And there is always something they haven’t looked at yet. The only thing that can make a telescope obsolete is light pollution from encroaching city lights. Even then, it can still be used for legitimate research, although perhaps less effectively than if it were located in a more remote location.

But facilities and mechanical devices require maintenance, and that is expensive. Also, there are demands for new technologies that can restrict older instruments to specific roles, perhaps research that has fallen out of fashion. In the case of the Hubble, maintenance and upgrades are very expensive due to its location. There is also a funding problem, and competition for that funding (remember, in a space telescope, you not only have the platform, but the communications and ground facilities that are needed to support it), that can exceed the cost of the instrument. Many astronomers would rather spend the money that is going to operating and maintaining it on another instrument, one perhaps optimized to work in the far infrared or ultraviolet region.

The problem with the Hubble is not that dissimilar than the problems of many of the great California observatories that are now becoming obsolete because nearby city lights and pollution are ruining their performance (Mt Hamilton, Mt Wilson, Mt Palomar). The scopes work fine, retrofitted with modern detector technology, they are still capable of doing cutting-edge research, but they can’t be properly used, and it costs more to move them than it does to just build a new one somewhere else. So these magnificent instruments will probably be abandoned, or converted to museums.

It is sad, because the technology for grinding lenses hasn’t improved substantially for about a century now, these instruments are often very difficult to duplicate today. Once you grind a piece of glass to 1/10 wavelength of light precision, you simply can’t make it any better. Its perfect.

The Hooker 100 inch reflector at Mt Wilson is over a century old, but its mirror is figured to a precision most instruments aren’t ground to today where big light buckets are the primary design spec. This is the instrument Hubble used to prove that the spiral nebulae were actually external galaxies and that the universe was expanding. He could actually see individual stars in the Andromeda Galaxy! Surrounded by modern hardware in a modern facility on top of a mountain in Chile or Hawaii it could still be doing valuable work, not all research requires super-large mirrors. And even today, there are only a handful of telescopes bigger than 100 inches.

Obsolescence doesn’t mean the gadget don’t work, it usually means
it just can’t be used without spending too much cash on it. Also, no telescope ever “surveys the entire sky”. You can always look again at a higher resolution, or use improved film emulsions or better imaging CCDs, or hang a better photometer or spectrometer or some other instrument on it, even to go back and take a second look at the same object. The lens or mirror is just a bucket used to scoop up light. What you do with that light is what really matters.

Hubble at the Hooker

I don’t know if that’s true or not, I doubt it, there are always events coming along, but it IS an interesting thought.