Prism astrolab

This article or the following section is not adequately provided with supporting documents ( e.g. individual evidence ). Information without sufficient evidence could be removed soon. Please help Wikipedia by researching the information and including good evidence.

As Prismenastrolab extremely high precision measuring instruments for the Geodetic Astronomy referred, with which one many in a short time star passes in a constant zenith distance can observe most of 30 °. Their constancy (to at least 0.5 ") is guaranteed by a right-angled optical prism that has the appropriate cut angle.

The prism is attached in front of the objective of a horizontally aligned theodolite or an automatic leveling device . Precise inclination sensors such as a bubble level , an automatic height compensator or reflections on a wind-protected mercury surface are required to ensure that the target axis is exactly horizontal .

With the same height method, the measurement is carried out by setting the calculated direction in which a star reaches the desired zenith distance, and then stopping the star at several lines of a network of threads mounted in the telescope .

Three types

In the course of the last few decades three types of instruments have been developed, two of which are light enough to be suitable for night-time field use for geoid and location determination (rapid measurement of astronomical latitude and longitude on TP points). The third type is intended for stationary use in observatories or on massive measuring pillars (precise measurement of star locations or local sidereal time , monitoring of the earth's rotation).

1. Theodolite with prima and mercury horizon : the target axis of the theodolite is horizontal, the prism deflects the measuring beam up and down by 60 °. The star passage is measured directly in the upper beam , while the lower beam is reflected in a mercury bowl mounted in front of it and its image runs towards the direct star. When the images coincide, the exact time is recorded on a chronometer . This type of construction was developed in the 1950s by Wild Heerbrugg for the triangulation theodolite Wild T3 , measuring accuracy about ± 1 ″ .
2. Automatic leveling device with a 60 ° prism in front. The sighting axis is held horizontally by an exact inclination sensor to an accuracy of 0.3 ″, the prism points at a zenith distance of 30 °. The star passages are observed directly in the thread network and registered with a digital stopwatch . The first device of this type is the Ni2 astrolabe developed by Zeiss around 1960 . It weighs only a few kg, can be used on a normal surveying tripod and provides location or plumb direction to ± 0.2 ″ to ± 0.5 ″.
3. The Danjon astrolabe is only transportable to a limited extent . It was developed in the 1960s for fundamental astronomy and time services to monitor the rotation of the earth . The heavy instrument requires a stable pillar and works according to the 1st principle, but combines all components in a shielded housing. The two images of each star are kept in coincidence with a recording micrometer and their times are automatically recorded ( accurate to a few milliseconds ). Accuracy of the perpendicular direction about 0.05 ″.