I dont understand how it worked. It does not mention any telescope but does mention largest "quadrant principle device" with not further explanation on what this is and how it worked.
The observatory predates the invention of refracting telescopes. It was used to determine the position and movement of stars and planets, not to observe them. The "quadrant" is a device to accurately measure the angle of an observation:
It's essentially a very big 'protractor' used to measure angles (of stars or the sun over the horizon). The bigger such a device is, the more accurate the measurements.