1. That the polar axis be placed at the altitude of the Pole. 2. That the index of the declination circle point to o° when the optical axis of the telescope points to the equator. 3. That the polar axis be placed in the meridian. 4. That the optical axis of the telescope be at right angles to the declination axis. 5. That the polar and declination axes be at right angles to each other. 6. That the index of the hour circle point to oh when the telescope is placed in the meridian; that is to say, when the declination axis is horizontal. 1st adjustment. To bring the polar axis to the altitude of the Pole. Put on a transit eye-piece, or some efficient substitute. Select from the Nautical Almanac, or some other catalogue, a star whose position is very accurately known and which at the time in question happens to be on or very near the meridian. Bring the telescope on to it, and read the declination circle; turn the polar axis half-round and again read the circle; the mean of the two readings is the instrumental declination of the object: correct for refraction, and compare the corrected value with the true declination. given in the catalogue. If the selected star be above the Pole, (which it is best that it should be, as chances of error from refraction are reduced to a minimum,) and the instrumental exceeds the true declination, the pole of the instrument is above the pole of the heavens, and vice versa. The polar axis must then be adjusted as needs be, by the screws provided for the purpose. Example. When Ursa Minoris was near the meridian, its declination was observed to be 82° 15′ 20" when, the face of the circle was E., and 82° 15′ 53′′ when the face was W. The mean of these two observations is 82° 15′ 36·5′′; the refraction was 52 8′′; so that the corrected declination was 82° 16′ 29′3′′. The true declination by the Almanac was 82° 17′ 19:3′′. Hence the polar axis was too low by 50". 2nd adjustment.-To make the index of the declination circle. point to o° when the telescope points to the equator. Take half the difference of the 2 readings obtained as above: this will be the index error of the declination-circle verniers, and they must be shifted accordingly by the proper screws. Several pairs of observations should be made, and a mean of them taken to secure an accurate result. Example. According to the above observation the index-error was 16.5"-additive to readings with the circle E., and subtractive with the circle W. When the error is small in amount it is often preferable not to attempt to correct it by the screws, but to keep the value thereof as a constant of correction, to be applied with the proper sign to every observation made. 3rd adjustment. To place the polar axis in the meridian. Turn the telescope on some known star 6 hours or so from or past the meridian, but removed as far as possible from the Pole and the horizon. Read the declination circle: correct for refraction, and compare the result with the value assigned in a catalogue. If the star is E. of the meridian, and its observed declination exceeds that given in the catalogue, the lower end of the polar axis will be to the W. of its true place, and must be moved accordingly. On the other hand, should the observed declination be less than that given in the catalogue, the lower end of the polar axis is too much E., and must be shifted. Should the star observed be W. of or past the meridian, the effects of the erroneous position will be reversed, and the adjustments must be reversed also. Example. The declination of a Ursa Majoris when 6h W. of the meridian was observed to be 62° 36′ 110", the face of the circle being W. Correcting this for the index-error found above (16·5′′), andfor refraction (30.8"), the result was 62° 35′ 237". The declination by the catalogue was 62° 35′ 16.3". Hence the lower end of the polar axis was 7·4′′ E. 4th adjustment. To place the optical axis (or line of collimation) of the telescope at right angles to the declination axis. Put in a transit eye-piece, and observe the time of the passage of some star over the centre wire, and read off the hour circle. Turn the polar axis half round; observe a second passage, note the time, and again read off the hour circle. If the interval of time between the two observations corresponds exactly to the difference between the two readings of the circle, all is right; if not, it is evident that one of the transits has been observed too early and the other too late, on account of the erroneous position of the wires. One half the difference between the interval as measured by the clock and that by the hour circle will be the error of collimation, as it is usually called. Example. The following observations were made upon d Ophiuchi: The interval in time was 6m 109: the difference of the circle readings 6 9:58. The two intervals differed, inter se, 14. Onehalf of this is 078, which is the error of collimation, to be added to the readings of the hour circle when the circle is E., and subtracted when the circle is W. The error may be corrected by the proper screws if the amount is considerable. It is desirable that the star chosen be situated as near as possible to the equator, in consequence of the apparent angular motion of the sphere there being faster than elsewhere. 5th adjustment.-To set the polar and declination axes at right angles to each other. Place a striding spirit-level upon the cylindrical pivots on which the latter axis turns, and by moving the hour circle bring the bubble to the middle of its run. The declination axis will then be horizontalb. Read the hour circle; turn the polar axis half round; again bring the declination axis into a horizontal position, and again read the hour circle. If the readings are the same (or where the circle is graduated to 24h, differ exactly by 12h) in both positions, the declination axis is in adjustment. If the readings do not agree, the declination axis is not perpendicular to the polar axis. If the declination axis is furnished with adjusting screws, place the hour circle half-way between the position it actually has and that which it ought to have in This supposes the level to be itself in adjustment. If not in adjustment, it must be put so before proceeding further. |