal sextant, where the reflected image is reduced to a narrow slot.

30 degrees

Figure 5: 90 + 30 degrees = 120 degrees.

Figure 6 shows how 180 degrees can be measured by setting the index arm to 90 degrees. The ability to measure large obtuse angles improves the strength of position lines when fixing the position of aids to navigation.

90 degrees

Figure 6: 90 + 90 degrees = 180 degrees.

Although the stout case could be mistaken for solid wood, it is in fact some sort of laminated wood, as witnessed by the delamination of the outer layers of the top and bottom. It seems strange that an instrument destined for use in a damp and sometimes wet atmosphere should not at least use marine grade laminates for its case. The corners are keyed mitre joints which give both a very neat appearance and very adequate strength. Note that the key should be sited as close to the inside angle as feasible, as shown in Figure 7.

Case corner

Figure 7: Keyed mitre joint in case.

Figure 8 shows the instrument less its telescope in its case. It cannot be stowed with the pentaprism in place, though with a little more thought, the pocket for the sextant handle could have been rotated anti-clockwise and moved to the left a little to give room for both the prism and the originally supplied prismatic monocular.

In case

Figure 8: Instrument in its case

The telescope with my sextant is a standard 4 x 40 C and P offering except that there is a glass in front of the objective lens that acts as an astigmatiser with stars, drawing out the point sources into lines (Figure 9). It has no effect on extended images. I cannot imagine how this works, so if any reader knows, I should be glad if they would share their knowledge with me.

I hope to be able to add two or three more posts to this blog before the end of the year, after which it will the turn of a marine chronometer to be described on my other web site, .






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