Dealing With Diffraction In Telescopes
At left is a computer simulated image of a star as seen at high power
through a telescope. Based on the Huygens Principle, the image was created by
forming a unit gain circle in 3D to represent a circular aperture, then
performing a 2D Fourier Transform on the pattern. The result is the
interference pattern shown here, and it represents a point source (star) as
seen through a telescope.
For illustration, this image is much larger that an actual star will look
through your telescope. Rather than seen as a single point of light, a star in
a high power telescope is seen as a small circle of light surrounded by one or
more rings. The rings go on forever, but lose amplitude quickly, with only
the first few being visible.
The ring pattern is a basic law of physics, but inconvenient for the user of telescopes. As you
can imagine, two stars very close together have overlapping rings, and that
blurs the distinction between the stars. The pattern of rings, called a
diffraction pattern, limits the resolving power of a telescope.
The pattern shown is what would be seen through an astronomical refractor
telescope. Refractors provide star patterns with the least amount of energy
(brightness) distributed in the rings surrounding the star. Reflecting
telescopes like Newtonian telescopes and Schmidt Cassegrain telescopes usually
have some kind of secondary mirror in the light path that complicates the
pattern, resulting in more energy being distributed to the surrounding rings
and less in the central disc that's the star.
The light that is scattered into the diffraction rings makes the central
star image a bit dimmer and the surrounding rings brighter. You can read more
details about diffraction and different telescope designs at the diffraction tutorial.
It isn't just star images that suffer from this phenomenon. An extended
image, like that of a planet, is equivalent to a large number of such
diffraction limited points. Each point has energy dispersed into the rings
surrounding the point. This reduces the contrast in an extened image such as
Jupiter or Mars, which are composed essentially of many bright points.
The superior nature of refractors with respect to diffraction is why many
telescope users prefer them. So why doesn't everyone simply buy a refractor and
minimize the effects of diffraction?
The Effect Of Aperture Size On Diffraction Image
Why don't most people just use refractors? A -- because
large refractors are expensive, and B -- there is another
The image above shows the relationship between the nature of the diffraction
image and the size of telescope. It may seem backward, but the star image
actually looks smaller in a larger telescope. The central star image and its
diffraction rings are both smaller with bigger aperture. That's what lets a
bigger telescope separate stars that are closer together, and have better
resolution on planets and other high resolution objects. The left image depicts
a star diagram from a small telescope, the right a star diagram from a large
telescope. In a larger telescope, the central peak is higher also, but that's
not depicted in this image.
This seemingly backward result happens because a star is essentially a point
source of light. It has no size, and a telescope can't magnify it enough to
give it a size. The central disc seen in a star diffraction pattern is the
smallest resolvable disc the specific telescope can create. Since it can't see
things smaller, star images look that big. When you use a bigger telescope
that is capable of seeing smaller images, the central disc of the star image
then looks smaller.
If you checked out the diffraction tutorial, then
you know that most design types of reflecting telescopes have secondary mirrors
in the light path. The result of the secondary obstruction is to decrease the
amplitude of the star, and increase the amplitude of the diffraction rings.
That's why refractors are generally a superior design.
But this diagram shows that one way to reduce the effects of diffraction on
resolution is to simply use a bigger telescope. Big reflectors are relatively
cheap compared to refractors, and a bigger reflecting telescope, even with its
central obstruction, will have an overall smaller diffraction pattern and
better resolution. So planets like Saturn, Jupiter, and Mars will be resolved
better, and as a bonus stars will be brighter with a bigger telescope.
The Effect Of An Apodizing On The Diffraction Image
But there is another way to have at least a small affect on the diffraction
pattern without moving to a bigger telescope, especially if your telescope is a
reflector of some type. The message that different sized apertures produce
different sized diffraction patterns leads to another possibility. The way to
take advantage of it is make a device called an apodizing screen that
essentially merges diffraction patterns from two different size apertures
together, causing some destructive interference -- at least within the first
couple of rings.
The images above represent the diffraction pattern through a refractor (left),
a reflector with central obstruction (middle) and a reflector with an apodizing
screen(right). Moving left to right, you can see that the introduction of an
obstruction in the light path causes energy to be pushed into the surrounding
diffraction pattern. The energy in the central disc is also reduced, though that
is not represented in this diagram.
By putting an apodizing screen of proper dimensions in place, the
diffraction patterns of the two synthesized apertures have destructive
interference in the first few (the brightest) rings, effectively reducing their
effect. Even though the apodizing screen also slightly reduces the brightness
of the central disc (the star), the respective difference in amplitudes between
the central disc and surrounding rings is improved.
So -- how do you make an apodizing screen?
How To Make An Apodizing Screen For Your Telescope
You can make an apodizing screen with a little poster board and some window
screen. You can make a simple one with just a single piece of window screen, or you
can add one or two more screens (with different sized holes) to fine tune your
The left image shows the simplest design. It's a black window screen with a
single hole cut in the center. The size of the hole should be 90 %
the size of your telescope objective. This single screen design is sufficient
for a small telescope, say 4.5 inch or smaller. You can first cut a piece of
poster board or tempered hardboard the diameter of your telescope tube, then
cut a hole in that the size of your objective. That will give you a mounting
surface for attaching the screen. Then make a collar for the board and slip it
over the end of your telescope.
The middle image shows a two screen design. Below it you can see the
resulting diffraction pattern. Note that with two screens, the second
diffraction ring is nearly eliminated. The 2 screen design works well on 4.5
inch to 6 inch telescopes. The two holes in the screens should be at 90
% and 78 % the size of your telescope's objective. Rotate the two
screens about 45 degrees with respect to one another before mounting.
The right image shows a three screen design. Again, below it is the diffraction
pattern, with the 2nd ring gone and the 1st ring reduced in amplitude. The holes
in the screens in this design are 90 %, 78 %, and 55 % the
size of your telescope's objective. Rotate each screen about 30 degrees with
respect to the previous screen.
How well does it work? I find it works well. When you first look in your
telescope using an apodizing screen, you may be horrified. What you'll see is
an awful array of scattered colors. But in the center of all that you'll see
about a 100 arc-second area of clear view. This points out that the apodizing
screen only works for observing small objects, such as double stars and
planets. Whatever is small enough to fit in that 100 arc-second region of clear
view is fair game.
I nearly always use an apodizing screen on my 6 inch f/5 Newtonian when looking
at planets. I find that the subtle contrast of features on the planets is improved
with the screen. In fact, I can get my apodized f/5 to perform nearly as well on
such small targets as my f/10 planetary Newtonian. I also use the screen when
observing bright double stars.
It costs little and is easy to construct. Go ahead and give it a try.