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Diffraction: What is it?

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Telescope Diffraction



If you've probed into purchasing a telescope for very long, you've probably ran into the term diffraction, especially when discussing reflecting type telescopes. I cut my teeth on reflecting telescope, learning much from my old copy of Making Your Own Telescope (Dover Books on Astronomy), still considered one of the best telescope making books out there. Many of the concepts introduced here are covered in more detail in Making Your Own Telescope.

What is diffraction?

No doubt you've heard of reflection, the optical phenomenon that makes reflecting telescopes work. And refraction, the phenomenon that makes prisms and refracting telescopes work.

Diffraction is also a phenomenon that works with light. It is the property of light to be slightly redirected when passing near the edge of an object.

In the telescope sense, diffraction is a bad thing. It causes light to be scattered from the desired focal point to concentric rings around each point. The fundamental pattern of diffraction in telescopes is caused by the circular aperture of the telescope itself.

Unfortunately, as the different clever optical designs add the necessary elements to facilitate the new design, they tend to add additional surfaces that cause more diffraction.

The following discussion tries to show the diffraction effects seen with the common telescope designs available to the amateur astronomer.


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Diffraction and the Refractor

The above left image is the circular aperture of a refractor, and on the right is the diffraction pattern that results.

If you look at a star with high power, you'll see the central peak and perhaps a couple of the diffraction rings.

I've suppressed the central peak a bit for these images to show more of the rings and the effects different telescope designs have on the distribution of light in the rings. When actually looking through a telescope, you generally only see the first 2 or 3 rings, but the others are there, just too dim to see.

This diffraction pattern, caused by the circular aperture, is superimposed on each point of light in the field of view.

Of course, the desired image through a telescope would be for a single point of light for a point source (which a star is), but that's not possible with a circular aperture without some unfortunate trade offs.

So the best star images easily available are those from a quality refractor, in which the only diffraction pattern contributions are from the circular aperture of the objective lens.



Diffraction and the Cassegrain

The above images illustrate the aperture of a Cassegrain and the resulting diffraction pattern imposed upon a star image. As you can see, the central obstruction of a Cassegrain, being the secondary mirror, can approach about 40% of the total aperture diameter. That's the one trade off of having such a handy short tube telescope. The light cone of the focusing beam from the primary has to be intercepted when it's still rather large, necessitating the large diameter secondary.

This effect leads to a considerable amount of light being scattered from the central peak of intensity to the surrounding rings by passing near the circumference of the secondary.

Compare the pattern with the tighter light distribution of the refractor.

This reduces the ability of a Cassegrain of the same size as a refractor to split close double stars, as an example.

Since an object with size, like a planet under high magnification, can be considered to be just many, many points of light, you can see how the light dispersion of a Cassegrain would interfere with the resolution of details on an extended object.





Diffraction and the Windowed Newtonian

The Newtonian telescope (see illustration above) usually has a secondary much smaller than a comparable diameter Cassegrain telescope. The secondary is typically on the order of 15% to 30% of the objective diameter.

In the case illustrated, the secondary is about 20% of the size of the main objective, and has no visible supporting structure, which is consistent with a Newtonian having an optical window supporting the secondary.

Diffraction is still caused by the secondary, and redistributes light away from the central peak and into the diffraction rings. The effect is just not as bad as with the larger secondary of the Cassegrain. Compare this with the Cassegrain diffraction pattern.

There aren't very many models of Newtonian telescopes that have such a window to mount the secondary. The Astroscan by Edmund Scientific Co. is an example, however.



Diffraction and the 4 Vane Spider

More typically, as illustrated above, the Newtonian has some spider vanes, as they are called, to hold the secondary mirror in place.

A commonly used design, especially for telescopes greater than 6 inches, is the 4 vane spider.

However, the vanes act as additional diffraction surfaces, and result in light being scattered around the vanes.

While noticeable only around bright stars and planets, the diffraction pattern contribution of the vanes is spikes around the image. I've seen considerable spikes around the planet Mars, which can be very bright during a favorable opposition.

If you look at star field time exposures, you'll often see spikes emanating from the brighter stars. This is caused by the secondary spider of the telescope that was used to take the photographs.



Diffraction and the 3 Vane Spider

Sometimes, as show above, a 3 vane spider is used instead of a 4 vane. The aperture looks like the image on the above left.

Not intuitively, the resulting diffraction pattern displays 6 spikes, not 3.

So while in fact the 3 vane spider has less diffracting surface than a 4 vane (and thus less total diffraction effect), the resulting 6 spikes can be a distraction.

In using a 3 vane spider in a telescope for Mars observing, I found the spikes to be quite distracting.

You'll notice that the 6 spikes in the diffraction image are less bright than the 4 spikes in the 4 vane diffraction image. This is because in each case, each vane causes 2 spikes -- opposite one another in the image. In the case of the 4 vane spider, the opposite spikes of each vane reinforce the initial spike caused by the vane on the opposite side.



Diffraction and the Curved Spider

The above pair of images illustrate the curved spider mount and the resulting diffraction pattern.

Even though there is a curved vane acting as another diffraction surface, the curved nature (a 180 degree curve) causes the light to be evenly distributed throughout the diffraction rings.

The result? No spikes. In fact, the image through such an equipped Newtonian is similar to that through a Cassegrain.

Actually, it's better, diffraction-wise, in that the thin curved vane causes less additional diffraction than the increased size of the Cassegrain secondary.

It's a bit like having your cake and eating it too.

So what's the tradeoff? In the case shown, none, really. The length of such a curved secondary is about equal to the sum of the lengths of the struts in a 3 vane spider. So the total diffraction surface is about the same, but the spikes are simply gone.

Compared to a 4 vane spider, this type of curved spider has less total diffraction.

The main issue is that this simple curve design is only adequate for reflectors up to 8 or 10 inches. Beyond that the secondaries get so heavy that one needs to resort to 3 or 4 S-curved vanes to support the secondary.

With the S-curve or similar designs, there is a tradeoff. Clearly 3 S-curved vanes are longer than 3 straight vanes. So once one is forced to have multiple curved supports around the aperture to support the secondary, eliminating the spikes comes at a cost of more total scattered light.



Refractor
4-Vane Newtonian
Curved-Vane Newt
Cassegrain

The above images put the refractor, 4-vane Newtonian, curved-vane Newtonian, and Cassegrain diffraction patterns side by side for more easy comparison.

One wants all the light to be in the center peak, but as diffraction elements are added (necessarily according to design), more of the light gets re-distributed.

If you look closely, you'll see the tightest distribution for the refractor, next tightest for the Newtonian, and most diffused for the Cassegrain. The curved-vane Newtonian diffraction pattern looks little different from a windowed Newtonian.

I have two 6" telescopes now, an f/10 and an f/5. Both have curved secondary holders.

The f/10 has a secondary whose size is only about 16% that of the main objective, and with the curved secondary holder gives near refractor-like images.



So, isn't the Refractor Clearly Better?

If diffraction alone is considered, yes the refractor is superior. That is, for a given apeture. To be fair, refractors also tend to give steadier images.

Presented above is the diffraction pattern for a refractor and a curved-vane Newtonian of the same aperture.

As seen before, the Newtonian splatters more light into the diffraction rings, leading to less contrast on high resolution objects.



Factor in Cost and Size

The above pair of images reveal why everyone doesn't rush out and buy a refractor. On the left is the refractor pattern again.

On the right is the pattern from a curved-vane Newtonian of twice the diameter as the refractor.

See how the rings, even though carrying redistribute light, are significantly smaller than in the refractor pattern?

So simply by using a larger reflector one can get a diffraction pattern much smaller than the cleaner, but more spread out pattern of a smaller refractor.

And it turns out, it's the economical solution. One can purchase a 12 inch Newtonian or an 8 inch Cassegrain for the cost of a 4" refractor. So that's what most people do, get a reflector of any type a size or two up, and yet spend far less than they would on a comperable sized refractor.



A Case in Point

Pictured above is a view down the eyepiece end of my Discovery 6" f/5 equatorial Newtonian. My telescope is no longer sold by Discovery, but it's basically the same as the Celestron 31057 Omni XLT - 150.

You should be able to easily make out the curved secondary holder I've installed. From this close view you can see that the metal used is very thin on the edge, and has a width of about 5/8" of an inch.

From a distance, all that shows of the holder is the thin edge, a bit over 1/16" of an inch.

The original spider was a 3-vane holder, and the vanes were nearly 1/4" thick (I kid you not). The resulting diffraction pattern of the original spider was a pretty ugly thing when a bright object like Mars was viewed.

The curved element, on the other hand, simply shows the bright ball of Mars surrounded by just a bit of glow. Vastly superior in my humble opinion.

I believe Discovery imported this model, but used their own manufactured optics (which seem excellent, by the way). I don't believe they sell it any more.

I chose it because it was mounted on the same tripod as an 8" model that Discovery sold. I figured that if the mount could handle an 8", however adequately, it would easily handle the 6". I hoped also that the total apparatus would be light enough to be moved around the yard without disassembly.

Both hopes were satisfied. The mount easily handles the short 6", and the entire construction weights in around 35 pounds.

I had two issues with the telescope when I got it. The tube is a bit undersized, measuring about 7" across (8" is recommended). This led to the eyepiece tube extending into the field of view with short focus eyepieces.

The second issue was the thick-vaned secondary holder, which gave some whopper spikes on bright objects.

I solved the first by removing the optics and cutting about 3/4" off the mirror end of the tube. Then I reassembled everything. I needed to cut off the tube end because the main mirror mount also served as an end cap, and couldn't be moved forward in the tube. The end result was that the eyepiece tube focused further out and no longer extended into the field of view.

I solved the second issue as shown, with the thin metal, curved vane spider.

Now I couldn't be happier with this instrument. It's as easy to move around as I hoped, has a clock drive that lets me do some photography, gives superb star field views, and provides acceptable planetary views. All for a cost of a few hundred dollars, and without the distraction of the spikes so often a part of the Newtonian view.

While I don't believe Discovery sells this model anymore, I believe Orion, Celestron, and possibly other suppliers do. Use the astro-customzed search engine below to search for a Newtonian telescope.

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