How can diffraction happen in the Hubble Telescope? I've seen people talking about the angular resolution of the HST is, if using Rayleigh's criterion, equal to:
$$\theta = 1.220 \frac{\lambda}{D}$$
My question is, since the diameter of the HST $D$ (2.4m) is wayyy larger than the wavelength $\lambda$, how on earth would diffraction happen? Wouldn't the light just go through without any effects? Thanks.
 A: It all depends on what you want to resolve and how far away it is. For the  Andromeda galaxy at a distance of about 2.5 million light years, HST will resolve visible light objects about 0.6 light years apart.  If we take our separation from Proxima Centauri (4.3 ly) as typical of separations in the spiral (certainly it's less in the center of the galaxy), the HST will easily resolve individual stars, but not planets in solar systems (too small). In fact, the latest (2014) HST images of Andromeda show individual stars. So the diffraction already limits us to stars.
If we push to galaxies at 25 million ly, the resolution drops to 6 ly and we can't resolve separate stars.  That limits us identifying specific  single stars that go into supernovas.  If we're watching [EDIT here] UV or gamma, it's better because of the shorter wavelengths (smaller $\theta$ is better resolution, and supernovas have very interesting UV and gamma profiles.  It's nice to have stellar spectra both before and after the supernova, but if we can't resolve the star it hurts the overall wavelength range analysis.
[End edit]
Most recently HST has looked at galaxies about 13 billion ly away. In 500 nm light, that would be a resolution of 3300 ly. That makes it difficult (but not impossible) to determine any shape or structure of the galaxy.
I wouldn't say the light goes through "without any effects."
Added: The reason there is diffraction is the circular aperture always interrupts the EM wave.  This partial sampling of the wave is what creates the diffraction effects. It's always there, but we usually don't notice it because other effects will blur an image first (atmospheric, optical aberration, etc.) While the diffraction is small, it is noticeable for images with small angular size. HST's huge aperture located outside the atmosphere allows it to have an impressive distance reach before Rayleigh diffraction is noticable.
