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ADJUSTMENT AND USE

OF

INSTRUMENTS,

IX.-XV.

ADJUSTMENT AND USE

OF

INSTRUMENTS.

IX.

GENERAL REMARKS ON ADJUSTMENTS.

1. Care should be taken in all instrumental adjustments, where crews work in pairs, to loosen one before tightening its opposite.

2. Remember that the eye-piece inverts the image of the cress-hairs, and that consequentiy añy movement of it, by weans of the small capstan head screws on the outside of the telescope-barrel, should be in the direction which would seem to increase the error requiring correction.

3. Before beginning the adjustments, screw the object-glass close home, and make a pin-scratch across its rim and the end of the tube, by which to mark its proper place; draw out the eye-piece until the cross-hairs are exactly in focus; that is to say, until no movement of the eye shall appear to displace them, and bring the object to be observed clearly into view.

4. Never permit the glasses to be rubbed with a gritty fabric. To remove the dust from them, use a soft, clean handkerchief, and change often the part applied.

23

X.

THE LEVEL.

TO BRING THE INTERSECTION OF THE CROSS-HAIRS INTO

THE OPTICAL AXIS OF THE TELESCOPE.

1. Set the instrument firmly, cast loose the wyes, and, by levelling and tangent screws, bring either of the cross-hairs to coincide with a well-defined object, distant from 400 to 600 feet, or as much farther as distinct vision can be had free from heat ripple. Gently rotate the telescope half-way around in the wyes. If the cross-hair selected for treatment then fails to coincide with the object, reduce the error one-half by means of the small capstan head screws at right angles to it on the telescope-barrel. Bring the spider-line again to coincide with the object by means of the levelling and tangent screws, and, if necessary, repeat the operation. Proceed in the same mallner with the other cross-hair. If the error is large, bring both nearly right before undertaking their final adjustment.

2. Having thus adjusted the line of collimation upon a distant point, requiring the object-tube to be drawn well in, select a point close by, which shall require it to be thrust out almost to its limit. If any error appears, correct half of it with the small screws provided for the purpose, a little forward of the diaphragm, and usually protected by a movable sleeve on the outside; correct the other half with the levelling-screws. After completing this adjustment, test the former one on a distant object, and, if necessary, repeat the operations.

3. In the transit, the small guide-ring screws used for this adjustment are covered by the bulb of the cross-bar in which the telescope is fixed, and are therefore inaccessible. The adjustment, however, is one not liable to become deranged in either instrument, and, in the transit, is of comparatively small importance.

4. The young practitioner should bear in mind that the intersection of the cross-hairs may coincide with the optical axis of the telescope, and yet be out of centre as regards the field of view. Such eccentricity does not affect the working

of the object-piece solely. It may be removed by manipulation of the small screws securing the inner end of the eye-piece.

TO BRING THE LEVEL BUBBLE PARALLEL WITH THE TELE

SCOPE AXIS,

5. Clamp the instrument over either pair of levelling screws, and bring the bubble to the middle of its tube. Turn the telescope slightly on its bearings, so that the bubble-case shall project a little on one side or the other. If the bubble slips, correct half its movement by means of the small lateral capstan head screws at one end of the case. Return the telescope to its first position, level up again, and repeat the operation until the erroneous movement ceases. This adjustment brings the telescope and level into the same vertical plane.

6. Next, the bubble being at the middle of its tube, carefully lift the telescope out of the wyes, turn it end for end, and replace it. If the bubble settles away from the middle, bring it half-way back by means of the capstan-heads, working up and down at one end of the case. Again middle it with the levelling screws, and repeat the operation until the error is corrected.

TO ADJUST THE WYES; OR, IN OTHER WORDS, TO BRING THE TELESCOPE INTO A POSITION AT RIGHT ANGLES TO THE

VERTICAL AXIS OF THE INSTRUMENT.

7. Close the wyes. Unclamp. Set the telescope directly over two of the levelling screws, and with them bring the bubble to the middle of the tube. Then rotate the telescope horizontally, until it stands over the same pair of screws, changed end for end. If the bubble errs, correct one-half of the deviation with the capstan head nuts at the end of the bar, and one-half with the levelling screws. Place the telescope over the other pair of levelling screws. Repeat the operation there; and continue the corrections, over one and the other pair of levelling screws alternately, until the bubble stands without varying during an entire revolution of the instrument upon its vertical axis.

8. The capstan head nuts on the cross-bar should be moved by gradual stress, not by pounding. They are a rude device. With so short a leverage as the length of the common adjust

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