One of the most powerful blank/missing or empty things in the history of astronomy that did come about may very well be Christiaan Huygen’s telescope without a telescope tube. Presented to the Royal Society in 1691, Huygens instrument (also known as an “aerial telescope”) was meant to perform the world’s most powerful optical observational instrument, having a focal length of 122'. Monster telescopes were not practical at that time given the weight and flexure and movement of what would be a massive multi-hundred-foot-long telescope tube. And so Huygens came up with the idea of doing away with the cumbersome part of the large telescope...The rest of the story is at Ptak Science Books, which appears to be a veritable cornucopia of TYWKIWDBI-like things.
...its missing section was perhaps one of the most important developments in astronomy in the second half of the 17th century. Its arrangement was very elegant, and simple–the mast held the object glass whose position could be changed with a series of simple pulleys and pulls; the eyepiece was placed on a table, aligned with string and rope. And that was it. By the first third of the 18th century, James Bradley was able to construct a telescope of this type with a focal length of 212 feet, and used it to measure the diameter of Venus.