Transit and its adjustments

An engineer's transit has two spindles, one fitting inside the-other, to each of which is fastened a horizontal circulаг plate.Another spindle is attached to a lower plate, on which is a graduated circle, and the inner spindle carries an upper plate, to which the verniers and standards holding the telescope are attached. The motion of these horizontal plates is controlled by clamps and tangent screws. On the upper plate are two spirit levels, the plates are levelled by 4 (sometimes 3) levelling screws.

Transits having a vertical arc are called engineer's (or surveyor's) transits A plain transit lacks these attachments.

A telescope has a compound objective and an eyepiece composed of 4 lenses, if the instrument is an erecting transit, two lenses if inverting. An inverting instrument gives a much better illuminated field. Between the objective and eyepiece is the cross-hair ring held in its concentric position by four opposing screws, the heads being outside the telescope tube.

Special attachments are; a) a dark eyepiece for observing the sun; b) a diagonal or a prismatic eyepiece for sighting high altitudes; c) a reflector attached to the telescope for illuminating cross-hairs when working in the dark; d) stadia hairs which are two extra horizontal cross-hairs for measuring distances by stadia.

Adjustments of the transit are: a) to make the plane of plate bubbles perpendicular to the vertical axis of the instrument, b) to make line of sight perpendicular to the horizontal axis; c) to make the horizontal axis perpendicular to the vertical axis of the instrument.

Bubble tubes. Bring plate bubbles to centers of respective tubes by turning levelling screws. Turn 180° and correct half of the apparent error by turning capstan screws of bubble tubes. Adjust each bubble independently.

Line of sight. First, adjust the vertical cross-hair to lie in the plane perpendicular to the horizontal axis. Set up and level the instrument. Sight the vertical hair on a well defined point. Clamp both plates, and move the telescope up and down. If the point does not appear to travel along a vertical cross-hair loosen screws holding a cross-hair ring, and by tapping lightly on one screw, rotate the ring until the above condition is fulfilled. Then tighten screws and proceed with the second part of adjustment as follows: sight the telescope at point A (200 to 300 ft. away), and clamp the vertical axis; invert the telescope on its horizontal axis and set point B at about the same elevation as A, in line of sight, and about the same distance from the instrument as A. Loosen the clamp, turn horizontally and sight A again; damp horizontal motion, invert the telescope on the horizontal axis and if line of sight falls on B it is in adjustment. If not, set point C in line of sight beside point B. Mark or note point D one-fourth the distance between C and B, measured from C. To adjust, move the cross-hair ring until D is intersected, by loosening the screw on one side of the telescope and tightening an opposite one.

Дата добавления: 2015-07-19