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

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?

• What kind of answer are you looking for here? There is a physical optics limit that depends on the diameter of the telescope that you can't beat (except by using a bigger telescope or getting nearer to the star). – Rob Jeffries Oct 25 '18 at 14:57
• Just the answer to the question. If its no then its no. How I've understood it, turning the brightness down makes it possible to view the actual body of stars, is that true? Could that be used in the context of the question? – user24634 Oct 25 '18 at 14:59
• Your question would be specific (enough) if you (e.g.) asked something like How can we reduce diffraction in the lenses of a telescope?. Currently the question makes answers in all directions possible. If someone points that out to you by saying that the effect in lenses and atmosphere are quite different, use your energy to improve your question instead of protesting. – user1569 Oct 25 '18 at 18:58
• You have no interest in any answers or in learning about anything. You are just here to start arguments. I won't be answering any further questions from you or responding to your comments. – Rob Jeffries Oct 25 '18 at 19:32
• I'm voting to close this question as off-topic because it is evident from your reaction to the (good and deleted) responses from Rob Jeffries (a professional astrophysicist) that you are trolling. – StephenG Oct 25 '18 at 20:13

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.

• being in space is good too, the atmosphere diffracts a lot – user24634 Oct 25 '18 at 15:51
• @user24634 I didn't add that because the issue above trumps it. The issue with the atmosphere is turbulence and actually most of that problem can be removed using clever adaptive optics techniques - but the problem above remains. – Rob Jeffries Oct 25 '18 at 16:20
• a Hubble employee mentioning that the atmosphere is the main limitation, ucolick.org/~mountain/AAA/aaawiki/… – user24634 Oct 25 '18 at 16:39
• @user24634 you continue to mis-use the word "diffraction" in many ways. You will find it a lot easier to learn about optics if you start at the beginning, reading some simple online tutorials or equivalent. – Carl Witthoft Oct 25 '18 at 17:52
• Hey, @user24634, Rob's really trying to help you, and he knows what he's talking about. Please be open about receiving feedback, especially when people are putting a lot of their time into trying to help you. – HDE 226868 Oct 25 '18 at 23:45