Definition and Light - Grasp Can You Afford Both?
There's usually a tradeoff that faces amateur astronomers. When you set out to acquire a telescope, you want images that are as sharp as possible, along with the greatest possible light-gathering. Refracting telescopes are renowned for their high definition. Reflectors are less costly, which allows larger mirrors with more light gathering. The problem is that large optics that give excellent definition may not be affordable. This generally forces a compromise that was highlighted when one of our members made a side-by-side comparison between two different scopes - a Coulter Odyssey 13.5" and a 5" refractor with an objective by D&G Optical. Both were aimed at the Lagoon Nebula in Sagittarius. The image in the Odyssey was brighter, as you'd expect, but the image in the refractor was notably sharper, even though this was a nebula - a diffuse object which normally isn't a good test for sharpness.
A discussion over the merits of a refractor versus a reflector really boils down to how you intend to use your telescope most of the time. Since a refractor doesn't have the central obstruction of a secondary mirror, the image presented will usually be sharper than in a reflector, assuming the optics are of good quality. But inch-for-inch, a refractor is up to five times as expensive as a fully-equipped reflector. For the same price, you'll get much more light gathering in a reflector. There's a saying among amateur astronomers: given a tradeoff between image quality and light gathering, go for the light gathering. What good is image quality if you can't see the image? As with most generalities, this is true only up to a point. Better optics may be the difference between seeing a fuzzy blob and seeing some detail, and details make an object interesting. It also assumes that your intended targets are faint objects and that you'll be observing from reasonably dark skies. But if you observe objects that are fairly bright, then definition becomes more important. And if your site is light polluted, then the light-gathering ability of large optics can be self-defeating, since they tend to gather more light pollution along with light from the objects observed. The refractor often gives better results in less than optimum atmospheric conditions, because there are fewer air currents within the closed telescope tube and a smaller telescope is looking through a smaller column of air. In certain cases, this makes a compelling argument for a refractor. (That's why we recommended a refractor for the Lake County Museum.) But now we're talking about a lot more money for a larger-sized refractor, more than most of us can afford. Can you get light gathering ability and sharp images for an affordable price?
Long-focus Newtonian reflectors can provide images that are virtually identical to those in refractors of comparable size. A high focal ratio Newtonian isn't as sensitive to slight optical imperfections and allows the use of a very small secondary mirror (diagonal). By minimizing the size of the central obstruction, the image quality is improved.
In addition, using a low-profile focuser allows the secondary to be placed slightly farther from the primary mirror because the light cone doesn't have to extend as far out of the tube. As a result, the size of the secondary can be reduced even further. The following diagram shows a general formula for the placement of components in a reflector, and it helps to illustrate how the secondary can be positioned optimally.
Many planetary observers have adopted the six-inch, long- focus (f/8 and up) reflector as a mainstay of their observing arsenals. Such an instrument often outperforms four-inch refractors that cost many times more. Of course, as you increase the diameter of the primary mirror, long focal length brings the liability of a large, ungainly telescope tube.
Off-axis diaphragms permit reflectors (including Schmidt- Cassegrains) to mimic the performance of a refractor by restricting the incoming light to a smaller opening that bypasses the secondary mirror and the legs of the spider. The procedure is simply to make a mask that fits over the opening of the telescope tube. The mask has a circular hole (diaphragm) cut out so that light entering the tube won't encounter the secondary mirror. As an example, a ten-inch mirror with a 2.14" secondary requires a cutout slightly less than 4" in diameter. In effect, that telescope is now gathering the light equivalent to a 4" unobstructed scope (i.e.: a refractor). When positioning the off-axis diaphragm on your scope, try it in different positions. I recently read that some mirrors (primarily the cheaper ones) have "sweet spots" - sectors of the mirror that are slightly better corrected.
Use of a diaphragm dims the image somewhat and since it changes the effective focal ratio, you must re-focus the eyepiece slightly. A diaphragm works well for examining surface details on the brighter planets and for close double stars, which become easier to split cleanly. But there is one drawback - by restricting the aperture, the telescope is less able to pick up colors, which require as much light gathering as possible. Often, the ability to see subtle colors on a planet is an important factor in distinguishing details. Depending on the type of mirror you have, you might want to consider a large on-axis diaphragm. An article in the publication Clear Skies dealt with the problem of edge defects common in mirrors of lesser quality. In this case, the author, David Cortner, installed a permanent 16" mask over his Coulter 17.5" mirror. Says David:
"This large aperture mask hides the mirror's outermost zone and prevents its slightly misfocused light from distorting star images and washing out extended objects with soft, unfocused light. Images are almost as crisp as with the 6-inch off-axis mask and I can't see the slight dimming the big mask must cause...I now consider my Dob a 16-inch f/5 rather than a 17.5-inch f/4.5 because that's how it is always used."
An added consideration is that if you use a large mask, then the secondary mirror should also be sized accordingly so as not to unduly increase the percentage of central obstruction.
Highly accurate mirrors compensate for the effects of the central obstruction and shorter focal ratios. Light is a precious commodity in astronomy, so you want to make the best use of what's available. An accurate mirror concentrates more light into the Airy disk - the image itself - rather than dispersing it around the field, so you get a sharper image and make better use of the existing light-grasp. You can actually end up seeing objects at fainter magnitudes, since the available light isn't being wasted. (Recall that David Cortner was unable to detect a reduction in brightness; the defective 11/2" he masked off from his mirror probably was not contributing anything to the image.) A better mirror also provides higher contrast, and it is contrast which enables you to see details more clearly.
The best mirrors are often made by skilled amateurs who take the time to do the job right. You might upgrade your scope with a better mirror, but refiguring a thin mirror is extremely difficult and the results seldom equal the quality of a good full-thickness mirror. If you buy a commercial product, look for a supplier who's likely to stand behind the performance claims and, better yet, is willing to provide surface accuracy data specifically on the mirror you purchase. Although perfection is elusive, an article in S&T awhile back stated that beyond a certain point, an increase in the surface accuracy of a mirror is almost imperceptible to the eye. Of course, we're referring to true accuracy, not what some claim in their ads. Going from a system accurate to 1/4 wave to one accurate to 1/10 wave may not be worth the effort or cost. But here "system" refers to the total wavefront accuracy, which is the combined accuracy of the primary and secondary mirrors at the point of focus. Since a mirror doubles its error at the wavefront, a primary mirror with a surface accuracy of 1/8 wave is really accurate to only about 1/4 wave in actual use at the wavefront. And that still doesn't take into account any further degradation by a less-than-perfect secondary.
Over the last several years, there seems to have developed a tacit acceptance of poorer quality mirrors, all in the name of making a lot of light grasp affordable. For example, an article on observing faint planetary nebulae in the May, 1994, S&T commented, "Abell planetaries lend themselves well to light-bucket telescopes. Since they are fuzzy, they will look okay through less-than-perfect optics." This tolerance probably originated because thin mirrors are more difficult to produce to an accuracy of at least 1/8 wave. Independent tests have indicated that some mirrors on the market have a considerable number of defects and are no better than 1/2 wave; that means at the wavefront you'd be getting no better than one wave accuracy! But there really is no reason why light buckets should have to suffer from third rate optics. The experts seem to agree that once you've decided how much you can afford, look for the best quality mirror in your price range, even if it means having to settle for a smaller telescope than you'd really like. Don't be blinded by the siren call of the light bucket! If you're simply interested in seeing fuzzy blobs, then get the very largest you can afford. However, higher quality optics will generally let you see more detail than with a larger mirror of lesser accuracy, and you'll have a scope that's easier to handle and more satisfying in the long run.
If cost is no object, then go for the biggest and the best, but practically speaking, everything is some sort of compromise. Knowing where to compromise is the trick.Published in the April 1994 issue of the NightTimes