If you can get the resolution to be good enough you can probably tell what kind of radioactive material it is. So X-ray machines will look very different from nuclear reactors which will look very different from Nuclear weapons.
I hope you can understand the immense value in being able to determine where any nuclear weapon on the planet is via spy satellite.
So why hide it in a research telescope? Because if I wanted to use anti-satellite weapons to take out spy satellites I might not suspect this humble little research telescope.
Just a theory and I could be wrong who knows. I did hate optics and found it kind of boring and stupid.