According to TFA the slotted lens would be much lighter but also MUCH MUCH larger than traditional setups. Also, the distance between the lens and the camera is so large that a second spacecraft is needed.
Trying to maintain the alignment of two spacecraft would be difficult at best. Now consider that they would need to fire thrusters and move one of them every time they need to focus on a different object. How much fuel would be used up just looking at 10 different objects? I would expect a lot. Add to this what micro-meteors and the random castaway screw driver would do to the foil over time and you can see some real limitations to this.
Not saying it's impossible, just that they need to work out some ways around the obvious problems.