This is not quite true. These telescopes were built by the military to look at things a few hundred miles away, not millions of miles away, so the optical dimensions are different. They are still useful, but they are not clones of Hubble.
Furthermore, they lack the expensive part: the one-of-a-kind scientific instruments. These will still have to be developed from scratch.
NASA is developing one of them into a telescope called WFIRST, and by some estimates, little to no money will be saved by using the donated military telescopes.
The NRO instruments are just optics, optics make up a tiny fraction of the cost of a space telescope. NASA was not gifted fully functional space telescopes they could just launch and then start observing. They still need all of the components to actually turn them into functional instruments, spacecraft systems, cameras, attitude control, etc. The estimated cost for WFIRST alone is $3 billion, only slightly less than if the instrument had been built from scratch, if at all.
Basically, yes. The mirror is the same size as Hubble, and the field is much wider. However, there are some technical differences that make it less useful for imaging extrasolar planets and the most distant galaxies. Details are harder to come by, as this is ex-military hardware, so certain things are still classified.
I doubt there's any way to easily estimate the cost, but the two HST-superior telescopes NASA was given by the NRO, need to be made science-ready. There are no instruments on them.
Say for example that the cost would be $500m to $1 billion over several years to get them ready, then another $1 or $2 billion over ten years to operate them both. If you're NASA and you have $23 billion to spend per year average over that time, do you invest into the two telescopes and divert resources, or stick with HST until its end and focus on Webb. Obviously if the US Government were behaving more rational, NASA would be given the resources to prep these telescopes and launch them. They're exceptional gear going to waste, surely we could task something interesting for them.
I was unaware we had more, so this is pure speculation, but I would imagine that having continuous coverage is considered critical, so if we launch another one and it experiences a failure soon after the current Hubble fails, we’ve lost that margin of error.
How would having the backup in orbit reduce the amount of coverage that an HST-type device could provide?
It seems like having another telescope in orbit could basically be a "hot-spare" for the highest priority tasks while continuing to do lower priory work in the mean time.
Because it's safer on the ground. If you put everything you have up in the sky, and they all fail for various reasons (solar storm, collision, wear & tear) then you're screwed.
Regardless, per another comment it sounds more like a budget problem, so my speculation was crap anyway.
Yeah, I guess I understand that, it just seems like most things that aren't a design failure are unlikely to affect more than one orbiting telescope.
In other words, to return to your example, it seems unlikely to me that a collision would happen to more than on telescope ever, let alone in a time window too tight for a third (or fourth, etc) telescope to be built and launched.
(and, anyway, it looks like you are right: this is all budgetary anyway, so whatever)
Satellites don't break significantly faster in orbit than in storage. Storage can easily cost $5 million a year, but on top of cost savings there's other benefits to sending your payload up early (assuming you've budgeted for a bit more battery wear, fuel consumption, etc.)
The most dangerous time for a satellite is by far the first couple months. This is when most design and QC failures become apparent. If you launch before you need your satellite, you can do your on-orbit checkouts and not be in a rush to deal with any anomalies. When your preexisting operational satellite dies, you can have one ready to go immediately instead of needing to beg borrow or bribe your way to a better position in a launch provider's manifest.
