The Diffraction Limit
In the design pages of this site
the term diffraction limitThe maximum resolution a telescope may theoretically have.
is mentioned quite a bit and should be understood for one to
appreciate the advanced performance of the TMT.
First of all, diffraction is an inherent property of light
limiting the resolution at which we can observe things.
This 'diffraction limit' is the minimum angular resolution a telescope
or any optical device may have. To explain this, we
need to define angular resolution - the minimum space between two point sources of light that a telescope can separate
or distinguish between.
Imagine you're spending a night observing with a telescope. You point it at a patch of stars and notice three pairs of stars. The first pair are obviously separated, the second appear to
be just barely separated and for the third pair you can't tell where the stars begin and end.
Let's blow the image up a little so it is easier to see. It might look something like this:
The obviously separated stars on the left don't tell us much about the angular resolution. All we can say is
that it is better than the
separation of the other two. We can't see the separation of the stars on the right, so we know the telescope's angular resolution is worse
(greater) than the angular separation of those stars. The stars in the centre, however, meet what is known as the Rayleigh Criterion (read more technical
information about the Rayleigh Criterion here). We can just tell that the stars are separated, meaning that whatever
angular separation they are at is the angular resolution of the telescope. To find that out in numbers we then look
up those stars and see how far apart they really are. It should be noted that astronomers measure distances on the sky in angular measures (like degrees)
not the distance measurements we are more familiar with (such as metres).
The diffraction limit of a telescope is determined by the
size of its aperture and the quality of its optics. Optics that are not manufactured
perfectly will negatively affect the image quality of
the telescope. Aperture is the diameter of the primary mirror,
and the wider the primary mirror, the smaller the diffraction limit gets. The diffraction limit can be determined mathematically by creating
an imaginary triangle where two of the sides are the length of the diameter of the telescope and the third side is the wavelength of the light in question. This is illustrated in the image found below. Ideally an optical system should operate at the diffraction
limit, meaning that the system should be producing images at the highest quality the instrument is capable of and would therefore show
the most detail possible for its design.
It should be noted that a telescope can never give a 'perfect image' due to the wave nature of light. The term 'diffraction limited' means
that the telescope's image is only affected by the wave nature of light and nothing else. In
other words, it means that the image is as good as it is going to get.