Clocks and Flashing Comparator

People have been measuring time with sundials, water clocks, candle clocks and hourglasses for thousands of years, but it was not until the 17th century, with the invention of the spring drive and the balance wheel, that clocks became precise instruments: they play a very important role in observatories, especially because the exact time is needed to measure the locations of objects in the sky.

Therefore the observatory’s instrument room harbors a total of three astronomical clocks, including the master clock from the Anton Rapf company, which presumably comes from the Viennese private collection of Oppolzer's father.

In addition to telescopes and clocks, the instruments for analyzing the photographic plates are also an important part of the Historical Observatory. The largest and most important of these instruments is the Zeiss stereo comparator acquired by Oppolzer, which was later converted into a blinking comparator: When we look through the microscope and turn on the built-in motor, it alternately directs the image of the left and the right flashing photo plate onto our eye. If these are two photo plates of the same area of the sky taken on different nights, then those celestial objects will catch our eye if they have moved across the sky in the time between the first and second image, in contrast to most others. Many small planets, for example, have been found in this way using such blink comparators.

However, the device can also be used to find stars with variable brightness. As their brightness is different on the two photographic plates, these celestial objects immediately stand out from the mass of stars by "blinking".

Even today, the same basic method of comparing two images is still used to evaluate images taken with telescopes, but now advanced computer technology is used for this purpose.