Attach a visible light telescope to the outside of the ISS 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.