Are there any optical method of viewing stars without "diffraction" making them appear larger? [closed]

Question

In the 1700s, Edmond Halley and others discovered that stars appeared larger than they are, and attributed it to diffraction in the lens of an observer (telescope, eye) and/or in the Earth's atmosphere. Are there any ways to view stars without that effect?

Answer 1

There is a physical limit to the angular resolution of any imaging system. For a circular aperture it is often quoted that the angular resolution corresponds to around $\theta_{\rm min} = 1.22 \lambda/D$ (in radians), where $\lambda$ is the wavelength of observation and $D$ is the diameter of the telescope.

A similar limit applies in interferometry but now $D$ might represent the separation between your telescopes.

To translate an angular resolution to an actual physical resolution of size you would have to multiply by the distance to the source.

So the ways in which you can improve matters are:

1. Use a bigger telescope or use an interferometer with a longer baseline.
2. Work at shorter wavelengths.
3. Get closer to the source.

There are of course technical challenges with each of these. In particular you might have thought just increasing the baseline would be easy enough - but you need to know the separation of your telescope to a fraction of the wavelength you are using and that turns out to be tough for larger separations. Shorter wavelengths are problematic for optics - normal lenses and mirrors don't work at X-ray wavelengths for example. Getting closer to the source requires interstellar travel...

The OP wants to argue with me about whether the atmosphere is a significant factor. Of course the atmosphere degrades stellar images (it's not diffraction, but refraction). Going into space overcomes this and in the past this would have been the limiting issue. However, now it is commonplace for ground-based telescopes to achieve close to their diffraction-limited performance using adaptive optics techniques or "Lucky Imaging", both of which can and have got better images than the Hubble Space Telescope (examples below).

Example 1: The middle picture is from HST, the right hand image is Lucky Imaging from the ground based Palomar 5m telescope. The larger aperture of the Palomar produces better resolution despite looking through the atmosphere. See https://www.ast.cam.ac.uk/research/lucky for further details.

Example 2: HST image of Titan vs an image taken using adaptive optics with the 10m ground-based Keck telescope. Larger aperture wins again because the effects of the atmosphere can be avoided.