Freiberger Yacht sextant

24 06 2011

This post is preceded by “Inside the Freiberger Skalensextant” and “The Freiberger Skalensextant.”

Freiberger Präzisionsmechanik of the then German Democratic Republic, produced three sextants over the years: the uncommon Skalensextant; their main line, the Trommelsextant, aimed at professional navigators; and in the late 1970s they introduced their Yacht sextant. The latter was aimed at “Western” yachtsmen in competition with Plath and Tamaya at a price of around US$600, when a full-sized sized sextant by these makers could scarcely be had new for under US$1000. It had few features in common with its big brother, the Trommel or Drum sextant which retailed for about US$200 more, but presumably the pressure die cast aluminium alloy frame with rack cut directly into it, that they both had, was one feature that enabled Freiberger to undercut their Western rivals, or it may be that they were heavily subsidised.

Another feature which is also unfortunately in common with the big brother is that some examples have large arc errors, sometimes up to 90 seconds. As the arc is calibrated every ten degrees, it is not difficult to make a graph of the errors to help in applying corrections. Others would argue that on a pitching and rolling yacht, errors of observation are likely to swamp errors of the instrument. Many Freiberger instruments, it must be said, have the negligible errors of their competitors.

In a contemporary handbook, written in a charming Engleutsch, the Yacht sextant was “destined for use on sporting and luxury ships… with conditions and requirements for accuracy …completely maintained though weight and volume of sextant have been reduced  down to approximately half their former values. In consideration of aspects of beautiful shape and representativity, graduated arc and arc webbing have been modified that dimensional stability at extreme temperature differences and mechanical stress can be expected at the same time.”

In my description that follows, I will comment mainly on points that distinguish this sextant from the conventional, touching only briefly on parts that follow standard practice.

General arrangement (Figure 1)

The frame is certainly austerely functional and the limb has a radius of 142 mm against that of the drum sextant, which is about 170 mm, while its weight with its permanently mounted telescope is 860 G, compared to, say, the 1300 G of an alloy-framed Tamaya with a star telescope mounted. There are only three index shades and two horizon shades, quite enough for most purposes. The mirrors, which are interchangeable, both measure 59 x 29 mm. This is not the advantage that it might seems, as the horizon mirror is fully silvered, that is to say, there is no plain glass half, the horizon being viewed directly rather than through a thickness of plain glass. The edge can be fully protected with paint, whereas the junction between the silvered and unsilvered parts of the traditional mirror was always a weak point for sea water to attack the silvering. However, each half of a Galilean telescope “sees” only its own field of view: if half of the objective lens is covered over, half of the field of view is lost. With the traditional arrangement of mirrors, some of the light from the index mirror is reflected off the front surface of the plain part of the horizon mirror into the telescope, so that the images of the body and of the horizon to a large extent overlap. In this instrument, with the fully silvered mirror and a Galilean telescope, there is overlap of about one sun diameter (about half a degree) as opposed to about two and a half diameters for a full glass, and this may make for difficulties for  an inexperienced or occasional observer.

Most modern sextants seem to have Galilean telescopes with about x 4 power with an objective of 40 mm, while the Yacht sextant has one of x 2.4 power, with an objective of 22 mm, so the field of view is about the same. Early yacht sextants had ordinary, second surface mirrors. One that I have seen from 1985 has first surface mirrors which give a slightly brighter image, imperceptible to the eye (which can just detect a doubling in brightness), but at times of poor contrast between the sky and the horizon, even small differences in brightness can make for a useful increase in contrast.

Figure 1 : Front view of Yacht sextant.

The arc is a  sliver of screen-printed aluminium secured to the frame with glue and two pins which, rather surprisingly are made of steel. Predictably, in my example which had seen a lot of sea use, they had rusted and the glue had also given way. The telescope is a 2.4 x 22mm Galilean that focusses by rotating the objective lens mounting rather than the eyepiece and is  less prone to go out of focus as the eye presses against the scope. The eyepiece is provided with a deep flexible rubber pad, a perhaps necessary adjunct for a lightweight telescope used on board a pitching and rolling yacht.

Figure 2 : Rear view of Yacht sextant

The rear view (Figure 2) shows that Freiberger retained the same arrangement for the index arm as in the Trommelsextant (and the Soviet SNO-T) siting it behind the frame out of harm’s way. To avoid having to have a bridging piece, the black anodised alloy handle is attached to the frame only at the top, by three screws. The release catch at the lower end of the index arm swings the worm out of engagement in the plane of the rack and, as will be seen, has a much simpler (and undoubtedly cheaper) arrangement than in its larger brother.

Index arm bearing

Figure 3 : A cornucopia of screws.

Removal of the handle reveals a sextet of screw heads beneath. Two pass through the bearing (strictly speaking, the journal) and attach the index mirror mounting to the deep flange on the other side. Four attach the index arm to the journal (journal : the part of a shaft that is enclosed by a bearing). Removing these allows the journal to be withdrawn. The fit is necessarily a very close one and care has to be taken that it does not capsize and jam. Figure 4 shows the bearing dismantled and it is seen that the large parallel bronze journal runs directly in the frame. There is no provision for taking up wear as none is to be expected in a slow-moving part with such large bearing surfaces.

Figure 4 : Index arm bearing details.

Worm and release catch

The Trommelsextant swung the worm out of engagement with the rack by having a relatively complex eccentric bearing that rotated the worm in its bearing out of the plane of the rack. Elegant though the method was, it must have been very expensive to produce. In the Yacht sextant, Freiberger have managed to give a large degree of protection to the worm by enclosing a substantial bronze swing arm with integral worm shaft bearing in a U-shaped alloy casting, that is attached to the index arm via a tongue and three screws (Figure 5). The shaft, immediately on emerging from its bearing, is enclosed by the micrometer drum and the knob for rotating it, so that a common consequence of dropping the sexant, a bent worm shaft, is avoided.

Figure 5 : Worm enclosure

The release catch is a cranked metal stamping. When it  is operated, its end bears on a pin that passes through an oval hole in the enclosure casting into the swing arm, and presses the swing arm down agains a leaf spring that lies between the swing arm and the bottom of the casting. Upon detaching the casting from the index arm, some more details are revealed (Figure 6).

Figure 6 : Swing arm from above.

The bronze swing arm rotates about a pin for an axis (seen in Figure 5) and has a long and substantial bearing for the worm shaft. End float of the latter is prevented by trapping the worm between an L-shaped phosphor-bronze  pre-load spring and a PTFE washer. The aluminium alloy drum, divided into minutes and attached to the knob by two screws, was heavily corroded and pitted in my example, and I was obliged to make a new one.  The knob is retained on the worm shaft by a grub screw and a pin nut. Figure 7 perhaps allows a better appreciation of the internal workings of the release catch and swing arm.

Figure 7 : Side view of worm mounting.

Shades

The index shades are mounted directly into the frame, as shown in Figure 8.

Figure 8 : Index shades mounting.

They are mounted on a screw that has a longitudinal key way machined in it. The screw passes through a narrow tongue of the frame, through the shades and separating washers, through a Belleville washer (a washer having the characteristics of a short, stiff spring) and finally into the body of the frame. The separating washers have an integral tongue that fits into the keyway, so that forces from the rotation of one shade cannot be transmitted to the next. The Belleville washer provides enough friction for the shades to stay in the positions in which they are put. It might be thought that the friction could be varied by tightening the screw, but a transverse steel (!) taper pin prevents it from rotating. Driving it out places at risk the slender tongue of the frame which would pose great problems to replace it, if broken off.

The two horizon shades are mounted on a shouldered screw which also has a keyway and the separating washer also has a tongue that fits into the keyway to prevent it from turning  (Figure 9). The screw is driven into a mounting block hard against its shoulder and it is only this that prevents the screw from undoing. A Belleville washer between the rearmost shade and the mounting block provides friction. Close inspection of Figure 9 shows that over time the convex side of the washer wears into the soft metal of the shade, resulting in loss of friction and a very annoying tendency for the shades to flop downwards out of position. This can be countered  by means of a thin shim washer, whose internal diameter must be a little greater that the diameter of the unthreaded part of the screw, and care must be taken that the shim does not get trapped between the shoulder of the screw and the mounting block. Alternatively, the height of the Belleville washer can be increased slightly by making a depression in the end grain of a block of hard wood using a 12 mm ball bearing, resting the washer in the depression, placing the ball bearing on top and hitting it smartly with a hammer. This is likely to make the ball fly off into a remote recess of the workshop, but as it cannot now be reused for any other purpose, the loss is not a great one. When replacing the shades, they and the washers are mounted on the screw and it is started into position, with the concavity of the Belleville washer facing the mounting block. If the sextant is held with that washer facing upwards, it is then possible to wangle the hole in the washer over the shank of the screw before driving it home.

Figure 9 : Horizon shades mounting exploded.

Adjusting screws

The screws themselves are standard M2 screws with squared heads. They pass through  split collets that are threaded on the outside and which screw into bosses projecting from the back of the mirror mountings. Each collet has a tapered tip that fits into a matching female taper at the bottom of the boss, so that when the collet is tightened, it closes up around the screw, thus increasing the resistance to the screw’s rotation or locking it if required (Figure 9). There is a clearance hole through the mirror bracket for the screw

Figure 10 : Adjusting screw, collet and tool.

Though the adjusting screws for the SNO-T and the Freiberger Skalensextant are superficially similar, anyone who treats them the same is in for an unpleasant surprise; and jammed, broken or stripped screws are a common occurence. In these sextants,  the collets, which have coarse threads on the outside are not split, and the adjusting screws pass through threaded holes both in the collet and in the back of the bracket. The screws are locked by unscrewing the collet a little.  The screw can be withdrawn by rotating the collet a little to find the position at which the screw is unlocked. Any attempt to fully unscrew the collet before the screw has left the thread in the mirror mounting inevitably causes damage. Some early Trommelsextants may have the same arrangement, so it would pay to proceed with caution if backing off the nut by half a turn does not release the adjusting screw.

If you have enjoyed reading this account, you will I am sure enjoy reading my book, The Nautical Sextant, available through booksellers, from Amazon or direct from the publishers, Paradise Cay and Celestaire.

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C Plath Yachting Sextant

14 06 2015

This post was preceded by “Making a shades adjusting tool” and “Eighty years of Carl Plath Sextants”. Other posts on C Plath sextants may be found by entering “C Plath” in the search box on the right. All figures may be enlarged by clicking on them. Return to the text by using the back arrow.

Several makers, including C Plath, made sextants directed at the yachting market with more or less success. There seems to be a fair number of Freiberger yachting sextants around, but I have only ever seen two Plath Yachtsman sextants. In the years after WWII, many full-size sextants must have flooded the market, especially the USN Mark II sextant and those made by Henry Hughes and Son. The latter also made half size sextants for use in sea planes and presumably they were attractive to yachtsmen, as some have survived. A variety of plastic sextants derived from the Maritime Commission version for lifeboats came on to the market and evolved into instruments that looked like “proper”metal sextants, though few were rigid enough to behave like one. Francis Barker produced a box sextant labelled “Small Craft Precision Sextant” intended for sale to yachtsmen, but despite having been provided with a horizon shade and an eyepiece shade in addition to the usual index shade, I doubt that it found much favour with nautical users. A box sextant is a fiddly instrument at the best of times and it is difficult enough to take sights from a rolling yacht. Ilon industries made an ingenious little micrometer sextant provided with a tiny prismatic monocular (https://sextantbook.com/category/ilon-industries-sextant/) that may have found favour with the well-heeled and Tamaya made a light weight 5/6ths sized micrometer sextant. The French firm of Roger Poulin made an interesting little sextant that was plainly aimed at the yachting market and I have described it here: https://sextantbook.com/?s=Poulin .

It is not clear whether the yachtsman wished a smaller sextant because of lack of space aboard yachts or because a smaller sextant might be cheaper than a full-sized version. At any rate the saving in space and weight must have been insignificant, and the savings made by buying a smaller sextant cannot have been great when compared with the cost of the vessel.

Unlike the Freiberger Yacht Sextant (https://sextantbook.com/?s=Freiberger+yacht), which attempts in a way to echo the full sized instrument, the frame of the C Plath sextant is monolithic and exceptionally rigid. Figure 1 shows a general view of the front. The bases of the index and horizon mirror brackets are identical though the horizon mirror itself is half silvered. Both are circular, presumably because it is easier to seal the mirrors against the intrusion of salt water behind them, but as can be seen in some of the later figures, the index mirror has suffered around the edges. The two index shades and one horizon shade are adequate in most circumstances. Their brackets are simple and no provision is made for adjustment of friction. A notch in the edge of the frame allows the horizon shade to be folded completely out of the line of sight.

The rack in which the micrometer worm engages in machined into the edge of the limb, together with a slot for a keeper to keep correct engagement. The radius of the rack is about 140 mm (5.5 ins) and the instrument weighs 1260G (2lbs 12 oz).

Figure 1: General view of front.

Figure 1: General view of front.

The telescope has a simple draw tube for focusing, and  has an aperture of 25 mm and a power of about 2.5 diameters, giving a field of view of a little over 6 degrees. This is about the same as one gets from a 4 x 40 mm telescope of a full-sized instrument. Though a C Plath leaflet says the aperture is 30 mm with a magnification of x 4, the inside diameter of the tube in front of the objective lens of my sextant is only 27.5 mm and it has to sit on a shoulder, so the aperture behind the lens is only 25.1 mm. The measured magnification is about x 2.5.

The telescope is not demountable, a disadvantage on a small vessel when it is rolling and pitching, as with a standard field of view it can be difficult to acquire the heavenly body and bring it down to the horizon. Removing the telescope altogether makes it much easier to find the body and to bring it down, when the telescope can be replaced and the horizon swept to re-acquire the body. However, the telescope mounting is very robust so that it is not only resistant to knocks, but the sextant can safely be picked up by the telescope without fear of damaging or displacing it. The micrometer mechanism is well protected against knocks and the release catch is simple to operate. Figure 2 shows a rear view of the instrument.

Figure 2: Back view.

Figure 2: Back view.

The frame is closed off at the back by a back plate, which is attached to the frame by three screws and a leg. The handle, adapted from a full-sized instrument battery handle, is attached to the back plate via pillars by two countersunk screws. Removing the back plate reveals the index arm as shown in Figure 3. Note that if the sextant gets drenched in salt water, it is an easy matter to rinse out the interior with fresh water without necessarily removing the back plate.

Figure 3:  Rear view without back plate.

Figure 3: Rear view without back plate.

The index arm is in two pieces: a stout rectangular bar attached to the index mirror bearing at the top; and  a plate that I have christened the index arm expansion at the bottom. This plate carries the micrometer mechanism. I have labelled the screw for attaching the horizon mirror and the swing arm keeper in Figure 3 for future reference below. Also seen are the two stout screws that attach the telescope to the frame.

Figure 3: Index arm bearing.

Figure 4: Index arm bearing.

The anatomy of the index arm bearing is revealed in Figure 4. A micro-finished journal runs in a parallel bearing machined directly into the frame, with two PTFE washers acting as spacers and also taking any minor thrust forces that may arise. A flange above the journal carries the index mirror in its bracket, while a spigot below attaches the index arm. Figure 5 shows how the upper end of the index arm is split, with a pinch screw to close it around the spigot. This allows adjustment of the mirror in the horizontal plane as well as axial adjustment to take up any axial movement in the bearing.

Figure 5: Upper end of index arm.

Figure 5: Upper end of index arm.

Figure 6 shows how the index mirror is adjusted for perpendicularity and the horizon mirror for side error (the horizon mirror is illustrated) . The mirror bracket is rocked by means of two screws about two ball bearings sitting is depressions to form an axis of rotation.

Figure 6: Mirror bracket adjustment.

Figure 6: Mirror bracket adjustment.

As the reflective surface of the index mirror lies a little ahead of the axis of rotation of the index mirror it is necessary to use two vanes to raise the line of sight to somewhere near the centre of the mirror, as otherwise a minor error in perpendicularity may be introduced. Figure 7 shows how two small dominoes have been used, but any two identical objects objects of about the right height may be used, such as pieces cut from aluminium or steel angle, large hexagonal nuts or large rollers from a scrapped roller bearing. One is placed on the limb of the sextant at zero and the other at about 90 degrees. The index arm is then rotated until a reflected view of the second vane is seen alongside a direct view of the first, when the mirror is adjusted to bring their tops into line as shown. In many sextants, including this one, it may be necessary to remove the telescope and/or index shades to obtain the required view.

Figure 7: Adjusting index mirror for perpendicularity.

Figure 7: Adjusting index mirror for perpendicularity.

When adjusting the horizon mirror to remove index error, the screw arrowed in Figure 3 is slackened and a tommy bar used in the hole visible on the right in Figure 7 to rotate the whole base. This is a relatively coarse way of adjusting and may involve much trial and error, but once done, the whole set-up is rigid and not likely to drift out of adjustment in a way that is so annoying with plastic “instruments”.  Removing side error has already been mentioned in the paragraph following Figure 5. Note that index error cannot be removed by using the sun, as the single horizon shade is not dense enough for this method. There is no adjustment available for collimating the telescope, but quite large errors of collimation have relatively little effect on the accuracy of readings, especially for the class of sight likely to be made with this instrument. In any case, this is taken care of at manufacture and would require very rough handling indeed to disturb.

The micrometer mechanism is robust and well-protected. Figure 8 shows it detached from the index arm. The black release catch on the right in fact remains stationary when disengaging the worm and it is the horn extending down and to the left  on the plate that rotates when it and the black catch are squeezed together.

Figure : Micrometer mechanism detached from index arm.

Figure 8: Micrometer mechanism detached from index arm.

In Figure 9, the front plate which carries the fiducial lines for the degrees scale and the micrometer has been removed to show the swing arm chassis. This carries the micrometer worm in a plain parallel bearing, the axial play of which is taken up by a leaf spring. A swing arm extends upwards and to the right to a bearing in the form of a shouldered screw, about which the chassis rotates. A stout helical spring keeps the worm in engagement with the rack machined on the edge of the limb of the sextant.

Figure : Front plate removed to show interior of micrometer mechanism.

Figure 9: Front plate removed to show interior of micrometer mechanism.

Figure 10 shows these parts more clearly. In addition, there is a rectangular keeper that guides the index arm expansion and keeps the worm in correct engagement. It slides in a slot machined in the limb below the rack.

Figure : Micrometer mechanism exploded.

Figure 10: Micrometer mechanism exploded.

A further, circular, keeper ensures that the swing arm chassis cannot lift off the face of the index arm expansion. The spigot on the keeper slides in the oval slot and the keeper is retained in the chassis by means of a screw whose tapped hole is shown in Figure 11, centre, which illustrates the bearing surfaces for the swing arm chassis. The keeper can be seen in place in Figure 3, above.

Figure : Swing arm bearings.

Figure 11: Swing arm bearings.

The sextant frame, being made of aluminium alloy, is inherently resistant to corrosion, but parts that do not run together have a tough coating of blue paint. Other parts are made of bronze and all the screws and springs are of stainless steel. If the sextant should receive a soaking, it is a simple matter to rinse it with fresh water and allow it to dry, as all the parts of the interior are accessible. Nevertheless, at overhaul it would be wise to use waterproof marine grease  for all moving parts except for the rack, which should receive SAE 30 lubricating oil, brushed into the rack with surplus being brushed and wiped off.

The case provided was, like so many other sextant cases over the last fifty years, made of plywood. Quite why the makers did not usually specify marine grade ply is a mystery, as many of them, including those from C Plath, suffered from delamination if stored damp. It was stored face down in the case, leaving the handle ready for use, but as it cannot be set down on a table face down, this is a limited advantage. Perhaps though, it was to discourage users from leaving it in a position on a table to slide onto the floor. The general rule is that a sextant should be in the user’s hand or in its case, relatively easy to follow on a yacht, but more difficult on the bridge of a large ship. All in all, this is a robust sextant, well suited to its task.

Dr Andreas Philipp writes that at least 900 of these sextants were made from 1968, starting with a serial number of 101. They were sold mainly in the USA.





Freiberger Drum Sextant (Trommelsextant)

10 08 2011

This post is preceded by one on the Freiberger Yacht sextant and two on the Freiberger Skalensextant

Readers looking for a manual that helps with maintenance and repair of the Freiberger Trommelsextant (drum sextant) will find my SNO-T Sextant Manual very useful, as the design of the one is based on the other. While the manual describes the SNO-T, it also gives an account of the Freiberger drum sextant where its design details differ. See under “The USSR SNO-T sextant” and “Buy”

The firm of Freiberger Präzisionsmechanik has been in existence since  late in the eighteenth century and by the 1870s had a large workshop employing over eighty people in the manufacture of surveying instruments. At the end of WW II it was overrun by  Soviet forces and dismantled, leaving only fifteen workers to carry out maintenance on surveying equipment. It was refounded in 1950 and in that decade the trommel sextant was developed. As far as I know, the firm had never previously made sextants.

The sextant is unusual in several respects. While the over-all shape and placement of shades and mirrors is conventional, it has a die-cast aluminium alloy frame, which combines lightness with a strength and hardness near to that of mild steel. The ladder or three circle patterns of its main competitors were ignored. Material is concentrated around the edges and the whole stiffened by a central web (Figure 1). The worm runs in a rack machined directly into the edge of the limb, thus avoiding the complication of attaching a bronze rack to an aluminium frame.  The substantial but unseen bronze worm seems to run very well against the alloy rack.

Figure 1 : Freiberger drum sextant, front view.

The very substantial index arm lies behind the web of the frame and is bridged by a casting to which is attached the handle (Figure 2). The upper end of the index arm is screwed to a large diameter bronze journal (the part that rotates in a bearing) that rotates in a bearing machined directly in the frame, thus abandoning the narrow, tapered journal and bearing in use since the third quarter of the eighteenth century. C Plath later dallied with such a bearing in their bronze-framed instruments, but they soon reverted to the tapered form.

 

Figure 2 : Freiberger drum sextant, rear view.

 The micrometer mechanism is concealed within an alloy casting attached to the lower end of the index arm. The cylindrical worm runs in eccentric bearings in a bronze casting that itself rotates against the force of a helical spring in bearings machined in the alloy casting. Thus, when the bronze casting is rotated, the worm swings out of engagement with the rack. The closeness of this engagement can be adjusted by means of a tangential screw whose head is just visible in Figure 2 to the left of the drum. While Freiberger chose to swing the worm out of the plane of the rack, nearly every other maker swung it out of engagement in the plane of the rack, following the pattern devised by C Plath in about 1907.  The latter method must certainly have been cheaper to manufacture, even allowing for the unecessary complexity of the worm shaft bearings in some pre-war marques. However, Freiberger’s method totally encloses the worm and solidly supports the shaft at both ends, so it is hard to imagine the shaft getting bent by an accidental knock, as had happened to at least three conventional instruments that have passed through my hands.

The sextant was usually provided with a 3½ x 40 Galilean telescope only. My own instrument has a 7 x 35 monocular, which gives a superior field of view as well as making the point of coincidence of the body with the horizon easier to determine. The vee and flat of the mounting are the reverse of all other makers, so their telescopes cannot be interchanged.

The USSR imitated Freiberger’s design in their SNO-T sextant, albeit in an instrument of slightly smaller radius and one provided with an unusually full complement of tools and spares. The edge of the SNO-T frame is  8 mm thick (compared to 3 mm), making it an even more rigid and robust instrument than the Freiberger. The bare sextants weigh 1300 and 1200 grams respectively.





A French Hydrographic Sextant

13 01 2019
2 a case inside

Figure 1: Sextant in its case.

I recently acquired for a relatively modest sum the three-circle vernier sextant shown in Figure 1. Attached at the front corner of the frame is a plate engraved with the letters “S.H.” or “Service Hydrographique (de la Marine)” or French Naval Hydrographical Service, formed in 1886 as successor to the “Dépôt des cartes et plans de la Marine”, founded in 1720. The plate seems to serve no other purpose that I can think of  than as an identifier.

3 a e bouty name

Figure 2: Front of the tangent screw mechanism.

Engraved on the front of the tangent screw mechanism is the name “E. Bouty”. Edmond Bouty (1845 – 1922) was a physicist in the Science Faculty at Paris, but I cannot find that he was an instrument maker, nor is there any other name on the sextant. It may be that his contribution was the design of the scale lighting system, about which more later. It is not even clear that the sextant is of  French manufacture, as at the left end of the limb are the letters “D.S.” indicating Deutsche Seewarte, the German Hydrographical Service, but the frame, of about 180 mm radius, differs in detail from that of C Plath’s Dreikreis sextant.

2 b frame turning marks

Figure 3: Turning marks on front of frame.

The bronze frame is of no particular interest except that when clearing old and perished paint from the frame during restoration I noticed marks (Figure 3) that showed that it had been faced in a lathe, giving a small clue to the manufacturing process.

3 c spring nut

Figure 4: Spring box detail.

Returning to the tangent screw mechanism, the spring box is shown exploded in Figure 4. A tongue on the sliding block is trapped between the end of the tangent screw and a long spring mounted on a guide and retained by a nut. The end of the guide can be seen on the right of Figure 2.

3 b clamp

Figure 5: Exploded view of index arm clamp.

The sliding block is retained in its slide in the lower end of the index arm by the retaining spring on the upper right of Figure 5, while the clamp screw and its leaf spring bears on the back of the limb. In use, the clamp is slackened and the index arm moved approximately into position, when the clamp is tightened, thus fixing the sliding block to the limb. Turning the tangent screw thus moves the index arm about the sliding block against the pre-load of the helical spring as a means of fine adjustment. In truth, it is the index arm that slides rather than the sliding block, but as no one else had given it a name, I decided to do so when writing “The Nautical Sextant.” This system of applying pre-load was used in many vernier instruments such as vernier theodolites and gun aiming systems. as well as in several makes of sextant.

4 perp adjust

Figure 6: Index mirror bracket.

The index mirror is held against a vertical bracket by means of a clip which is tightened against the bracket by means of a screw bearing on the back of the bracket. The mirror is made perpendicular to the arc of the sextant by a system that seems  to have been used only by French makers. Two screws attach the radiused feet of the bracket to the upper end of the index arm and the end of a screw held captive in the base of the bracket can then rock the bracket to bring the mirror square to the plane of the arc..

5 side error

Figure 7: Horizon mirror bracket.

Figure 7 shows a somewhat similar method of adjusting out side error of the horizon mirror, but in this case a deep slot cut nearly through the base of the bracket gives flexibility to the the adjustment by means of another captive screw.

7 horizon mirror

Figure 8: Horizon mirror detail.

The detail shown in Figure 8, as well as making clearer how the mirrors are held against their brackets, shows that the horizon mirror bracket can be adjustably rotated about an axis vertical to the plane of the sextant, in order to adjust out index error. Note that the mirror is fully silvered, which means that the direct view of the horizon does not pass through glass and that the edge of the silvering of the mirror can be given better protection against corrosion. It does however result in a smaller area of overlap of the direct image of the horizon and the  reflected  image of the observed body when using a Galilean telescope. Enter “Freiberger yacht sextant” in the search box at the top of the page for a discussion of why this is so.

6 index error

Figure 9: Detail of index error adjustment.

Figure 9 gives more detail on the index error adjustment. There is a boss as an axis on the underside of the horizon mirror bracket that passes through the frame and is held by a retaining screw. A further boss passes through a clearance hole in the frame  and has an internal thread tapped in it as a nut. The index error adjusting screw, held captive in the frame by a screw and clamp, engages with the “nut”, so that when the adjusting screw is turned, the whole mirror bracket rotates. When adjustment is complete, the bracket is locked in place by a  clamp screw..

This is a rather complex means of adjustment of the horizon mirror, which had long been achieved much more simply by means of   a pair of screws bearing against the back of the mirror, while lugs on the mirror clamp provided spring loading. Elegant though it may have seemed to its (?) French inventor, it is unnecessarily complex., though perhaps no more complex than the solution adopted by Brandis and its US successors.

9 battery handle

Figure 10: Interior of battery handle.

This sextant represents perhaps one of the earliest ones to light the scale in poor light. Scale lighting had to wait for the development of suitable dry batteries in the 1890s and of miniature flashlight bulbs with robust tungsten filaments in about 1904.

Figure 9 shows the interior of the Bakelite handle which accepts a 3 volt 2R10 battery.  A screw at the lower end holds the negative pole of the battery firmly in electrical contact with the frame of the sextant and at the upper end a spring loaded switch plunger makes contact with the positive pole. The top end of the lid is bevelled and the lid itself is slightly bowed, so that when rotated closed it remains in place.

10 b handle to bearing

Figure 11: Wire from handle to foot.

A wire passes from the body of the switch to the foot (Figure 11), inside which is a spring loaded brass plunger (Figure 12).

10 a switch to contact

Figure 12: Inside of foot.

The index arm journal is hollow and a wire passes up its centre to an insulated contact on the end, to make electrical contact with the contact inside the foot (Figure 13).

11 a journal contact

Figure 13: Insulated index arm contact.

The other end of the insulated wire passes down the index arm in a machined groove to a clip held on an insulator block (Figure 14).

12 lighting system

Figure 14: Lighting bulb holder.

The clip makes contact with the outside of the bulb holder and thence to the central contact on the bulb. The outside of the holder is insulated from the brass interior, which is threaded for the bulb. The brass interior fits snugly in the cylindrical shade which is attached to the index arm and hence the frame, thus completing the electrical circuit. Most subsequent makers contented themselves with a simple loop of insulated wire to conduct electricity to the bulb, but this more complex and no doubt more expensive system has the merit of not flexing any wire. Like most complex systems, however, there is more to go wrong.

13 rising piece in situ

Figure 15: Rising piece.

The telescope rising piece (Figure 15) is simpler than that of many of its early 20th century competitors and it has a rectangular mortice machined in its face to engage closely with a tenon on the telescope bracket, so that it can be slid up or down to vary the amount of light from the horizon entering the telescope. Collimation is standard, by means of a tilting telescope ring held in place by two screws.

8 index shades

Figure 16: Shades mounting.

The shades make none of the usual provisions to prevent movement of one being transmitted to its neighbours. Resistance to rotation is given by means of a Belleville washer, a conical washer with the characteristics of a short, stiff spring. Since these date from about 1870, they add no clues to the age of this sextant.

15 telescopes

Figure 17: Telescope kit.

The kit of telescopes shown in Figure 16 is for the most part standard, with a 4 x 24 mm Galilean “star” telescope for general use and a 6 x 16mm Keplerian “inverting”  telescope. By the twentieth century, this latter probably received little use except for artificial horizon sights to rate chronometers in out-of-the-way places of known longitude. The large 3 x 36mm Keplerian telescope is of interest as it has a wide angle eyepiece with an eye lens of 25 mm aperture. This gives an image nearly as bright as the 4 x 24mm telescope (the extra lens in the eyepiece causes some loss of light) and with a field of view about four to five times wider.

1 a case exterior

Figure 17: Case exterior.

The mahogany case was much battered and stained, and with several shrinkage cracks, so it was gratifying to be able to restore it to the state shown in Figure 17. It looks decidedly English and placing the handle on the side follows Henry Hughes and Son’s practice, but neither the sextant frame nor the mirror mountings  are consistent with this.

If you enjoyed reading about this sextant, you may also enjoy reading my “The Mariner’s Chronometer“, also available via Amazon.com.

 

 

 

 





ALPHABETICAL LIST OF POSTS

29 12 2013

This alphabetical list of posts may help you to find what you want. When you have found a post of interest, enter the part of interest as a search term in the search box.

A10 vapour pressure bubble chambers, Sealing

Admiralty pattern vernier sextant

Admiralty pattern micrometer sextant

AN 5851-1 bubble sextant averager., Gummed-up

AN 5851-1. Jammed shades carrousel

Battered Observator sextant, A

 Battery Handle Structure, C Plath

 Box Sextant, A

Broken legs

 Bubble Horizon Attachment, C Plath

Bubble illumination of Mk V and AN 5851 bubble sextants

Bubble Sextant Restoration Manual, A12

 Bubble Sextant Restoration Manuals, A10 and Mark IX series

bubble sextants, Aircraft

Bubble sextant, Hughes Marine 

 Bubble Horizon, A Nautical Sextant

 Byrd Aircraft Sextant, Update on

Byrd Sextant Restored, A

Carl Plath’s Earliest Sextant

C Plath Sextant Lives Again

 C. Plath Drei Kreis sextant, Restoring a

 C. Plath Vernier Sextant, A Fine

C Plath Yachting Sextant

C18 sextant named J Watkins

C19 sextant restoration

C Plath Sun Compass

Carl Plath micrometer sextant

 Carl Plath Sextants, Eighty Years of

circular sextant mirrors, Making

Compass, a C Plath sun

Compass, an improvised sun

Damaged Rising Piece, A

 Dip Meter, A Russian Naval

 Dip Meter, An Improvised

Distance Meter, A Stuart

Distance Meter, Fleuriais’ Marine

Drowned Husun Three Circle Sextant, A

Ebony quadrant, restoring a

Errors, Backlash and Micrometer

Faking it., Is it a SNO-M or is it a C Plath?

 Filotecnica Salmoiraghi of Milan, A Fine Sextant by

Freiberger Drum Sextant (Trommelsextant)

Freiberger Skalen Sextant

Freiberger Yacht sextant

A French Hydrographic Sextant

Half-size Sextant by Hughes and Son, A

Half-size Sextant by Lebvre-Poulin, A

Heath and Company’s best vernier sextant

Heath Curve-bar sextant compared with Plath

Heath Vernier Sextant Restored

Hughes Marine Bubble Sextant

Hungarian sextant via Bulgaria, An

Hydrographical sextant, a French

Ilon Industries Mark III sextant

Jesse Ramsden and his Dividing Engine

 Keystone Sextant Case, Making a

 LEDs 1: miniature screw bases., Adapting to

LEDs 2: Plath bubble horizon unit, Adapting to

Lefebvre-Poulin, a Half sized sextant by

left-handed sextant, Unusual

Legs, broken

Mark V / AN5851 sextant bubble chambers, Refilling

 Markk V/ AN5851 sextant bubble chamber, Overhaul of

mirrors, How flat are sextant ?

 monocular mounting, Making a sextant

Observator Classic sextant, restoring a

Observator Mark 4 sextant

 Prismatic Monocular, Making a

 Quadrant Restored, An Early C19 Ebony

Quadrant Restored, an old wooden          June 2018

 Scale Lighting Systems, Later Tamaya

Sextant ‘scopes for myopes

Sextant Mirrors, New  for Old

Sextant Calibrator, A

Sextant Frame, Evolution of the

sextant shades, Polarising

Sextant, 210 years on,

Shackman sextant and a link to Jesse Ramsden

 Shades-adjusting Tool, Making a

Simex Sextant(s

Skalensextant, Inside the

SNO-T Mirror Bracket Repair

 SOLD KM2 Bubble Sextant

Sounding Sextants 1

Sounding sextants 2

Sounding sextants 3

Spanish Vernier Sextant, A Late

Spencer, Browning and Co sextant

Sun compass, a C Plath

Sun compass, an improvised

switch overhaul, Tamaya

Tamaya Collimation Blunder

The Case of the Broken Screw

Troughton and Simms Surveying Sextant

Turn-of-the-century French Sextant

US Maritime Commission Sextant, A

USN BuShips Mark II sextant: some design oddities

USSR SNO-M sextant, The

USSR SNO-T sextant, The

 Vernier Sextant, British Admiralty

Watkins, J, A C18 sextant named

Which lubricant?

Worm with wrong thread angle?

Worm Turns, A





A: AN INDEX TO POSTS

6 03 2010

INDEX

 As the pages of the blog posts are not numbered, I have given the dates of the index entries as the easiest way of retrieving the information they contain. Don’t forget that WordPress also allows you to search this site specifically (see search box in top right-hand corner of the home page).

adhesives, industrial       27 Mar 11

Admiralty, British, vernier pattern of sextant     27 Mar 11,  24 June 11

Admiralty, British, micrometer version of sextant    15 Feb, 18

alcohol, filling of A10 A bubble chambers   13 Oct 09

angle, vertical sextant     13 Jul 12

Araldite, for cementing lenses       24 Jan 10

refractive index of                    24 Jan 10 (Comment)

arc, dangers of polishing                                    24 Jan 10

platinum                                                           10 June 10

silver, letting in to limb                               10 June 10

astigmatiser shade             20 Mar 11

autocollimator, principle of                                6 July 09, 13 Feb 11

accuracy of                     12 Mar 09,  13 Feb 11

calibrating dip meter with             5 Apr 2012

setting wires of                13 Feb11

view through eyepiece of      13 Feb 11

backlash     12 Mar 09

beam splitter     13 Feb11

bearing,

index arm           18 Feb 10, 13 Jul 12

adjusting     12 Mar 09

of US Maritime Commission sextant    15 Dec 10

swing arm

conical centres     12 Mar 09

plain     12 Mar 09

of US Maritime Commission sextant     15 Dec 10

Belleville washer                                               18 Feb 10, 11 Aug 09

Blish prism                                                      5 Apr 2012

Bracket, mirror,

bent     20 Oct 16

slotted                1 Oct 14

shade                                14 June15

brass, hammered                                              10 Nov 09

bubble chamber

overhaul, of AN5851                            20 Dec 08

refilling, of AN5851     20 Dec 08

bubble sextant

A  Coutinho  Pattern                   6 May 16

restoration,

A10 series                                            25 Feb 09, 13  Oct 10

screw closure of     13 Oct 10

Mk IX series                                        25 Feb 09

British Admiralty pattern sextant      24 June 11

Broken legs, mending  7 Oct 16

Byrd sextant                                                     30 May 09

Byrd, Commander Richard                               30 May 09

calibration, of C18 sextant                        10 June 10

calibrator, sextant

calibration of           13 Feb11

construction of        13 Feb 11

mounting sextant on    13 Feb 11

Carbonara, Victor             1 Oct 14

Carl Path’s earliest sextant (see also C Plath)       21 April 2017

case

latches                                                      18 Feb 10

keystone                                                  17 Dec 09

furniture, of US Maritime Commission sextant  15 Dec 10

plywood, delamination of                           14 June 15

casting, pressure die-                                     14 July10

C Plath (See also Carl Plath and Plath)

production numbers       14 July 10 (see also Plath)

sextants, eighty years of     13 Nov 12

yachting sextant                    14 June 15

centres, as bearings     12 Mar 09

chassis, swing arm     12 Mar 09

of La Filotecnica sextant 5 Oct 10

CHO-M sextant. See SNO-M sextant

Chronometer, marine    See www.chronometerbook.com

cleaning sextant parts             18 Feb 10, 23 Nov 08

clearance, axial of worm     12 Mar 09

collimating rising piece     2 September 11

collimation, faulty     2 September 11

collimator      13 Deb 11

horizontal     13 Feb 11

Commission, US Maritime   15 Dec 10

Cooke and Son                       28 Apr 10

Coutinho Pattern Bubble sextant   6 May 16

Dip                                        5 Apr 2012

abnormal                  5 Apr 2012

meters                       5 Apr 2012, 23 June 2012

calibration of             5 Apr 2012

Dollond, John                           10 June 10

Dollond, Peter                           10 June 10

double sextant                                                  16 Jul

micrometer mechanism of                     16 Jul 09

handle of                                              16 Jul 09

mirrors of                                             16 Jul 09

optical paths of                         16 Jul 09

telescope of                                          16 Jul 09

distance meter,

Fiske     13 Nov 12

Fleuriais     4 Nov 12

Stuart      13 Nov 12, 13 July 12

drawings, of SNO-T mirror adjusting screw and bush 17 april 10

drill, slot, use of  17 April 10

error, index, adjustment            1 Oct 14

side, adjustment            1 Oct 14

perpendicularity, adjustment            1 Oct 14

errors, of worm     12 Mar 09

Entandrophragma cylindricum, see sapele

eyepiece

micrometer                                           22 Mar 09

monocular, disassembly                        18 Nov 09

positioning                                18 Nov 09

of C Plath’s earliest sextant          April 21 2017

fake C Plath sextants                   14 July 10

La Filotecnica Salmoiraghi        5 Oct 10

Filby, David      13 Nov 12, 21 April 2017

float glasss                                                        11 Feb 09

Florez, Luis de                                      30 May 09

Francis Barker ,sextant, small craft precision     14 June 15

frame,

        aluminium alloy          29 Sept 10, 13 Nov 12, 14 June 15

        bent     20 Mar 11

braced                                                            10 June 10

bronze                    29 Sept 10, 13 Nov 12

         curve bar pattern      28 Jan 10

          Dreikreis                                             24 Jan 10

of C Plath’s earliest sextant    21 April 2017

evolution of                                        29 Sept 10

of Freiberger Skalensextant                                   22 Mar 09

of La Fil0tecnica               5 Oct 10

plastic                                                      29 Sept 10

pressure die cast                                  26 Oct 08,  29 Sept 10

stiffness of Heath sextant                      28 Jan 10

of Plath sextant                        28 Jan 10, 14 June 15

                      of  tulip pattern                                                10 Nov 09

of US Maritime Commission sextant  15 Dec 10

Freiberger , yacht sextant              14 June 15, 10 Aug 11,

skalen sextant 4 Apr 10

drum sextant   22 Mar 09

Gavrisheff dip meter                                         5 Apr 2012

Gilbert and Company                                      10 June 10

glass, float                                                        11 Feb 09, 27 Jan 09

graduations, sextant, refilling                              18 Nov 09

grinding paste, for circular mirror     27 Mar 11

Hall, Chester Moor                                                    10 June 10

handle

detachable            1 Oct 14

kinematic mounting of       20 Mar 11

of double sextant     16 July 09

of Gilbert sextant   10 June 10

of La Filotecnica sextant 5 Oct 10

of Troughton and Simms sextant   28 April 10

of US Maritime commission sextant  15 Dec 10

of WWII C Plath sextant                     14 July 10

Heath curve bar sextant,

frame stiffness                          28 Jan 10

tangent screw                                      28 Jan 10

Heath

micrometer shaft bearings                      24 May 09, 5 May 09

rack                                                     24 May 09

swing arm bearings                               5 May 09

vernier sextant                                      23 Nov 08

Hughes

Admiralty pattern sextant, vernier    27 Mar 11,  24 June 11

micrometer    14 Feb 18

lamp switch assembly                            18 Feb 10

micrometer mechanism              18 Feb 10, 18 Feb 18

rack                                                     3 Jan 09

telescope mounting                               18 Feb 10

index arm

Ilon Industries Mark III sextant            1 Oct  14           14 Jun 15

index arm, bearing                             18 Feb 10, 22 Nov 08, 14 June 15

C Plath                                          14 July 10, 13 Nov 12, 14 June 15

SNO-M                                           14 July 10

US Maritime Commission    15 Dec 10

index mirror turntable                                10 June 10

sealed     24 June11, 14 Jun 15

joint, corner rebate                                           11 Aug 09

Keeper, index arm                 14 June 15

, swing arm                14 June 15

lacquer, see paint

lamp assembly, making new for Husun  18 Feb 10

latches, of case                                     18 Feb 10, 24 June 11

left-handed sextant                                            2 Dec 08

legs,

broken, mending                           7 Oct 16

turning new                                           24 Jan 10

placement of                                         6 May 09, 26 Apr 09, 2 Dec 08

lens,

auxiliary                                                30 May 09

recementing                                          24 Jan 10

swaging                                                24 Jan 10

achromatic, airspaced                   10 June 10

Leupold and Stevens see US Maritime Commission sextant, 15 Dec 10

level, spirit                                                        30 May 09

sensitivity of                                          30 May 09

light path, in Ilon Mark III sextant              1 Oct 14

lighting, of scale                                                30 May 09

limb, rivetted to frame                                  10 June 10

chromium plated                  5 Oct 10

logo, probable fake Plath                              14 July 10

of La Filotecnica Salmoirhagi       5  Oct 10

lost motion    See backlash

makers names                                                   10 Nov 09

mandrel, test     20 Mar 11

meter, dip, an improvised                 5 Apr 2012

Gavrisheff                                5 Apr 2012

Russian naval                        23 Jun 2012

micrometer

eyepiece                                         22 Mar 09, 13 Feb 11

sextant

bearing, by C Plath     20 Mar 11

bent         20 Mar 11

calibration of                                         6 July 09

drum, making new, dividing       20 Mar 11

numbering       20 Mar 11

evolution of                                           3 Jan 09

examination of                          6 July 09

index, bent          20 Oct 16

mechanism                          18 Feb 10, 14 June 15

of  Husun                                   3 Jan 09

of Ilon Industries Mark III            1 Oct 14

by Carl Plath                            3 Jan 09, 13 Nov 12

shaft, bearings,  by Heath                        24 May 09, 5 May 09

bent, repair of     20 Oct 16

by La Filotecnica     5 Oct 10

by C Plath     20 Mar 11

setting errors of     13 Feb 1

of US Maritime Commission sextant 15 Dec 10

version of Admiralty pattern sextant   15 Feb 18

worm, axial play in       14 June 15

Micrometre a reflexion, Fleuriais’     4 Nov 12

mirror,

adjusting screws, of SNO-T    17 April 10

auxiliary                                     30 May 09

backing of brass sheet,  15 Dec 10

blank, preparation of circular     27 Mar 11

bracket, Ilon Mark III sextant            1 Oct 14

bracket, SNO-T, repair of     17 April 10

Observator, repair of     20 Oct 16

bracket, C Plath casting numbers   14 July 10

circular, cutting       27 Mar 11

first surface            1 Oct 14

f latness of                                             27 Jan 09

grinding  edges                          11 Feb 09

horizon, C19 mounting assembly           10 Nov 09

Gilbert method of adjusting     10 June 10

Overcoated    13 Nov 12

Plath and SNO-M  compared  14 July 10

sealed  24 June11

index,              of Troughton and Simms sextant    28 April 10

,         , adjustment   14 June 15

adjustment by turntable      10 June 10

making new                                          11 Feb 09

removing silvering from             11 Feb 09

rounding corners                                   11 Feb 09

sealed                                                   6 May 09,  24 June 11

Mk IX A sextant restoration manual                  17 Nov 08

magnifier, scale, mounting in vernier sextant   24 June 11

monocular

eyepiece disassembly                           18 Nov 09

labyrinth seal of objective lens      20 Mar 11

La Filotecnica                     5 Oct 10

mounting,                                             18 Nov 09

Freiberger, boring                      18 Nov 09

marking out                  18 Nov

milling and shaping        18 Nov 09

squareness                                18 Nov 09

prismatic, in Ilon sextant            1 Oct 14

prismatic, making                                  18 Nov 09

removing objective                                18 Nov 09

removing prisms                                    18 Nov 09

separating halves                                  18 Nov 09

Tamaya etc                                           28 Jan 09

mounting, of Husun telescope                            18 Feb 10

of La Filotecnica telescopes     5 Oct 10

Navistar professional sextant     13 Nov 12

objective, removing from monocular                  18 Nov 09

octant, aeronautical, Mark I Mod 3                   2 Dec 08

O rings, Viton  13 Oct 10

Neoprene 13 Oct 10

paint

defective    15 Dec 10

for sextant finishing                                   24 Jan 10,10 Nov 09

photoluminescent                                  9 Dec 08, 26 Oct 08

wrinkle finish                                         18 Feb 10

Parallax, solar     4 Nov 12

Plath

Dreikreis sextant                          24 Jan 10, 13 Nov 12

Eighty years of

logo                                                      24 Jan 10, 14 July 10

optics of, post WWII     20 Mar 11

sextant frame stiffness                           28 Jan 10

sextant production, WWII                 14 July 10

shades adjusting tool                 6 Oct 13

tangent screw mechanism                      24 Jan 10,  24 June 11

telescope rise and fall mechanism          24 Jan 10, 24 June 11

polish, French                                                   24 Jan 10

Porro, Ignacio                              5 Oct 10

Poulin, Roger              29 June 14, 14 June15

pressure die casting                                           26 Oct 08,

working life of dies                                     14 July 10

prism, eyepiece, of AN5851                             9 Dec 08

Blish                             5 Apr 2012

Roof                            23 Apr 2012

removing from monocular                      18 Nov 09

protractor, three armed. See station pointer

quintant, vernier                                                30 May 09

rack,

Hezzanith                                              3 Jan 09

Hughes                                                 3 Jan 09

Heath                                                   24 May 09

Ilon Industries            1 Oct 14

C Plath Yachting sextant       14 June 15

radium

paint, decay of                                       9 Dec 08

half life of                                             9 Dec 08

release catch

of La Filotecnica sextant     5 Oct 10

of US Maritime Commission sextant   15 Dec 10

of C Plath yachting sextant         14 June 15

Repsold      13 Nov 12

rising piece

bent, repair of    28 Apr 10,      20 Oct 16

collimating,    2 Sept 11

faulty construction of Tamaya    2 Sept 11

structure of, in Admiralty pattern sextant  24 June11

of Carl Plath’s earliest sextant        21 April 2017

in Ilon Industries sextant            1 Oct 14

in La Filotecnica sextant      5 Oct 10

in Troughton and Simms sextant    28 Apr 10

Salmoiraghi, Angelo       5 Oct 10

sapele                                                               24 Jan 10

scales,

glass                                                     22 Mar 09

ivory                                                     10 Nov 09

screwcutting conical threads                              6 July 09

screws,

broken, removal of                    24 May 09, 17 April 10, 20 Oct 16

grub, replacement of                          4 Mar 10

pin    29 Apr 10

tangent, of  Troughton and Simms sextant    28 Apr 10

of C Plath’s earliest sextant      21 April 2017

Service, US Maritime    15 Dec 10

sextant

Admiralty pattern     27 Mar 11

calibration of         13 Feb 11

cleaning of                                            23 Nov 08, 18 Feb 10

double. See double sextant

fake                                             14 July 10

left handed                                            2 Dec 08

small craft            14 June 15

survey. See survey sextant

Yachting             14 June 15

shades,

assembly of horizon                              18 Feb 10

assembly of index                                 18 Feb 10

carrousel of AN5851                            14 May 09

remounting index                                   18 Feb 10

repainting                                              18 Feb 10

shellac, sealing bubble chambers with    20 Dec 08

secondary sealing bubble chambers with   13 Oct 10

Simms, William      29 Apr 10

Skalensextant, Freiberger                                  22 Mar 09

bearing of                                             22 Mar 09

dimensions of                                        22 Mar 09

optical path of                                       22 Mar 09

prism, cleaning of                                4 Mar 10

scale

lighting                                         22 Mar 09

focus screw, access to            4 Mar 10

focus of                                        4 Mar 10

SNO-M sextant (Navigation sextant with illumination, marine)

compared to C Plath                             26 Oct 08

origins of                                              26 Oct 08

telescope of                                          26 Oct 08

SNO-T sextant (Navigation sextant with illumination – tropicalised)

index arm bearing of                             22 Nov 08

instrumental accuracy of                        22 Nov 08, 13 Feb 11

micrometer of                                       22 Nov 08

mirror adjusting screws            17 April 10

use of                                               17 April 10

repair of                                          17 April 10

drawing of                                       17 april 10

origins of                                              22 Nov 08

position of index arm                             22 Nov 08

telescopes of                                        22 Nov 08

spring,

of tangent clamp                                    10 Nov 09

material of                                    10 June 10

of swing arm chassis     12 Mar 09

pre-load, of worm shaft     12 Mar 09,  15 Dec 10

Stanley, W F and Co, merger with Heath          26 Apr 09

station pointer                                                   26 Apr 09

survey sextant                                                   6 May 09, 26 Apr 09, 28 Apr 10

frame of                                                6 May 09

handle of                                              6 May 09

placement of legs                                  6 May 09

telescope mounting of                           6 May 09

use of                                                  26 Apr 09

swaging, of lens                                                24 Jan 10

swing arm

of Observator sextant       20 Oct 16

bearings, Heath                                5 May 09

chassis     12 Mar 09

Ilon Industries            1 Oct 14

keeper       14 June 14

switch, lamp, of Husun                          18 Feb 10

Tamaya, faulty rising piece,   2 Sept 11

tangent screw,

Heath                                                  28 Jan 10

Hezzanith endless                                 3 Jan 09

Hughes Admiralty pattern   24 June 11

of C Plath’s earliest sextant    21 April 2017

taper pin,

removal                                               18 Dec 08

replacement                                         18 Dec 08

sealing bubble unit with      5 Oct 10

telescope,

collimation of     2 Sept 11

making                                                23 Nov 08

Galilean, optics of                                 30 May 09

of C Plath’s earliest sextant     21 April 2017

of C Plath WW II sextant                  14 July 10

of C Plath yachting sextant           14 June 15

of Hughes Admiralty pattern sextant          24 June 11

of La Filotecnica Salmoiraghi sextant     5 Oct 10

of SNO-M sextant                               26 Oct 08

of SNO-T sextant                                 22 Nov 08

of US Martitime Commision sextant   15 Dec 10

Willson bubble                          2 Dec 08

telescope mounting

of survey sextant                                   6 May 09

Husun                                                   24 Jan 10

telescope rise and fall mechanism, Plath 24 Jan 10

Admiralty pattern  24 June11

thread, interrupted of telescope mounting     5 Oct 10, 24 June 11

tool, shades adjusting, Plath           6 Oct 13

Troughton, Edward    28 Apr 10, 10 June 10

Troughton, John     28 Apr 10

US Maritime Commission  15 Dec 10

US Maritime Service   15 Dec 10

US Navy Bureau of Ships   Dec 10

vial, of spirit level                                              30 May 09

washer, Belleville                                              18 Feb 10

lead, for sealing filling hole in A10 bubble unit  13 Oct 10

making                                                 11 Aug 09

Wild, Heinrich                                                  22 Mar 09

Willson bubble telescope                                  2 Dec 08

Wollaston, William                                10 June 10

worm

bent shaft of                                         6 July 09

burrs on                                               6 July 09

cutting new                                          6 July 09

errors     12 Mar 09

Hughes                                                3 Jan 09

Ilon Industries           1 Oct 14

periodic error of             6 July 09

progressive error of                              6 July 09

xylene,

effects on plastic                                   18 Dec 08

refilling bubble chamber with ,              18 Dec 08, 13 Oct 10

resistance of Viton to, 13 Oct 10