The Type III azimuth circle which has been a service stand-by for many years, plays an important part in navigation and is so well known that no description is considered necessary. Although sufficiently rugged to withstand ordinary use, accidental knocks or misuse will cause derangement. When out of adjustment the fault is difficult to determine and rectify with the facilities found on board ship. An error in the circle fittings affecting azimuths, affects the course laid by the amount of the error. As an example of this relation the experience of a certain ship is given. This ship while on a long voyage had a consistent, unaccountable set of about ten miles a day. As day after day elapsed with a set of approximately the same amount, it was suspected that the azimuth circle was out of adjustment. Comparison with other circles developed the fact that none of them agreed. Later when there was an opportunity to have the circle checked it was found to be out a degree and a half, which error almost exactly accounted for the set experienced.
Many navigators send azimuth circles to the Naval Observatory or to a navy yard instrument shop for checking when an opportunity is afforded. Some repair ships are now equipped to adjust circles. Were it possible to make a positive check on board ship, much of this work could be avoided.
The commonly accepted test is to check an azimuth by the sun mirror against a direct azimuth with the terrestrial vanes. This also is the instrument shop test, but when conducted on board ship it is not conclusive as it requires the presumption that the terrestrial vanes are in adjustment. It is true that the vanes themselves are not so subject to derangement as the sun-mirror prism arrangement, but the test usually requires the dark mirror to be used and the latter is more subject to derangement than any other part of the circle. At an instrument shop the adjustment of the terrestrial vanes and the dark mirror can be checked and the subsequent sun-mirror check is then conclusive.
As very few officers are familiar with the procedure of adjusting an azimuth circle, a description of the method used at the instrument shop, Navy Yard, Mare Island, and at the Naval Observatory may be of interest.
On first reading, the description will appear to be very technical, but any reader who is sufficiently interested to break out an azimuth circle for reference while reading will find that the procedure is easily understood.
Special Appliances Required in Making the Adjustment
The appliances used in making the adjustment are a table, a plumb line, and a small, specially fitted telescope. The table consists of a composition disc about 3/8-inch thick with a diameter exactly fitting the inside of an azimuth circle. Two diametrical lines at right angles are scribed across the upper face of the disc. Near the outer ends of each line the disc is cut through over an area of about a square inch. Under these openings horizontal plates slightly larger than the openings are secured to small studs screwed into the bottom face of the disc. The distance from each plate to the upper face of the disc is the same as from the card to the upper face of the bezel ring on a Navy type 7 1/2-inch magnetic compass. The top face of each plate is painted white and has a black index line which lies in the vertical plane through the diametrical line passing across the opening over it. The disc is fitted with a bubble level and is pivoted on a small stand equipped with four leveling screws and a clamp. The arrangement is very similar to that of a theodolite azimuth plate. The table can be used on any fairly level surface, but for convenience is mounted on a tripod.
The plumb line should be about six or eight feet long and may be made of coarse black thread. It should be so hung as to have the sky or a well-lighted, white-painted surface behind it.
The telescope is a sextant star telescope equipped with a clamp bracket so installed as to be vertical to the optical axis and of such size that it can slip over the sun-mirror frame.
Examination and Repair Preliminary to Adjustment
A circle under repair is first critically inspected to determine that it is not bent or otherwise deformed. It is then examined to see if it “quarters” correctly. This is done by placing it on the disc and ascertaining that the 80°, 180°, and 270° marks (there is no o mark) register accurately with the diametrical lines. At the same time see if there is any sidewise shake on the disc. If the marks do not register properly, the center of the circle is not coincident with the center of the disc and correction must be made by an appropriate alteration to one or more of the wearing lugs secured on the inner circumference of the circle flange. If there is any shake, examine the spring clip to determine if there is the proper tension. If so, the trouble is worn lugs which will have to be renewed. While making the above inspection level the disc and see if the level on the circle is correct. If it is not in adjustment, insert the necessary shims to correct the error.
The fittings are then carefully inspected for dirt, corrosion, stripped screws, broken or foggy mirrors, badly chipped prisms, etc. Occasionally prisms are a trifle under size and do not fit the prism boxes neatly. Cases have been known where the defect was overcome by the use of removable shims. This is not good practice because when the prisms are removed on board ship for cleaning, the shims are liable to be lost or incorrectly replaced. It is best to discard such prisms. If they have to be used, the shims should be of metal soldered in place. Defective parts are renewed after all parts of the circle have been thoroughly cleaned. A complete overhauling at an instrument shop will include repainting the black parts if necessary. It is almost always necessary to replace the vane wire as very few circles come in with a properly stretched wire. This replacement is not difficult after the knack is acquired of getting the wire taut, but not so taut that it breaks when the screw is set up.
Dark mirrors are frequently broken or thrown out of adjustment. Carelessly slamming a pelorus or binnacle cover in place before folding the mirror out of the way is the most usual cause. To replace, take out the four screws in the frame cover. Several strips of paper packing usually will be found between the cover and the back of the mirror. Note their positions. When the new mirror is installed replace the packing strips as they formerly were located. If they are in bad condition replace with new ones made from paper of similar thickness. The use of these strips is an approximation only and may have to be altered as described later. In no case use strips of such thickness that undue pressure is necessary to draw the cover fully down all around.
The sun mirror usually is so foggy that replacement is necessary. This mirror is ground with a cylindrical surface of such radius that the reflected image of the sun strikes the face of the prism box as a vertical beam of approximately the same width as the slot in the box. Examine the new mirror to make sure it is silvered on the convex side; otherwise it will not serve its intended purpose. Take off the mirror frame cover and put the new mirror in place. Pack it along the edges and across the back so that it cannot shift in its mounting. Use strips of hard-surfaced paper. Blotting paper will not do as it tends to absorb moisture which will quickly deteriorate the silvering. Mirrors are seldom ground exactly alike and it is not advisable to substitute a new mirror on board ship unless there is some means of checking the adjustment. Put the cover in place and set down the screws. The cover must go home without undue pressure being required and yet have the mirror held firmly in place. If the mirror can shift even slightly the circle will be thrown out of adjustment for taking sun azimuths.
There are four fittings which pivot on horizontal axes: the sun mirror, the dark mirror, and the two terrestrial vanes. They must pivot with sufficient friction to remain in whatever position they are left when finger pressure is removed. It is very exasperating to use a circle with a floppy fitting which requires one hand to hold it in place and it is next to impossible to use a circle with any degree of accuracy unless both hands are free to manipulate it in azimuth. The pivot screws screw into the movable fittings and turn in the mountings. With this method of securing, setting down on the screws has no effect on the tightness of the fittings and excessive pressure will only result in stripped screws or broken screw heads. To make the fittings pivot tightly, remove the screws and close the holes in the mountings slightly by squeezing lightly with pliers. On some circles the ends of the mountings are split as far as the holes, giving a spring grip as well as making it easier to close the holes when necessary.
After the foregoing has been done the circle is ready for adjustment.
Adjustment of the Terrestrial Vanes
Step 1.—Level the disc and place the circle on it. Turn the circle until the line of sight through the terrestrial vanes intersects a plumb line hung eight or ten feet away and set up the disc clamp. If the sight slot follows the plumb line, the rear vane is in a plane perpendicular to the plane of the circle. If the slot does not line up properly, bring the vane into adjustment by placing appropriate shims between its mounting and the circle. This vane is seldom found out of adjustment.
Step 2.—Prove in a similar manner whether or not the wire in the front vane is in the perpendicular plane and correct if necessary by means of shims between its prism-box mounting and the circle. This is seldom found necessary. It will be noted that the mount for the dark mirror and the front vane is apparently secured to its prism box by one small screw. In addition there are two dowel pins which cannot be seen unless the parts are disassembled. They keep the parts in alignment while the screw serves to hold the parts together. If the parts are disassembled for any reason, care must be exercised that the dowel pins are in place before reassembling.
Step 3.—Look through the rear-vane peep hole and move the dark mirror until the reflection of the 180° mark on the circle can be seen. The front-vane wire should line up with this reflection. If it does not, shift the mounting of the front vane about its vertical axis until the correction is made.
Step 4.—Line up the plumb line with the front-vane wire and the rear-vane peep hole. Move the dark mirror until the reflection of the front-vane wire can be seen. This reflection, the plumb line, and the wire itself should all closely coincide. If they do not it is necessary to adjust the dark mirror in its frame. This is done by taking off the frame cover and adjusting the paper packing strips until the required coincidence is obtained. Then recheck the 180° reflection (Step 3). It probably will be necessary to shift the front sight mounting to bring the 1800 reflection and the wire together. Again check on the plumb line. There should not be much error, but if there is it can be reduced gradually by repeating both corrections as often as necessary.
Step 5.—Turn the circle on the disc until the 180° mark registers with one of the diametrical lines. Look through the rear- vane peep hole at the reflection in the front- vane prism. If the index line on the disc and the index wire of the circle do not coincide, shift the latter to the correct position. This step completes the adjustment of the terrestrial vanes.
Adjustment of the Sun Mirror
Step 6.—Place the circle on the leveled disc and clamp the small telescope to the sun-mirror frame. Orient the circle in azimuth until the cross in the telescope coincides with the plumb line. Then revolve the mirror on its axis through an arc of about 45°. The cross should follow the plumb line. If it does not, it proves that the axis about which the mirror pivots is not parallel to the plane of the circle, i.e., one standard has been bent so it is not the same height as the other. The fault is remedied by bending one of the standards until the proper coincidence is obtained.
Step 7.—Try the side of the sun-prism box with a small square. If not vertical to the plane of the circle, correct by loosening the screws and moving the box on its vertical support. This adjustment is seldom necessary.
Step 8.—This step should be undertaken when the sun is visible and comparatively low. A strong electric light can be used with fair satisfaction, but the sun is so much better that it is advisable to wait for it if necessary. Place the tripod and table in the sun. Level the disc and put the circle on it with the terrestrial-prism index wire registering with one of the disc index lines. Use a shade glass to protect the eye and turn disc and circle together until the center of the sun is sighted through the terrestrial vanes. The dark-mirror reflection of the sun must usually be used as the sun seldom will be of sufficient strength to be of use in the sun mirror if low enough to be sighted directly through the terrestrial vanes. Clamp the disc and turn the circle clockwise on it until the terrestrial-prism index wire is in coincidence with the disc index line go° from the one first used. With the sun mirror throw the reflected beam on the sun- prism box. If in adjustment, the beam will impinge directly on the slot and be reflected to the disc index line below it. The adjustment seldom will be correct on first trial. If the reflected beam strikes the slot at an angle, the fault is due to the mirror being badly shifted in its frame, mirror standards bent, a poorly ground mirror, or the prism box off perpendicular. The fault will have to be determined and corrected. If the beam strikes parallel to, but to one side of the slot, loosen the two screws which secure the sun-mirror mounting to the circle and adjust the mount until the beam enters the slot. Set up the screws lightly and repeat the observation with the terrestrial vanes and sun mirror, adjusting the mirror as necessary, until an absolute check is secured. Then set the screws up firmly and make a final check. As the sun is moving more or less rapidly in azimuth all the time, depen4ing on the season of the year, the work described above must be performed with as little loss of time as possible between the observations by the two methods. If the reflected beam from the sun prism does not strike the disc index line radially, loosen the two screws which secure the sun-prism box to the circle and move the box slightly until the reflected beam and the index line are parallel. Set up the screws lightly. Then loosen the two screws which hold the prism box to its vertical support and move the box slightly until the index line is bisected by the reflected beam. It may be necessary to repeat these two operations several times until the final result is satisfactory. Then set up firmly all the screws involved. The adjustment of the sun mirror may have been affected by the prism adjustment. Its adjustment should be rechecked and readjustment made if found necessary. The above completes the adjustment of the sun mirror and the circle.
Testing and Adjusting with Ship’s Facilities
The tests and adjustments described above can be made with a 7 1/2-inch magnetic compass taking the place of the special table. The work must be done ashore with the compass blocked up so it is held in a level position. The fitting for a star telescope can be made on board ship. The procedure in principle will be the same as with the table. While the work is in progress due care must be exercised to prevent disturbance of the compass by movable magnetic material in the vicinity, especially on the person of the adjuster.
Approximate Tests
The dark mirror is not liable to shift in its frame if properly packed when assembled, but the frame may easily be bent or the whole mount be thrown out of alignment by misuse. If there is no reason to think that the frame is bent or the mirror itself is out of adjustment, the test of the reflected image of the 180° mark coinciding with the vane wire is proof of the terrestrial-vane adjustment.
A small straight-edge placed against the sides of the sun-prism box should intersect graduations on the opposite side of the circle equally distant from the 90° mark.
The perpendicularity of the fittings can be tested with a small machinist’s square.
When viewed over the middle of the top of the sun-mirror prism box, the reflection of the slot should appear in the vertical center line of the sun mirror. When the mirror is rotated slightly the reflection should travel along the center line. If the mirror is symmetrically ground this is an almost absolute test when made by one having an accurate eye; otherwise a considerable element of personal error will affect the result. If in doubt as to the accuracy of the estimate, lay off the center line of the mirror and mark it with water paint or soap and improvise a peep hole over the center of the prism box. Unfortunately many mirrors are not perfect and the test in such cases is only an approximation. When a circle has been placed in adjustment, it is well to make the above test and record the result. As long as the same mirror remains installed, subsequent tests can be checked against the recorded data and will be of value.
Hints on the Use of the Circle
A little lubrication on the edge of the compass or repeater will make an azimuth circle turn smoothly.
At night, bearings of distant objects or lights cannot be taken with the degree of accuracy that is possible with day-time observations, especially when a light with a short period of illumination is being observed. It is very difficult to get any result when the slot or peep hole in the rear vane is used as a great number of the already limited rays of light are cut off. Nor can the vane wire be seen unless considerable illumination is used. This illumination interferes with the observation. Equally good, or at least sufficiently good, and much more rapid results can be obtained by sighting over the vanes. It is a great help to turn the front vane in slightly so its top can be seen when only dim illumination of the compass or repeater is used.
Accurate azimuths of the sun when the ship is rolling or pitching are rather difficult for the novice. Practice develops skill and it is wise to get the knack before sometime being unexpectedly required to show expertness. The higher the sun the more difficult it is to obtain an accurate observation, as slight errors in holding the compass level have a greater effect.
Formerly it was the practice to equip storage boxes for circles with wooden blocks recessed to hold one or more of the fittings. These are being removed when received at an instrument shop for overhaul, as it has been found there is danger of the fittings being forced out of position when a circle is carelessly placed in a box so fitted. In their place two blocks are installed to grip the outside of the circle. Each block has a pivoted wooden button to clamp the circle.
When using a circle, do not turn it by exerting pressure on any of the fittings other than the three studs provided for the purpose.
Keep circles clean and do not let them get wet if it can be avoided. All dampness should be dried off as soon as possible. If corrosion is allowed to progress unchecked it will be difficult to take the prisms out for cleaning.
Do not allow any parts except the prisms to be disassembled for cleaning. Should other cleaning or adjustment be required on board ship, the person undertaking it should do so with full knowledge of the difficulties involved.
Remember that the fittings are secured in place by the clamping effect of light screw heads and are easily knocked out of adjustment if the circle is not handled carefully. The fine threads of the screws are easily stripped if undue pressure is used. Should occasion arise to make repairs on a circle, use screw drivers which fit the slots in the screw heads.
Different classes of ships have different numbers of circles allowed them. When the allowance is large enough to permit doing so, one circle known to be in perfect adjustment should be locked up and only used as a standard of comparison for the other circles. Defective circles can be very easily brought in adjustment if a standard is available. On every ship the navigator should reserve one circle for sun azimuths exclusively and permit no other person to handle it. If in adjustment originally, this careful use should preserve it unchanged.