Removing Error

Courageous seamen can often be reduced to quivering jelly at the thought of “violating” the sacred factory adjustments of their sextants. Time and use take a toll on sextant alignment, and the procedures necessary to remove adjustable sextant errors should not be a source of terror for the instrument’s owner. Adjustments are not difficult to execute on a modern sextant, nor do they require an engineering degree to undertake. In fact, the act of eliminating such errors can actually offer the owner an insight into the clever technical design of the Hadley/Godfrey sextant.

Any sextant can contribute at least seven possible sources of error to a sailor’s quest for his position at sea. Three of these centering error, prismatic error, and graduation errorare inherent in the construction of the sextant. Centering error is caused by the index arm not pivoting at the exact center of arc curvature. Prismatic error is caused by the ground surfaces of the filters or the mirrors not being mutually parallel. Graduation error is the result of inaccuracies in the machining of the arc, micrometer drum, and vernier of the sextant. Since there is little a sextant owner can do to get rid of these three unadjustable errors, they will not be discussed further. If you are fortunate to own a modern, quality sextant, these errors will probably be so small that they will cause no problems.

Before dealing with adjustable errors the reader should clean his sextant optics. Just touching the index or horizon mirror during cleaning may affect adjustment, so the most sensible approach is to clean these optical surfaces before proceeding. It is a good idea to purchase an inexpensive lens cleaning kit (available at most camera shops) and keep it with the sextant. Use new sheets of lens tissue for each surface cleaned. Apply a few drops of lens cleaning solution to several tissues which have been shaped into a ball. Clean the surface of a mirror, telescope element, or shade with a light, circular motion. If a surface is heavily encrusted, change tissues often to avoid scratching. Use new tissues for each surface.

Adjustable sextant errors are present when:

1. The frame of the sextant and the index mirror are not perpendicular to each other (called perpendicularity error).

2. The frame of the sextant and the horizon glass are not perpendicular to each other (called side error).

3. The index mirror and the horizon glass are not parallel to each other at 0° (called index error).

4. The telescope is not parallel to the frame (called collimation error).

Adjustable error checks should be carried out using a sea horizon. The horizon should have a sharp, well defined separation of sea and sky and the sextant’s telescope should be sharply focused. Error adjustments should be made in the following order.

Perpendicularity error

To confirm the perpendicularity of the frame and the index mirror, fold the shade glasses out of the way and move the index arm until it reads about 40°. Set the sextant on a stable table and look into the index mirror in such a way that you can see both true and reflected portions of the arc. If the two portions of arc coincide, the index mirror is perpendicular to the frame of the sextant and no adjustment is necessary. If, on the other hand, the reflected image appears to be above or below the true arc, adjustment is necessary.

Locate the small wrench or screwdriver supplied with your instrument and, with the sextant still resting on the table, place the wrench or screwdriver on the index adjustment nut or screw located on the rear of the index mirror frame. Slowly turn the adjustment point clockwise or counterclockwise until the arc and its reflection are brought into alignment. Your first adjustment is complete. (Your instrument may also have two smaller screw adjustments on the rear of the index frame. These normally need not be touched except during mirror replacement.)

Side error

To confirm the perpendicularity of the horizon glass to the frame of the sextant, set the index arm at zero and look through the telescope at a sharp sea horizon. Rotate the micrometer knob until the true and reflected images coincide. Then rotate the frame 45° to the right or left around the telescope axis. If the reflected and true horizon lines remain continuous during rotation, no adjustment is necessary. If, however, the reflected and true horizon lines separate as the frame is rotated about the telescope axis, the horizon glass and the frame are not perpendicular (side error is present) and adjustment is required.

You will note that there are two adjustment screws or nuts on the rear of the horizon glass frame (there may be a third, smaller screw that is used during glass replacement and need not be touched). The adjustment screw that is most distant from the sextant frame is the one you must use. Gently turn this screw clockwise or counterclockwise about an eighth of a turn and then again rotate the frame about the telescope axis while looking at a sharp sea horizon. The discrepancy between the true and reflected horizons will increase or decrease as the screw is rotated. Turn the adjusting screw in the appropriate direction until the real and reflected images coincide when the frame is rotated about the axis of the telescope. When this condition is reached, the horizon glass is perpendicular to the frame of the sextant.

Index error

This error is removed after the first two (perpendicularity) adjustments are made. Index error is expressed in minutes of arc and can be either positive or negative in value, depending upon whether the index indicates a value above or below 0° 0′ when the sextant is set to zero.

To confirm that the index and horizon mirrors are parallel to each other, set the index to 0° 0′ and look at a sharp sea horizon through the telescope. Turn the micrometer drum until the true and reflected horizons coincide. Then read the arc. Ideally, the index will indicate 0°, but in reality it rarely does. If the index indicates a negative value (something below 0°), the index error is called an “off the arc” error. If the index indicates a positive value (something above 0°), the index error is called an “on the arc” error. Off and on the arc errors of 1.5′ or less need not be removed. These small errors are normally accounted for by an index correction factor when working a sight (e.g., an index error of -1.2′ is corrected by applying an index correction of +1.2′ to your sextant altitude). When determining index error, a series of observations should be made, approaching coincidence of the real and reflected horizons from opposite directions (bringing the reflected horizon alternately up to and down to the real horizon).

To adjust the index error as close to 0° as possible, set the index to 0° 0′. Look at a sharp sea horizon through the telescope and note any discrepancy between the true and reflected horizons. Turn the horizon glass adjustment screw (the large screw nearest the frame of the sextant) until the true and reflected horizons coincide. Turn the micrometer drum until the true and reflected horizons separate and then bring them together again in coincidence. Read the index again. You will probably find that you are now within the plus or minus 1.5′ target. Since both the second and third adjustments involve positioning the horizon glass, side error should again be rechecked and adjusted if necessary, followed by a repeat index error check. Don’t be surprised if you must repeat adjustments for side error and index error three times.

Collimation error

An error in collimation occurs if the telescope is not parallel to the frame of the sextant. Collimation errors occur less frequently than the first three errors, but when they do occur, higher than normal sextant altitudes are measured.

To confirm that the telescope is parallel to the frame, set the sextant on a stable table and sight along the frame of the instrument toward a wall. Choose a distance from table to wall that will allow the telescope to focus sharply. Have a friend hold a ruler against the wall and guide her in bringing the ruler edge coincident with the plane of the sextant frame. When this is achieved, have her draw a pencil line at the ruler edge.

Then, with the ruler, have her place another pencil line on the wall, parallel to the first but above the first, and at the same distance from it as the distance from the center of the telescope to the frame of the instrument.

If the telescope is parallel to the frame, the second line will appear to be in the center of the field of view of the telescope. If not, a non-parallel condition exists. If the sextant has collar adjustment screws, keep the sextant stationary while looking through the telescope, and gently adjust the collar screws until the second line moves into the center of the telescope’s field of view. If your sextant does not have an adjustable telescope collar, then the collimation adjustment cannot be carried out.

One word of caution concerning some older sextants: these instruments may incorporate now obsolete “opposed screw” adjusters to move horizon glasses and index mirrors during alignment. If you have one of these antique beauties, you must release surface pressure on one screw before tightening the opposing screw to prevent damaging the glass surface. The procedures for removing adjustable errors remain the same.

So there you have it. A painless method (well, almost) to tune up your most reliable navigation instrument by eliminating its adjustable errors. (All of these procedures even work, for the most part, on plastic sextants.)

You can now join those select few who, like circumnavigators and singlehanders, have dared to test their personal limits. A well-tuned sextant will reward you with more accurate sextant altitudes; the first step to faultless celestial navigation.

Gregory Dill is an avid celestial navigator and marine antiquarian who lives in Dartmouth, Nova Scotia.

Categories: Navigation