Is a mirror-less telescope possible? I was reading about telescopes and the Hubble Telescope for example has a 2.4m mirror which reflects lights to a sensor. Other type of telescopes use lenses to focus light to the imaging sensor.
I was wondering, is it possible to have a telescope without a mirror or lens?
So in Hubble's case, instead of having a 2.4m mirror reflecting light to a small sensor, why not have a big imaging sensor (same size as the mirror - 2.4m). Would this type of telescope have similar capabilities as the mirror one?
I know that we use mirrors because it's way cheaper and easier than building large sensors but I'm curious if a mirror-less telescope would be better / worse or just the same.
 A: Systems other than mirrors can provide telescope function.
There's four other ways to make a telescope.   


*

*Like Galileo did, we can use lenses instead of mirrors (this does not work well at large scale).   

*One can employ diffraction structures (holograms, or gratings,
or zone plates), but there are significant problems in doing this
for a broad range of wavelengths.  

*More primitive, is the pinhole camera,
and variations using multiple holes, so-called "coded apertures"; that 
is how some X-ray imaging is done, but it takes a lot of work.

*Last, is to find a black hole or neutron star, and take advantage
of the gravitational bending of light in its vicinity.   That's more
of a discovery than a construction project, but several useful 
systems have come into view.Hubble gravity lens
A: A phase(d) array antenna is an imaging device without either a mirror or a lens. Of course, it needs a reference oscillator that is phase coherent with the incoming wave so when the two are mixed in the array, sensor element by sensor element, only the I and Q ("inphase" and "quadrature") components of the incoming wave are processed. I admit, this is conceptually easier for a radar antenna that generates its own illumination but I can imagine that, for example, by some magic if you could delay the incoming wave illuminating the array but ahead capture its carrier to phase lock your own oscillator then you could play the same game locally...
