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the telescope level by the levelling screws g. The horizontal plates and y will then be horizontal and the vertical axis vertical.

3. Focussing the Object Glass and Eyepiece, or as it is called, adjustment for parallax. The foci of the object glass and eyepiece glasses must be made to coincide with the cross hairs of the diaphragm. To adjust the eyepiece b, look through the telescope, and push the eyepiece out or in until the cross hairs are seen clearly and distinctly. Then direct the telescope to some well-defined object, and by means of the milled-headed screw at the side of the telescope move the object glass a out or in until the image of the object is seen clearly and apparently coinciding with the cross hairs of the diaphragm.

The adjustment of the eyepiece need only be made once, but the object glass must be focussed for each new object sighted on at different distances. A good test for parallax is to move the head slightly from side to side. If the adjustments are correctly made, the image will appear to coincide steadily with the cross hairs; if not, it will appear to move from side to side, as the head is moved. It is to be corrected by pulling the eyepiece out or in and focussing the object glass as already described.

PERMANENT ADJUSTMENTS.

1. Adjustment of the Supports of the Horizontal Axis of the Telescope.--The object of this adjustment is to make the horizontal axis of the telescope truly level when the instrument is levelled up, so that the telescope may revolve in a truly vertical plane.

Direct the telescope to a well-defined object of considerable altitude, such as the finial on the top of a church spire, the theodolite being placed close to the bottom of the church. Direct the cross hairs on to the top of the spire, clamping the horizontal plates and y, Fig. 52. Now depress the telescope to the bottom of the church, and there mark a point bisected by the cross hairs. Unclamp and reverse the position of the supports by turning the instrument through 180° horizontally or in azimuth, and again direct the cross hairs to the top of the spire. Clamp again, adjust exactly on to the top of the spire, and see whether the cross hairs still cut the same point previously marked on the bottom of the church when the telescope is depressed. If not,

mark where they cut, and also mark the point midway between the two marks. This middle point is the correct point in the same vertical plane with the top of the spire. Correct quarter of the total deviation between the first two marks by the screws ss at the top of the support and quarter by the tangent screw d' of the horizontal circle, thus bringing the cross hairs on to the middle point. Repeat the operation until the middle point and the top of the spire are both bisected in both positions of the supports, when the adjustment will be correct. Here we observe that the collimation line is again directed exactly on to the top of the spire before being directed to the bottom of the church for the second time. No error other than that due to the supports is therefore introduced, the telescope being merely depressed. This adjustment may also be tried by seeing if the cross hairs bisect an object of considerable altitude and its image as seen reflected in the horizontal surface of a fluid.

2. Adjustment of the Collimation Line.-The adjustment of the line of collimation consists in making it exactly perpendicular to the horizontal axis about which the telescope rotates. Let the cross hairs be directed on to some well-defined distant object P, and bring them to coincide exactly with it. In Fig. 158, let aob represent the horizontal axis of the telescope, and cod the collimation line directed as described on to the point P. If now the collimation line be perpendicular to the horizontal axis, when the telescope is lifted out of its bearings and replaced with the ends of the horizontal axis ab reversed into the position ba, Fig. 159, the collimation line cd will still be directed on to P. If, however, the collimation line cd be not perpendicular to the horizontal axis ab, as shown in Fig. 160, when the ends of the horizontal axis are reversed into the position ba, Fig. 161, the collimation line will no longer be directed on to the point P, but on some other point as P'. In this case the point " midway between P and P' is the correct position of the collimation line perpendicular to the horizontal axis ab. The method of adjustment is therefore as follows:Direct the cross hairs on to some distant point P, clamping the upper and lower plates by the screws a' and ', Fig. 52, and leaving the vertical arc unclamped, so that the telescope may be easily lifted out of its bearings. Now carefully lift the telescope out of its bearings and replace it with the ends of the horizontal axis

reversed. The cross hairs should still coincide with the object P. If they do not, but intersect some other point as p', Fig. 161, correct half the total deviation PP' by means of the diaphragm screws dd, Fig. 52, and half by means of the tangent screw d of the horizontal circle. The cross hairs should now coincide with the same point P when the horizontal axis of the telescope is reversed. If not, the operation must be repeated until they do. In the transit theodolite, in order to reverse the ends of the horizontal axis of the telescope without lifting it out of its bearings, which is liable to shift the instrument, turn the telescope over vertically, so that the object glass points away from the point P, and then turn the upper part of the instrument through exactly 180° by means of the vernier w on the upper plate t.

In the plain theodolite the adjustment of the line of collima

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tion may also be effected by twisting the telescope half round in its y's about its own axis, and observing whether the cross hairs still continue to coincide with the distant point P. Should they not, half the deviation is to be adjusted by the diaphragm screws d, d and half by the tangent screw d' of the horizontal plate. By this means the line of collimation is made to coincide with the axis of the y's, and in this case the adjustment of the axis of the y's perpendicular to the horizontal axis of the telescope is supposed to be correctly made by the instrument maker.

"PP'

Accurate Method of Adjusting the Collimation Line of Transit Theodolite.-An objection to the foregoing method is that in lifting the telescope out of its bearings and replacing it the instrument is liable to be moved, unless it is done with extreme care. Also when with a transit instrument to avoid this the telescope is revolved vertically and the instrument turned through exactly 180° in azimuth, any error due to reading the vernier or in the graduations of the horizontal plate, is introduced. The following is the best way to adjust the collimation line of a transit theodolite. Set up the instrument at A, Fig. 162, and direct the cross hairs to a well-defined mark at B. A fine point marked on a peg driven into the ground will do. Turn the telescope over vertically and note where the cross hairs cut a well-defined distant point P. Again direct the telescope on to B, this time with the ends of the horizontal axis reversed. This is effected by leaving the telescope still pointing on P, and turning the whole of the instrument through 180° in azimuth, then accurately bisecting в again by means of the tangent screw d. Again turn the telescope over vertically, and see whether the cross hairs still bisect P. If they do not, but intersect the object say in P', Fig. 162, then the point p" midway between P and P' is the correct point, i.e., p” is in line with A and B and BAP" is a true straight line. To adjust the collimation line, correct one quarter the total deviation PP' by the diaphragm screws d, d, i.e., P'p, and one quarter by the tangent screw d' of the horizontal limb, i.e., pp"; in other words, by means of the diaphragm screws d, d bring the cross hairs to bear on where P'p= P'P, Fig. 162, then by means of the tangent screw d' of the horizontal circle bring them to bear on p" midway between P and P'.

The cross hairs when the telescope is revolved vertically should now intersect both the point B and the point p" in both positions of the horizontal axis, and the correction must be persevered in until this is effected, when the adjustment will be

correct.

By using both faces of the instrument in this manner and taking the mean of the two points P and P' each time the telescope is

B

Fig. 162. Adjustment of Collimation

Line.

rotated vertically, a straight line may be ranged out correctly even when the collimation line is out of adjustment.

The points в and P should be selected as nearly on a level as possible. In any case the difference of altitude of B and P should be small, otherwise part of the error may be due to adjustment I being out, as the position of the supports is reversed. AP should be much greater than AB. Any error will then be increased, and a more accurate adjustment will be effected.

By this method it will be observed that although we may have moved the instrument when rotating it through 180° in azimuth, the cross hairs are again adjusted exactly on to the point в before the telescope is directed on to the distant point P for the second time. The instrument is not liable to be disturbed, therefore, as the telescope has simply to be turned over vertically.

If the ends of the horizontal axis of the telescope be reversed by lifting out the telescope and replacing it, there can be no error due to the supports, adjustment 1, introduced, as the position of the supports is not reversed. It is, however, not easy to lift out the telescope and replace it without shifting the instrument. In this case, however, it is better to reverse the horizontal axis by lifting out the telescope, as the cross hairs are always made to bisect the point B again by means of the tangent screw d before the telescope is turned over on to the point P. We may then be sure that none of the error is due to adjustment 1 of the supports being out.

3. Adjustment of the Level on the Telescope.This level should be parallel to the collimation line of the telescope. In the transit theodolite it is to be adjusted precisely in the same way as the level, adjustment 2, page 214. In the plain theodolite, level up the level by means of the tangent screw of the vertical arc. Take the telescope out of the Y's, and replace it with the ends reversed. If the bubble deviates from the centre of its run, correct half the deviation by the screws which attach the level to the telescope, and half the deviation by the tangent screw of the vertical arc. Repeat the operation until the bubble remains in the centre of its run, when the telescope is reversed end for end in the Y's. This makes the level parallel to the Y's, with the axis of which the collimation line is supposed to coincide. It is, however, more accurate to adjust

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