The double ring holds within itself a sighting tube with crosshairs.
An astronomical check on timekeeping was made by a sighting tube pointed to a selected star.
The men who put up Venus Equilateral had no idea that someone would be using the station for a sighting tube some day.
The left hand then gripped the sighting tube, giving it a gentle shake that caused it to extend beyond the doorway.
Many users prefer a simple sighting tube, which has a wider, brighter field of view and is easier to use at night.
Improving the 5th century model of the astronomical sighting tube, Shen Kuo widened its diameter so that the new calibration could observe the polestar indefinitely.
On the use of the sighting tube to fix the position of the pole star, Shen Kuo wrote:
The dioptra was a sighting tube or, alternatively, a rod with a sight at both ends, attached to a stand.
To allow more accurate astronomical observations and recordings, Shen Kuo improved the technical designs of the rotating armillary sphere, the gnomon, the clepsydra clock, and the sighting tube.
He was also responsible for proposing a plan of having a sighting tube mounted ecliptically in order for the better observation of celestial latitudes.