Telescope Diffraction And Apodizing Masks

It may seem counter intuitive, but increasing the size of a telescope decreases the size of the resulting diffraction pattern, as well as the central disc of the pattern. You might think that a bigger telescope would make a star image bigger, not smaller. The image you see above illustrates this principle. The image on the left shows a star pattern intensity plot as seen through a certain size telescope, and the image on the right is that of a telescope of twice the diameter.

Recall that 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 only the smallest resolvable disc the specific telescope can create. It can't see things smaller, so 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.

Note that on the right image, the first diffraction ring is only as big as the central star image on the left. So one way to reduce the effects of diffraction on resolution is to simply use a bigger telescope. As it happens, reflecting telescopes, even with their secondaries that cause more diffraction problems, are much less expensive than similar sized refractors. For example, a high quality 3 inch diameter refractor telescope might cost $1000, but for that you could easily get a 10 inch reflector of Dobsonian design. The larger size of a reflector can then resolve better than a smaller sized refractor, even though the refractor delivers a less objectionable diffraction pattern.

There is another lesson here. The size of an aperture determines the size of the diffraction pattern. This leads to a way one can improve the diffraction pattern for a given telescope. The device that does this magic is called an apodizing mask. An apodizing mask allows one to essentially merge diffraction patterns from two different size apertures, causing some destructive interference between the rings -- at least the first couple of rings.

Above you see three diffraction pattern presentations. The left image is that seen through a refractor. It's labeled clear, meaning that a refractor has a clear aperture. The second image is an intensity pattern one might see through a typical reflector that has a secondary about 1/3 the diameter of the main mirror. It's labeled obstructed because the secondary mirror puts an obstruction in the field of view. As you can see, the obstructed image has diffraction rings, especially the first one, that have more amplitude compared to that of the central disc.

The third image is like what would be typically be seen through a reflecting telescope that is using an apodizing mask. This illustration shows that the rings are reduced in amplitude. The cost of this magic is of broadening of the size of the central disk, and (not shown in the illustrations) a reduced intensity of the central disc. Even with these costs, however, the pattern seen through an apodized aperture has less negative impact from diffraction rings.

An apodizing mask to decrease the effects of diffraction can be made with simple materials. It's not a high tech device. All you need is some thin tempered hardboard or even cardboard, and some window screen. You don't even need to build the more complex version to get some advantage from it. The tempered hardboard or cardboard can make a hood that slips over the end of your telescope and holds the mask of window screen.

Above you see three apodizing screen designs, and below each the effect the screen has on star image diffraction. Moving from left to right, each mask has an additional layer of window screen with a larger hole cut out, and mounted so that the screen is rotated 30 degrees from the orientation of the previous layer. The result is that with each additional layer, a darker shading occurs.

The layers are generally cut to have hole sizes of 55%, 78%, and 90% of the diameter of the telescope's main objective. The star intensity graphs below each screen show its effect on the diffraction pattern produced by a telescope with an obstruction of 33%. As you can see, even one screen helps damp out the diffraction rings. Additional screens further impacts the diffraction rings, but also further dim the overall image.

If you decide to build an apodizing mask, be warned that only the center 100 arc-seconds or so of your field of view gives the improved image. Around that dark, low diffraction center is a horrible rainbow of scattered light caused by the window screen. Well, nothing comes for free.

Because of the scattered rainbow effect, it is clear that the apodizing mask is only useful on small targets, like a planet or double star. For large objects, like a star field, galaxy, or the moon, you'll not find the apodizing mask of use at all.

Some people believe the mask is only useful in helping viewing on nights of bad seeing. It may help on those nights, but probably mostly because it reduces the brightness of the images, thus making some of the light distortions less noticeable. The real advantage is as these graphs show, less scattered light into the diffractions rings, which help in the separation of close stars, and in providing modest enhancement of planetary contrast.

So how many screens do you need? Most directions suggest 3. I would modify that suggestion by taking into account the size (diameter) of your telescope. At 8 inches and above, I would also suggest 3 screens. But at 6 inches and below, my experience is that the lost light is a heavy cost. With my 6 inch reflectors I use just one screen, as in the leftmost image.

Does an apodizing mask help?

I think so, when used on small targets. A good example is the double star Rigel. Rigel is a rather bright star at the lower right of the constellation Orion. It happens to have a much dimmer partner. With my 6 inch f/10 telescope, specially designed for high resolution work and having a very small secondary, I can easily see the companion to Rigel. But with my 6 inch f/5 telescope, which has a 33% obstruction caused by its larger secondary, Rigel provides too much scattered light for me to easily make out the dim companion. When I slip on the one screen apodizing mask, I can then easily see the companion. Now quite as easy as with the f/10 telescope, but certainly an improvement over the non-masked f/5.

Apodizing screens cost little to make, and are easy to construct. Because of those factors, I suggest that you consider making one, at least one with the single screen. Give it try on the bright double star Castor in Gemini, or the challenging double star Rigel and see what happens. Try in on your next outing with Mars or Jupiter. I think you'll find at least modest improvement in your views.