# 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.

• Indeed, the effects are almost negligible. Calculating $\theta$ yields a very small number. – kristjan Feb 16 '15 at 21:59
• Hi @kristjan, I think the smaller $\theta$ is the better, as it stands for the ability of the telescope to differentiate objects to each other. – Lampard Feb 17 '15 at 9:37

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]