How to Adjust & Calibrate an Optical-Tooling Level

To Complete an Adjustment. In order to prevent an adjustment from being easily disturbed, the screws or nuts used must be firm when the adjustment is complete. Accordingly, the last movement in adjustment must always be to tighten a screw or nut when two screws or nuts oppose each other. When a screw is opposed by a spring, the last movement should be clockwise (i.e., against the spring).

After the final adjusting movement, the adjustment must be checked by the required test.


1. Target. A properly designed device that marks a single point.

2. Scale. When an optical micrometer is used, an optical-tooling scale. Otherwise an engine-divided scale like a high-grade drafting scale with graduations at about 3D-second intervals, as measured from the instruments being tested. With these, values to the nearest 3 seconds may be estimated.

3. Far target. A target at least 50 ft, preferably more, from the instrument under test, or the crosshairs of a collimator reticule.

4. Far scale. A scale at least 50 ft, preferably more, from the instrument under test, or the tilt reticule of a collimator.

5. Near target or scale. A target or scale almost as close to the instrument as the minimum focus permits.

6. High target or scale. A target or a horizontal scale at any convenient distance placed at a vertical angle of about 45° above the horizontal, measured at the instrument. (The reticule of a collimator must usually be used.)

7. Low target or scale. A target or a horizontal scale at any convenient "distance placed at a vertical angle of about 45° below the horizontal, measured at the instrument. (The reticule of a collimator must usually be used.)

8. Direct. With the telescope in its normal position.

9. Reversed. With the telescope turned upside down.

10. To traverse. To turn the line of sight left or right around the azimuth axis.

11. To transit. To turn the line of sight all the way over around the elevation axis.

12. To rotate. To turn on an axis that nearly coincides with the line of sight.

13. To aim at a collimator. When any line of sight is aimed at a collimator, it should pass through the objective lens of the collimator near its center, as well as appear to coincide with the collimator crosshairs. This eliminates the possibility of inaccuracies due to lens aberrations. Accordingly, part of aiming an instrument at a collimator consists of approximately bucking it in to meet this requirement. This can be tested by first aiming at the crosshairs of the collimator and then focusing on the displacement target which is just behind the lens. If the collimator has no displacement target, crossed threads can be arranged in front of the objective to indicate its center.

14. To aim a collimator. If a collimator has an eyepiece, it can be aimed parallel to any telescopic sight by bringing its crosshairs on those

Object 1. To make the bubble of the circular level center when the azimuth axis is vertical.

Test. Center the bubble with the leveling screws. Traverse approximately 180°. The bubble should remain centered.

Adjustment 1. Bring the bubble halfway toward the center by adjusting the vial.

Geometry. (See Fig. 15-11.) If the circular level is out of adjustment, when the bubble is centered, the azimuth axis is inclined from the vertical by the amount of the error. When the instrument is turned 1800 on the azimuth axis, the inclination of the axis is added to the error of the circular level. The error is therefore eliminated when the bubble is moved halfway toward the center.


Object 2. To make the horizontal crosshair lie in a plane that is perpendicular to the azimuth axis. This adjustment can be made on some levels. It disturbs all other adjustments except adjustment 1.

Test. Aim the telescope at the target by using the leveling screws and the tangent screw. Traverse the telescope slowly with the tangent screw. The horizontal crosshair should remain on the mark.

Adjustment 2. The reticule must be rotated in the direction it apparently should be turned. This is true for either an erecting or an inverting telescope because any objective lens system merely rotates the image 1800 and thus does not change the direction of rotation.

On most instruments, loosen two adjacent adjusting screws and tap them lightly to rotate the reticule. Tighten the same screws. Figure 15-9 shows the usual method of supporting the reticule.

Neutralization. In making an observation, use only the part of the horizontal crosshair that is near the vertical hair.


Object 3. To test the run of the micrometer.

Test. Set the micrometer at the end of its run (usually -100). With the leveling screws, sight a graduation on a near scale placed in a vertical position. Turn the micrometer to 0 and to +100, successively. The scale readings should be 0.100 and 0.200 less, respectively.

Object 4 Preliminary. To make the axis of the bubble tube lie in the same plane as the line of sight.

Test. Level the instrument. With the leveling screws, tilt the instrument so that the center of the circular bubble is at the right-hand side of the centering circle. Recenter the tubular bubble. Read a near and a far scale graduated in the same units and compute the difference in the readings.

Repeat with the instrument tilted so that the circular bubble is on the opposite side of the circle. The difference in the readings should be the same.

Adjustment 4. Most instruments have screws which move one end of the tubular bubble laterally. Adjust with these screws until the difference in the readings is the same.

Neutralization. Always center the circular bubble with great care.

Object 4. To make the main bubble center when the line of sight is horizontal.

This test and adjustment can be made by the so-called "peg" method which requires no collimator, and by two methods with a collimator, depending on whether or not the collimator can be leveled.

The Peg Method for Object 4

1. Establish two optical-tooling scales held vertically on two firm supports about 100 ft apart (Fig. 15-12).


2. Set up the level equidistant from A and B. Level and read A. Turn the instrument toward B, relevel, and read B. Assume A = 7.538 and B = 7.213, as in the figure. If there is an error in the instrument, the line of sight will always slope at the same angle when the bubble is centered. Therefore it will rise or fall the same amount in the equal distances. Thus the two readings at A and B must be at the same height, and any two points on the scales at A and B which have the same height must have readings which will differ by the same amount, in this case 0.325:


This is the value which, when subtracted from a reading on A, will give that reading on B which is at the same height as the A reading.

3. Set up the instrument as near one of the scales as possible and take a reading on it (A' = 5.647). If the instrument is in adjustment, it ought to read 5.322 on B:


4. Take a reading on B. Assume it is B' = 5.391. Evidently the line of sight slopes upward, since this reading is too great. The problem now is to find a reading on B which is at the same height as the instrument. Measure the distances from the instrument to each of the two scales. Assume that the distance to A is 20 ft and to B, 80 ft. From the figure a = b = c and, by similar triangles,


which is the reading on B that is at the same height as the instrument.

5. Aim the line of sight at this reading (5.299). To do this, set the micrometer at zero. Aim approximately at 5.299 with the tilting screw. Turn the micrometer toward 5.2 on the scale until the drum reads + 99. Aim accurately at 5.2. Adjust the bubble until it is centered. The same principle applies if the line of sight slopes downward. A downward slope is indicated when the reading on B' is too small. Assume that it was 5.229, as shown in the figure. The arithmetic is shown below.


It is evident that if permanent supports can be found for the scales, they can be adjusted so that the readings on both scales will be the same for all points of equal height. This reduces the required arithmetic.

With this arrangement, all arithmetic can be eliminated by setting up the level as near one scale as possible and adjusting the bubble until, when it is centered, the readings on the two scales are the same.

Method for Object 4 with Collimator Equipped with a Leveling Device. First the collimator must be accurately leveled.

Test. With the tilting screw and by traversing, aim the level at the collimator. The bubble should center.

Adjustment. Adjust the bubble until it centers. It is evident that once the collimator has been leveled, it can be used as long as its leveling device indicates that it is level.

Method for Object 4 with Two Collimators without Using a Leveling Device. This is illustrated in Fig. 15-13. Two collimators are so placed that they aim at each other. The level is set up between them.

Test. Level the level instrument and aim a collimator at it. Traverse the level to the other collimator, relevel the level, and aim the second collimator at it. Remove the instrument. The horizontal crosshairs of the two collimators should coincide.

Adjustment. Aim the two collimators at each other by taking up half the difference with each. Replace the level, aim it at one of the collimators, and center the bubble by adjustment. Check on the other collimator after releveling. It is evident that once the two collimators have been properly aimed, they can be used as long as they remain aimed at each other.


Object 5. To test the effect on the height of instrument of errors in centering the circular bubble.

Test. Place two optical-tooling scales in vertical positions, facing each other, held in firm supports, and separated by slightly more than twice the minimum focus of the level instrument. Set up the instrument halfway between them. Level the instrument so that the circular level bubble is just tangent to the circle on the vial at the point that is nearest one of the scales (scale 1). Aim at scale 1, center the main bubble, and read the scale. Aim at scale 2, recenter the main bubble, and read the scale. Find the difference in the two readings.

Now relevel the instrument so that the circular level bubble is tangent to the circle on the vial, but at the point that is nearest scale 2. Read both scales as before, center the main bubble for each reading, and find the difference in the two readings. Their difference should be the same as the difference previously determined.

If there is a discrepancy between the differences, it means that there will be errors in the work with this instrument because the tilt axis does not intersect the azimuth axis.

Excerpt From:

Kissam, Philip, Optical Tooling for Precise Manufacture and Alignment, New York, NY: McGRAW-HILL BOOK COMPANY, INC., 1962