Aperture Isn’t Everything

By Bill Pellerin
Houston Astronomical Society
GuideStar Editor

When amateur astronomers get together and talk about their telescopes, they usually discuss the aperture of the telescope first. Why? Because it is easy to determine, in fact, it is probably the parameter that is of most interest to knowledgeable telescope buyers.

Professional observatories make great efforts, and spend a lot of money, to increase the effective aperture of their telescopes. The Keck telescopes in Hawaii combine the light from two mirrors (two telescopes, really) providing an aperture of 280 feet and a resolution of 5 milli-arcseconds. At Mount Wilson, there is the Infrared Spatial Interferometer – three 65” telescopes, spaced up to 279 feet apart to get high-resolution infrared images.

The next most discussed parameter is focal ratio, which is stated as f/10 or f/5, whatever it is. This number defines the focal ratio, the aperture = the focal length (f) divided by the focal ratio. Or, the focal length = the aperture * the focal ratio. The focal ratio is of great importance to those who image the sky.

For example, a f/10 SCT (a common telescope in use by amateurs) of 8” diameter has a focal length of 10 * 8 = 80 inches. It is more common that the focal length is talked about in millimeters, so applying the correct conversion (1” = 25.4 millimeters) we get the focal length = 2032 mm. It’s peculiar that astronomers talk about diameter in inches and focal length in millimeters, but that is often the case.

By Bill Pellerin
Houston Astronomical Society
GuideStar Editor

This article is not intended to be an introduction to telescopes, but is intended to identify one parameter of telescopes that receives scant attention — the optical quality. The resolving ability of a telescope is determined by aperture, of course. More aperture equals more resolution, all things being equal. The problem, of course, is that all things are never equal, and the optical quality of telescope is what determines the quality of the image. Quality is a vague notion, but refers to the sharpness of the image, the absence of image artifacts, and the clarity of the image. It includes the optics’ ability to focus all the light from a star to a single point.

None of this says anything about the limitations of resolution imposed by the ‘seeing’ (the turbulence of the air between the observer and space). Sub-arcsecond seeing is generally rare and usually fleeting. That is, you may get it from time to time, but you won’t get it for long. Professional telescopes often use adaptive optics to reduce the effect of the seeing, but this is not a capability in wide use by amateurs.

Most advertisements, and web sites, for telescope makers tell you the resolution of the instrument, but this is based on the aperture only, and not on the average real-world performance of the telescope. Do you ever see something in telescope specifications about the optical quality? Not usually. There are often general statements about how the wonderful the optics are, but there is not much you can use to compare telescope A to telescope B. The term ‘diffraction limited’ is often associated with commercial telescopes. The manufacturer is telling you that the resolution of the telescope is limited by the aperture, not the quality of the optics.

In the end, as a practical matter, the quality of the view is a function of the optics. It should come as no surprise that high quality optics come at a premium price. While excellent optical systems can be made with any design, some designs make the realization of optimum optical quality difficult. Clearly, the number of optical elements in the design imposes the requirement on the optician to make each of the elements of first-rate quality. While this can be done it’s not easy.

One example of a telescope with poor optical quality, familiar to most of us, is the early Hubble Space Telescope. Do you remember that when it was initially put in service that the images were poor because the primary mirror had been made to the wrong shape? The surface of the mirror was exceedingly smooth, but with the wrong shape the mirror did not focus all the incoming light to the same place. A subsequent fix to the telescope (by adding corrective optics) resolved the problem and the extraordinary images that we get from Hubble are the result.

In our Houston Astronomical Society newsletter (the GuideStar), Clayton Jeter does interviews with people of interest to amateur astronomers. In the June, 2010 issue, Clayton interviews Carl Zambuto (Zambuto Optical Company). Here’s what Carl has to say, “And another thing – I always prefer quality over aperture. Yes, I like aperture, but quality comes first. Contrast in a smaller instrument will outperform larger, mediocre optics. Not everyone realizes this. … Everything is measured in contrast, first. Without contrast, you see nothing. There is no such thing as lesser quality large aperture being better for deep space.” Carl says that contrast “is more dependent on quality of wavefront, which we produce with a combination of surface smoothness and accuracy.”

You can read the entire interview at www.astronomyhouston.org. Look for the GuideStar link and then the June, 2010 issue.

What are we to do, since optical quality is not something we can easily compare based on published specifications? The answer, I believe, is to become intelligent consumers of telescopes and other astronomical products. No telescope design is best for all purposes; every design is a compromise in some way or another and you need to decide what compromises are acceptable to you. You may want a highly portable and easy to set up telescope instead of one that is, say, larger, but harder to transport and more trouble to put together (sometimes in the dark). All of us have limitations on how much money we are able to spend on a telescope.

So, how do we resolve (pun intended) the problem that there is not a good metric available to us to compare the quality of the optics used in telescopes we might be considering?

The answer is to look through a lot of telescopes, and you can do this if you live in a large enough city to have an astronomy club and a club observing site. You can also do this at star parties. Check out the image quality and the light gathering capabilities in high-end telescopes of all designs, refractors, catadioptrics, reflectors, and any other design you can get a view through. Look at a planet or a deep sky object through each of the designs and see which view pleases you the best. The planet (or the moon) will, assuming good seeing, reveal something about the quality of the optics. Look through the telescope long enough to get one of those (rare) moments of excellent seeing. Look for the planets, the moon, or stars to ‘snap’ into focus. There should not be any ambiguity about where the correct focus point is. If the best focus point is uncertain, this doesn’t speak well for the telescope optics. A deep sky object will give you a sense of the affect of the light gathering capability of the telescope.

Look for color correction when you look through a refractor. A well color corrected refractor will bring all colors of light to the same focus point simultaneously. A poorly corrected refractor will not – you will see colors at the edge of the moon or at the edge of a planet or a bright star.

The book Star Testing Astronomical Telescopes, by Harold Richard Suiter (published by Willmann-Bell) goes into substantial detail about how to evaluate telescope optics, with no instruments other than your eyes.

One more thing – your eyes are part of the optical train, and if you have astigmatism, like I do, your view of the universe will be the worse for it. You can get astigmatism correctors or eyeglass lenses that correct only astigmatism to solve this problem, and if you need one of these solutions, get one. Whether you need one or not depends on your optical configuration, and the degree of astigmatism that you have. It will make a substantial improvement in your observing experience. Check with your eye-care provider to get your astigmatism correction requirements.

The bottom line? If you get a good quality instrument of any design it will provide you with excellent observing opportunities for a long time. As your needs and plans change, you may want something else. Not a problem. Many of us who have been observing for a while have had several telescopes, and some of us have several now. Enjoy the sky with whatever optics you have – your unaided eyes, binoculars, or a telescope and develop the ability to recognize excellent optics when you see them.