Spire Test for Axis Perpendicularity

The primary goal is to ensure the instrument's horizontal axis (also known as the trunnion axis) is precisely perpendicular to the vertical axis (the axis around which the instrument rotates horizontally). This perpendicularity is fundamental for accurate measurements.

Understanding the Spire Test

The Spire test is specifically designed to check and adjust the perpendicularity between the horizontal axis and the vertical axis.

• The test involves observing specific points or targets at various orientations.
• By performing specific observation procedures and analyzing the results, adjustments can be made to make the horizontal axis truly perpendicular to the vertical axis.

Why Other Tests Are Not Applicable

The other options check different aspects of instrument calibration:

• Azimuth test: Focuses on directional accuracy and horizontal circle readings.
• Cross hair ring test: Verifies that the horizontal and vertical crosshairs within the telescope are themselves perpendicular to each other.
• Vertical arc test: Deals with the accuracy of angle readings on the vertical circle.

Therefore, the Spire test is the correct procedure for ensuring the horizontal axis is perpendicular to the vertical axis.