The Refractor..."Now That's a Real Telescope!"
The general populace has a problem with Newtonians and SCT's -- they look odd. At public star parties, we're forever hearing the question, "How does this thing work?" But in the public's mind, a refractor actually looks like a real telescope. To us amateur astronomers, outward appearance doesn't mean much; what's important is how the scope performs. Quite apart from the popular conception, if you want a good image, many experts will tell you to get a refractor. The images are generally sharp as a tack, with good contrast. But refractors suffer from small aperture; a larger refractor is simply too cumbersome and expensive for the vast majority of amateur astronomers. Expense is probably the biggest detriment. This means that most refractors in the hands of amateurs do not have anywhere near the light-gathering ability of a larger reflector, which is portable and more economical per inch of aperture. For this reason, most people consider a Newtonian or Schmidt-Cassegrain when upgrading.
Gone is the day of the refractor as an observatory instrument. This is notwithstanding that some are still used for important work. The massive size of the Lick 36-inch refractor shown on the previous page stands in stark contrast to a reflector of equivalent light-gathering ability. In fact, a few amateurs have fielded 36-inch Dobsonian-mounted telescopes. (But I maintain that any scope larger than about 16-inches is portable only in the same sense as a kitchen refrigerator is portable!)
Various claims have been made about how refractors outperform reflectors of larger aperture. But to some extent, that's comparing apples and oranges. Granted, for planets and double stars, the sharpness of the refractor's image makes it superior to a somewhat larger reflector. The central obstruction of the reflector's secondary mirror effectively blocks the light gathering of the primary mirror in direct relation to the area covered by the secondary, and it induces diffraction spikes. Unless the primary mirror is of the utmost accuracy, the central obstruction further contributes to a degradation of the sharpness of the image. But in the end, aperture is the name of the game. For deep sky objects, the demand for light gathering clearly favors a large reflector with good optics. Nowadays reflectors are the primary instruments at observatories.
We live in an era when 121/2-inch reflectors seem to be losing their status as light buckets, and many observers have a lust for even larger telescopes. Nonetheless, a few have recovered from aperture fever. One rationale is that especially in our area, atmospheric effects (mostly suspended particulates and water vapor) seriously limit the number of nights when a large telescope can be used effectively. The larger the column of air through which you are peering, the greater will be the image distortion. Light pollution also takes its toll -- a large reflector gathers more light pollution, as well as starlight. But the advantage of a more compact telescope is also obvious. Several years ago I made little, short-focus refractors using copy machine lenses ("copyscopes") for casual backyard observing. The drawback of copyscopes is that they can only be used at low magnification, but they served as the hook that got me into refractors once again. As a kid just starting in astronomy, my first telescopes were refractors. But my goal always was a larger reflector. Today I use both.
The veteran deep sky observer, Walter Scott Houston, had an observatory equipped with a vintage 5-inch Clark refractor, which he used for almost all his observing. A 4-inch refractor will resolve the brighter globular star clusters, and with a 3-inch under dark skies you can spot the Veil Nebula in Cygnus. But nebula filters, especially the narrow-band kind such as the Oxygen III and UHC, require enough aperture to compensate for the light loss induced by those filters. With a 3-inch refractor, there was no improvement to nebulae with the Oxygen III filter and only a slight improvement with a broadband filter. With the somewhat greater light grasp of the 4-inch, there is a noticeable improvement even with the Oxygen III. Visually, deep sky objects don't have the dazzle that they do in a larger reflector. But take a look at double stars, the moon, or planets. The sharpness will blow you away. Also, compare the photos in astronomy magazines. Those taken through refractors have more "sparkle" -- sharp, pinpoint star images.
A few years ago, I was so impressed with the view through Michael Cain's old homemade 3-inch refractor with a Jaegers lens that I bought a Celestron 3-inch as a backyard scope. It was the enjoyment from that scope that eventually persuaded me to move up to a 4-inch refractor. My 10-inch reflector is used almost exclusively at darker sites where the deep sky truly beckons, while the refractor has provided a new appreciation of solar system objects. I've used it more often than the reflector simply because it's an easy-to-handle scope that presents reasonably good views of backyard-type objects. With the clock drive it also serves well as a platform for piggy-back photography. Perhaps the situation would be different if we lived in another part of the country with more ready access to dark skies, where a big light bucket would feel right at home. From places like New Mexico, the view through a large reflector is awesome. Yet even a modest refractor performs just that much better under a dark sky.
Note that we've been referring to commercially-made scopes. Some amateur telescope makers grind their own long-focus mirrors with such precision that the resulting images rival those in a fine refractor. And there are some interesting new telescopes on the market that are a combination Newtonian-Maksutov design. These are very compact instruments that are reported to equal the performance of the best refractors. They command a premium price, but end up less expensive than refractors of comparable aperture. It's something else to consider.
This article is intended simply to suggest that a refractor could be an alternative to the much more common Newtonian or SCT. It cannot be said that a refractor is a "better" telescope than a reflector. Their strengths lie in different areas. But if you subscribe to the belief that the best telescope is the one that's used most often, well...
(In a subsequent issue, we'll review a few of the different refractors that you've seen advertised.)