Visible light imaging of the heavens is limited on Earth primarily due to the atmosphere. I know the ISS is moving at 17000mph, but given the delays to the James Webb telescope, is there any merit in attaching a space-rugged visible light telescope to the outside of the ISS? Could it ever approach the visible light performance of Hubble?
A self-sufficient orbiting telescope is basically Hubble mkII and would never get off the ground, literally and metaphorically
Hubble was expensive because it was state-of-the-art, requiring development of many new systems. The systems it needed to function as a standalone satellite (compared to being attached to the ISS) were cheap by comparison (reaction control, power, communications) because they mostly didn't require new development.
So a Hubble successor attached to the ISS would not be much cheaper than a free-flying one.
Any telescope attached to the ISS has to deal with:
- high vibration levels which degrade performance
- outgassing which degrades performance
- severe pointing constraints because it's surrounded by a large structure
- bandwidth constraints because it has to share a transmitter with ISS operations
- end of mission in 5 years when the station reaches the end of its life.
The advantages of attaching to the ISS:
- easier servicing,
- you get the option to store lots of data on the ISS and bring it down on a capsule. Downside is a latency of several months.
IMO you'd be better off building a standalone space telescope.
Attach a visible light telescope to the outside of the ISS
This is a reasonable idea and it has been thought of before, but usually for other-than-visible light. Other answers do a good job of explaining why the disadvantages substantially outweigh the advantage. The cost to put something on the ISS large enough to outperform the top few dozen telescopes on Earth or approach the performance of the Hubble would be... Astronomical!
However, there is a moveable telescope mounted on the outside of the ISS right now but read further to see why.
Had the Apollo missions gone on longer there might have been an Apollo space telescope. See answers to How would the Apollo telescope have worked in the Apollo command module? Where would it be located and how would it be operated? and the YouTube video Missions we Lost When Apollo was Cancelled
The Skylab space station did have a space telescope!
See for example How did Skylab's electrographic camera work?. This telescope took advantage of being above Earth's atmosphere in order to photograph using ultraviolet light, something that you simply can not do on the Earth.
The Apollo astronauts also had an ultraviolet telescope on the Moon (see answers to How was the Moon's first telescope used? (Apollo 16))
The ISS also has a space telescope!
Finally, NASA has indeed attached a robotic, point-able telescope to the outside of the ISS. It is called NICER or Neutron Star Interior Composition Explorer and it is an X-ray telescope, also something that you can't see from he ground and need to put your telescope in space.
See this answer and links therein.
I think there is nothing wrong with your idea in principle, there is already a modified Boeing jumbo jet specifically for the purpose of carrying a large telescope above the densest layers of the atmosphere in order to minimise atmospheric interference and absorption by atmospheric water vapor.
However, the telescope and the personnel who man it use up a considerable amount of space, though on the aircraft there are no competing activities going on beyond those necessary to fly the aircraft. On the ISS the main problems would be in finding the space to accommodate the telescope and its crew and making the necessary modifications to the fabric of the space station.
I don't think such a telescope could compete with Hubble, but that doesn't mean it couldn't do anything useful.
One advantage would be that if anything should go wrong,it could be somewhat easier to repair than it is with Hubble. Now that the Space Shuttle is no longer available, a crewed mission to Hubble would be extremely challenging and a robotic mission would require technology not yet available.