C Plath Bubble Horizon Attachment

26 06 2012

I have placed this post in the Aircraft bubble sextant category because, although the C Plath bubble horizon instructions imply that it can be used at sea with a nautical sextant, this is not the case. Bubble horizons are of use on land and in the air, where accelerations either do not occur or occur in a well-understood way. However, accelerations at sea are too great and unpredictable for them to be useful, except perhaps on a very large vessel in calm waters or in ice where no horizon is visible. In 1960, the German Navy was supplied with these attachments as were US submarines. It would be unsurprising if they were never used on aircraft, as much better, dedicated aeronautical sextants had long been in use (see Comment by Dr Andreas Philipp).

This post is preceded by  “A gummed up AN5851-1 averager”, “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  ”Refilling Mark V/AN5851 bubble  chambers” ,  ”Overhaul of MkV/An5851 bubble chamber” ,  ”AN5851-1 : jammed shades carrousel” ,  ”A Byrd sextant restored” ,  ”Update on Byrd Aircraft Sextant”, “A nautical sextant bubble horizon” and “Sealing A10 vapour pressure bubble chambers.”

When C Plath resumed sextant production in the early 1950s, they applied their experience with the SOLD bubble sextant to designing an artificial horizon attachment for nautical sextants. They had been instrumental in producing a bubble sextant for Admiral Gago Coutinho of Portugal, who made several long over-water flights in the 1920s, but that sextant was an adaptation of a nautical sextant employing two linear spirit levels at right angles to each other, both of whose bubbles had simultaneously to be aligned with the observed body.  The SOLD sextant had a circular bubble chamber, adjustable in size, and contained in a rigid bronze casting. The bubble unit used in the attachment for the post-war nautical sextant was based around a bubble unit almost identical in design and execution to the SOLD units. Figure 1a shows a general view of the units, while Figure 1b shows the bubble chambers from above, with the SOLD-type units on the right. Figure 1 c, from a war time manual shows a slightly modified design that is clearly the same design as the post-war unit.

Figure 1a : General view of bubble units

Figure 1b : View of bubble chambers from above.

Figure 1 c : Drawing of bubble unit (modified from SOLD manual).

When the adjusting screw is rotated anti-clockwise, the bellows are compressed and the pressure in A rises. It is transmitted via the drilled passage 1 to the bubble chamber and, if the bubble lies correctly within the triangle etched on the glass at the front of the unit, air is forced into the air chamber and the bubble size decreases.  Conversely, if the pressure in A is caused to fall by rotating the adjusting knob  clockwise, air is drawn from the air chamber, via passage 2, into the bubble chamber. The correct position for the bubble when observing is shown by a central square. In use, to adjust the bubble size, the sextant is tilted backwards to bring the bubble within the area marked by the triangle. After long storage, the bubble chamber may be nearly empty but nearly always this seems to be that the air and the fluid , alcohol in the SOLD, redistribute themselves, rather than being due to leakage. In this case, most of the fluid can be shaken out of the air chamber by repeatedly flicking the wrist with the air chamber upper-most. The air in the fluid chamber can then be displaced into the air chamber by patiently rotating the control knob slowly back and forth with the triangle up, until the bubble reaches the required size.

Figure 2 shows the ray path through the attachment. Rays from the observed body are deflected by the horizon mirror of the sextant  into a window in front and pass through a semi-reflective mirror into a 3 power Galilean telescope. The bubble formed in the bubble chamber above lies at the focus of a concave mirror below, so that rays forming the image of the bubble emerge from the mirror parallel, and the image of the bubble appears, like the observed body, to be at infinity. These parallel rays are deflected by the semi-reflective mirror (in fact a piece of plane, unsilvered glass) into the telescope. Thus, the image of the bubble is superimposed on that of the body and it lies to the observer simply to bring them into coincidence, a by-no-means-easy task.

Figure 2 : Ray path through attachment.

The bubble chamber is illuminated above by a bulb contained in a screw-on holder. Light from the bulb is diffused by a red diffusing screen (Figure 3) and its intensity varied by a variable resistance (“potentiometer” or “rheostat”) of about 10 ohms resistance. If the bulb is replaced by an LED, then the potentiometer will also need to be changed for one of about 1000 ohms, as described for my first post of September 2013.

Figure 3 : Bubble illumination.

Power for the lamp comes from the battery handle of the sextant. Current flows through the plug and lead to the wiper on the variable resistance (black wire, Figure 2) and thence to the brass ring (red wire) in the top of the attachment. The ring is insulated from the body of the attachment and, when the bulb holder is screwed home, another, brass ring, threaded for the bulb and insulated from the bulb holder, makes contact to carry the current to the bulb. The return current is carried from the central contact of the bulb to the body of the bulb holder, through the body of the attachment and thence, via the mounting fork, to the body of the sextant.

There is no provision for illumination of the bubble by daylight, so for sun observations, the shades of the sextant are dispensed with and a dark shade attached to the front window, so that the bubble can be seen at the same time as the sun. For star and planet observations, of course, no shades are used with the sextant or the attachment. Daytime moon observations may be possible  by removing the attachment’s shade and obscuring the horizon with all the sextant’s horizon shades and by using an index shade that just allows the moon to be seen, but I have no personal experience of this. The unit was originally supplied with only one shade.

I am grateful to Murray Peake for entrusting me with the overhaul of his attachment.


To expand on the paragraph after Figure 1c, when the bubble cannot be reduced in size, and small bubbles seem to enter the chamber at random:

Holding the unit upright, turn the bubble control fully anticlockwise (looking from above) until it will go no further. This will make the bubble big, as it will be sucking air out of the air chamber.
Hold the unit upright in one hand at arm’s length and swing it vigorously towards the floor as if a pendulum, but stop suddenly at its lowest point (this is a bit hard to describe, so I have added two photos; I am not slack-jawed in the first, I am saying “Take it now”). This is intended to force any fluid remaining in the air chamber into the bubble chamber and is much more violent than a flick of the wrist.
Tilt the unit backwards about 20 degrees and turn the bubble control slowly clockwise. The bubble may at first be so big as to appear invisible, but should then slowly disappear into the air chamber.
A tip : rather than having to attach the unit to a sextant in order to see the bubble as you manipulate it as above, unscrew the top lamp holder and illuminate the bubble chamber as necessary with something like a mini-mag-light.
Postscipt 1: Beginning of swing. Note position of unit in the hand.

Postscipt 1: Beginning of swing. Note position of unit in the hand.

Postscript 2: End of swing.

Postscript 2: End of swing.

3 October 2014. Patrick Fitzhorn writes:

“You may already know this, but I thought I would pass it on as it may be of interest to others.  I have (finally) fixed my C. Plath artificial horizon telescope (similar to that as shown in 26 June 2012).  Mine has been missing the primary telescope lens for several decades, rendering the whole thing useless. Years ago, I pulled the telescope assembly off, measured the inner diameter of the primary lens holder (29.2 mm), sized the secondary lens (a double concave), and then fired up a software tool for the design and analysis of optical lens systems for lasers.  Looking at the telescope, it seemed pretty clear that the lens would have been an achromatic doublet (to minimize color distortion). I modeled the system in the software and found roughly that the focal length of the doublet would have been about 56 mm.

I then called the trusty large optical lens maker in Denver, CO who said they would finish sizing the primary, and then because an achromatic doublet has four finished surfaces (both sides of two lenses then glued together) the lens would cost $4,000 US.  They would do the lens analysis for free!

Aha I thought.  I’ll just buy the lens from one of the two major US lens suppliers – Edmund Optics, or ThorLabs.  BUT – the closest I could come to off-the-shelf optics (at $80 to $120 US) was 30mm D and 60mm EFL.  Wouldn’t work.

I then carefully shelved the project.

Just recently, I picked the project back up, thinking there must be some other solution.  I figured there might be cut-rate optical companies with odd sized lenses, and surfing the web (like any good sextant soldier) I found two within the US.  One with student-quality lenses at $4 to $8 US in a huge variety of sizes.  The other with coated quality optics, less sizes, and a range of $20 to $30 US.  From the cheaper company I ordered 5 lenses with diameters within +0 and -2mm from my 29.2mm, and focal lengths from 45mm to 100mm.  One at a time I tried them in the telescope, finding that a 60mm EFL was a bit too long, and 51.77mm quite a bit short.  Using the 51.77mm set to good focus on the moon, I measured the telescope length with calipers.  I then turned the focuser out until the tick marks lined (where it should have been in focus) and remeasured.  The difference was 5.08mm – an ideal EFL of 56.77mm.  From the more expensive company I found a 58mm D achromatic doublet with 57mm EFL for $22 US.

Got it, installed it, cleaned it up, tried it – works great.  I now have my artificial horizon.  By the way – the story of how the primary was lost will take longer than this discussion!”

http://www.surplusshed.com is a good source of surplus lenses of all shapes and sizes. Knowing that the magnification of the telescope is 3 times, the focal length of the objective lens could be deduced if the focal length of the negative (concave) eye lens could be discovered. You cannot of course cast an image on to a screen with a negative lens, but a rough idea of its focal length can be had by combining it with a positive (convex) lens of known focal length and greater strength, so that the combination is positive, and then casting an image of a distant object on to a screen and measuring the focal length directly. The sums are slightly easier if you take the lens’ powers in dioptres, the inverse of the focal length in metres. Then, if you combine, say, a 10 D positive with a 5 D negative, the power of the combination is  + 5 D. Once you know the (negative) focal length of the eye lens, -f ,and the magnification, m, the required focal length of the objective lens is – -f  x  m or f x m. A quicker alternative is to take the negative eye lens to your friendly local optometrist to have its focal length measured in less time than it takes to write this sentence. WJM

A Russian Naval Dip Meter

24 06 2012

This post was preceded by  “An Improvised Dip Meter”

In the preceding post, I wrote a little about the Blish prism and the Gavrisheff dip meter and pointed out that if the angle between two horizons opposite to each other could be measured, the local dip could be deduced. Through the kindness of Alex Eremenko, I have recently been able to examine in detail an N 5 Russian dip meter which has several interesting design features. It is probably easier to appreciate these if an idea is had of the optical path of the instrument (Figure 1). I explained a little about dip and its importance in celestial navigation in the preceding post.

Figure 1 : Light path of Russian N5 dip meter.

The horizon is viewed simultaneously to the left and right of the observer. Taking the rays from the left side first, shown in green,  they pass through an adjustable slit, used to make the brightness of the two horizons equal, through a watertight window in the side of the instrument and thence to a roof prism, where they are deflected downwards at 90 degrees. They then pass through a semi-reflecting junction, and are again deflected by a second prism through 90 degrees in a plane at right angles to the first, into the objective of a x 4 Keplerian (inverting) telescope.

The rays from the right hand horizon also pass through a window and then through a weak positive (convex) achromatic lens of 1.5 dioptres power. They then enter a negative (concave) lens whose power is such that the two lenses together have no net power, so that they behave like a piece of plane glass. However, the positive lens can be moved up or down from a central position, so that the light path also moves up or down – but not by very much. In fact, the total travel of the lens is 12 mm and the total deflection of the rays is 15 minutes of an arc each way. The right rays continue on through the right angled prism and are reflected off the common, semi-reflective face, off the opposite face and thence into the telescope objective, where they are combined to be viewed through the eyepiece. A yellow filter can be attached to the latter to reduce glare. Figure 2 shows the practical realisation of Figure 1.

Figure 2 : Practical light path

Projecting from the bottom of the slide that carries the sliding lens is a boss which carries an adjustable cam follower. This cam follower is held by two anti-backlash springs against a large cam (Figure 3). The cam is rotated by means of the adjusting ring, which carries a scale graduated plus or minus 15 minutes from zero, each minute being subdivided to 0.2 minutes (Figure 4).

Figure 3 : Adjusting cam.

Figure 4 : 3/4 Plan view.

Figure 5 shows the 4-power telescope. The images of the two horizons are seen to be vertical and, by rotating the adjusting ring they are brought into coincidence, when the dip can be read off the scale. The zero is set using two autocollimators, aligned as described in the preceding post An Improvised Dip Meter.

Figure 5 : The telescope.

The instrument is well-sealed against the ingress of moisture by greased felt rings where there are rotating parts and by heavy wax at metal-to-metal and glass-to-metal joints. It is provided with a stout leather carrying case. Figure 6, modified from a drawing in the original Russian handbook, shows the general arrangement of the parts. It can be seen at a larger scale by clicking on the picture. Use the back arrow to return to the text.

Figure 6 : General arrangement drawing.

To use, the window with the slit is directed to the brighter horizon and the slit adjusted to make its brightness equal with the opposite horizon. The adjusting ring is rotated to bring the horizons (which will be seen to be vertical) into coincidence and the reading noted.. The observer then rotates through 180 degrees (about a vertical axis, of course) and also rotates the instrument through 180 degrees on a horizontal axis. A second reading is taken and the dip taken as the mean of the two.

I have provided this post to support my book The Nautical Sextant, which covers solely the nautical sextant. If you have enjoyed reading this and others of my posts, I am sure you will enjoy reading the book, available from the publishers, from Amazon and from good booksellers.

Simex Sextant(s)

16 06 2012

In about 1963, Captain Svend Simonsen, a merchant seaman officer who had retired from the sea, is said to have tired of selling shoes instead, so he set up a navigation school, at first teaching students in their own homes and then by correspondence. By 1971, it was a thriving business, trading in Santa Barbara, California, as the Coast Navigation School. Captain Simonsen arranged with Tamaya of Tokyo to provide him with sextants named “Simex”, for him to on-sell to his students and others. There were, it seems, several models for sale, with various telescope, shade, mirror and frame options, reflecting the options that Tamaya offered under its own name, hence the plural in the title of this post. Recently there came into my hands a sextant named “Simex Mariner”. It had plainly been dropped, as a leg and the telescope stud had been broken off, but it had been a high end sextant, as it has a bronze frame, polarising shades, scale illumination and a micrometer vernier reading to 0.1 minute. Figures 1 and 2 show it after overhauling and  checking, and replacing broken parts.

Figure 1 : Front (LHS) of Simex Mariner sextant

Figure 2 : Rear (RHS) of Simex Mariner sextant.

The bronze frame is of a standard 162 mm radius with a black crackle finish. The index mirror measures 32 x 48 mm and the round horizon mirror is 50 mm diameter. A  contemporary German sextant has mirrors of 41 x 56 mm and 56 mm diameter respectively. The design of the latter’s micrometer mechanism is almost identical to that of the Simex, following the design principles devised by C Plath. While most post-1950 sextants either omit  a micrometer vernier altogether or divide it to 0.2 minutes, the Simex vernier is divided to 0.1 minutes (Figure 3). Given the uncertainties of the dip of the horizon and that the absolute limit of the sextant’s measurement accuracy may be no better than 12 seconds (0.2 minutes), the vernier is probably an unecessary extra. Possibly it was added to attract traditionalists.

The scale lighting system follows Tamaya’s earliest practice and does its job perfectly adequately. It is easy to change the bulb and batteries. The latter lie diagonally across the width of the shaped hardwood handle and the handle itself is canted at 20 degrees to the vertical to give a slightly more comfortable grip. The switch mechanism is simple and easy to access by removing two woodscrews, unlike in many later battery handles, particularly those by C Plath, where is is nearly impossible to remove the switch when it is in need of servicing.

Figure 3 : Detail of micrometer and illumination.

The shades use crossed polarising filters to give continuously variable darkening of the view. The idea was first mooted in a patent of about 1938 and some versions of the US Navy Mark II sextant were fitted with polarising shades, but the material seemed to fade after a few years. In my Simex, the index shade worked perfectly well, but one of the filtersof the horizon shade had lost all its polarising properties. Access is easy, by unscrewing the retaining ring shown below in Figure 4. The material is sandwiched between two sheets of plain glass, and by soaking the filter in acetone overnight it is usually possible to slide the two apart and clean off the remnants of the polaroid material. As mine had faded uniformly, I simply stuck a layer of self-adhesive polaroid film to the outside of the glass, a solution that has worked well.

My Simex also has an astigmatiser added to the index shade. This is simply a weak cylindrical lens whose axis is horizontal when swung into place. Its purpose is to draw out images of stars or the sun into horizontal lines and it probably finds its main use in combination with a bubble horizon though, at least with stars, it may be helpful to remove tilt of the sextant by lining up the extended image of the star with the horizon.

Figure 4 : Astigmatiser and polarising shade.

There is quite a variety of ways of attaching shades to their mounting brackets and the method may appear mysterious to someone seeking to overhaul or tighten a shade that has become annoyingly loose so that it does not stay where it is put. Figure 5 shows the horizon shade in its mounting and Figure 6 shows it exploded. At a casual glance, the two grub screws may be overlooked and the heads of the mounting pin or adjusting screw mutilated in futile attempts to turn them. Each grub screw must first be backed out, using a well-fitting 1.5 mm screwdriver to do so, as if the slot is destroyed, the overhauler will be faced with an even greater problem.

Figure 5 : Horizon shade and bracket.


Figure 6 : Horizon shade and bracket, exploded view.

Mirror mountings, index arm structure and index arm bearing are all conventional.