Sunday, August 22, 2010
Uncle Rod’s Telescope Academy: Telescopes
Complicated choice? A little, but it really boils down to just this: reflector or refractor. There are plenty of subcategories, but there are only two basic kinds of telescope, ones that use mirrors to collect light, and ones that use lenses to collect light, If you are confused already, if’n you ain’t read a basic book on telescopes yet, I suggest you stop rat cheer and do so now. Which book? My time-honored selection is Sam Brown’s All About Telescopes. No, it ain’t modern, and it ain’t gonna educate you about go-to, but you don’t need that at the moment. It will tell you everything you need to know about scope basics—mirrors, lenses, focal length, focal ratio, and all the rest of that forbidding stuff. Go do some book-reading. I will wait.
Hokay, you now know the basics. You know the prime function of a telescope is to gather light. That any telescope can, with the proper eyepiece, magnify as much or as little as you want, but that gathering that light is the important thing. You know shorter focal length telescopes produce wider fields and lower powers eyepiece-for-eyepiece than longer ones. You know that aperture-for-aperture “fast” focal ratio telescopes, telescopes with low focal ratio numbers, have shorter focal lengths than “slow” ones with higher focal ratio numbers. A slow scope is usually considered to be one with a focal ratio higher than about f/10. A medium is around f/6 – f/8, and a fast one is f/5 and below. If you have glommed onto these few facts, you really know all the telescope theory you need to get started.
Still, there is that initial leap in the dark, that major decision: “Do I get a telescope that uses a lens to collect light, a refractor, or one that uses a mirror for that purpose, a reflector?”
Aside from comfort, refractors offer two pluses: lack of required maintenance and more efficient, better performing optics. If you own a Newtonian reflector, you will eventually, if not constantly, be faced with two maintenance tasks: keeping the optics clean and keeping the optics aligned (collimated). The Newtonian’s open tube allows its two optical elements, the primary mirror (the big one at the bottom of the tube) and the secondary mirror (the little one at the top) to be exposed to the air. Eventually, they will get dirty and require cleaning, a delicate task if damage is to be avoided. After a while, they will have to be sent out for recoating, re-aluminizing. A refractor’s single optical element, the big objective lens, is not prone to degradation and will require only gentle cleaning, an easy process compared to cleaning a mirror.
Almost any type of reflecting telescope will need to be collimated regularly-to-occasionally. In fact, larger truss-tube type Newtonians may need to have their optics realigned before every use. Refractors? Their optics rarely—if ever—need adjustment.
There’s also the quality factor. Most reflecting type telescopes need a secondary mirror to divert images to the eyepiece for looking. This mirror, placed in the light-path, doesn’t do too much to reduce the brightness of objects, but it can and does diminish contrast. How much? That depends on the telescope in question, but it will always be “some.” The refractor has no obstruction and is thus able to deliver the best images its objective lens is capable of producing.
Add to those pluses the fact that even large refractors can be ready for critical viewing almost immediately. Reflectors need to adjust to outdoor temperatures before they can deliver their best images. In contrast, while larger refractors may need a little while to acclimate to the outdoors, it’s a quick process compared to a reflector’s “cooldown. That’s why small-medium size lens scopes, especially, make such superb spur-of-the-moment-observing “grab ‘n go” telescopes.
As you may have heard, and if you haven’t, I’ll tell you right now, no telescope is perfect. Refractors suffer from two major deficiencies: lack of light gathering power and either cost or chromatic aberration. What’s important in a telescope? Its ability to collect light from distant objects. The size of its lens or mirror determines how many faint fuzzies in the Great Out There you will see and how much detail you will see in them. On the planets, light is important as well, as the eye has an easier time making out bright details. The size of the objective or mirror also determines the telescope’s resolving power, how small a detail it is capable of revealing. Again, bigger is better. An 8-inch reflector is a common, inexpensive telescope. An 8-inch refractor is a huge and seldom seen instrument due to size and cost.
“Even” a six-inch refractor, even one of fairly short focal length, is a large and heavy telescope. Much heavier than a comparable reflector. That doesn’t just mean it’s more of a pain to carry around; it means a heavier duty mount is needed to support the telescope adequately. A shaky telescope is an annoying telescope; really no telescope at all, as you’ll soon get tired of your objects dancing all over the eyepiece. Not only is a larger refractor heavier than a comparable reflector, it is either very expensive, or possessed of a bad optical problem, chromatic aberration.
You can get a reasonably priced 4 – 5 – 6-inch refractor (though it will still be more expensive than a reflector of equal aperture). The catch is that you may not like its images. The least expensive astronomical refractors are usually achromats, whose objectives are simple lenses made from two “elements” of different types of glass, crown glass and flint glass.
Early refractors had even simpler single element lenses, lenses made of a single piece of glass. And that was not a good thing. Due to the light-refracting properties of a lens, all the rays of light from a celestial object do not reach focus at the same point, with the focus points of blue and red rays being significantly different. For that reason, the in-focus image of a single-element objective looks like something you’d see in a kaleidoscope. Any bright object will be surrounded by a glaring purple halo.
The color purple can be reduced by the use of an achromatic objective. This type of lens, patented by John Dolland in 1758, is, as above, composed of two elements, two separate pieces of glass. Two types of glass with different refractive properties; they bend light rays differently. This can lessen, if usually not eliminate, chromatic aberration. Unfortunately, the achromatic objective can only do its best job at high focal ratios, with a 6-inch needing to be at f/20 before color really becomes unnoticeable. That means a 6-inch telescope will need a tube ten feet long for good color correction. Chromatic aberration increases with the diameter of the objective lens, but even a 3-inch glass (of reasonable focal length) will show considerable color on the Moon, the bright planets, and bright stars.
‘Course, I often hear achromat lovers say, “Unk, the color really doesn’t bother me.” OK, B-U-T. The major benefits of a refractor, perfectly sharp high-contrast images, are somewhat lacking in manageable tube-length achromats. Chromatic aberration, you see, doesn’t just throw up ugly purple rings around everything; it steals sharpness and reduces contrast.
If all there were to the refractor story was achromats, the design would have likely died out for serious use due to chromatic aberration. By the end of the 20th century, however, that nut had been cracked. It turned out it was possible to produce color free refractors by using lens elements made of exotic glass, including fluorite crystal. These telescopes, perfected by Takahashi in Japan and TeleVue and Astro-Physics in the United States, literally saved the refractor from the dust bin of history. Since, with no color problems to worry about, they could be made in fast focal ratios with resultantly short focal lengths, “APOs” became renowned as photographic instruments and are favored today by many imagers.
There is always that catch, though, ain’t there? The catch is cost. APOs are expensive to do well. Their lenses have to be just right to eliminate color, and it often takes three lens elements to do the job. For that reason, a 6-inch can cost ten-thousand dollars or more. It will be a fine instrument, but—and here is the rest of the catch—visually its images will not be worlds different from those produced by a 6-inch Newtonian costing three-hundred dollars. Oh, they will likely be better, sharper if not noticeably brighter, but thousands and thousands of dollars better?
And all of that is true, which is one reason the Newtonian was for decades the most popular telescope for amateur astronomers. It is still the most popular home-built telescope design, and, in Dobsonian form, it’s probably just behind CATs in the commercial telescope popularity contest. For years and years, from the 1920s till the 1970s, at least, the “standard” amateur telescope was the 6-inch f/8 Newtonian.
Yeah, the hordes of Newtonian loyalists will tell you it is the perfect telescope, but when you pin ‘em down even they will admit it ain’t quite that. There are always the “buts.” Starting with the optics. No, there’s no chromatic aberration, but escaping that demands tradeoffs. Starting with the secondary mirror. As I tole y’all in the refractor section, placing that in the light path does do some harm, reducing contrast. If the secondary is kept as small as possible (nothing close to 100% field illumination is needed for visual observing), the damage can be kept to a minimum, though.
Replacing the lens with a mirror does away with chromatic aberration, but it causes another problem. In any but the highest focal ratios (about f/10 for a 6-inch), the mirror must be parabolized; it must have a slightly deeper curve than a sphere, a parabola. Fail to do that and the telescope will suffer from the same problem the Hubble did when it was first launched: light rays from the edge of the mirror will focus at a different distance than those from the center. Resulting in lousy, fuzzy images.
The solution for this problem, spherical aberration, is to use a parabolic mirror. But there is no such thing as a free lunch. As parabolic mirrors get faster, go below focal ratios of about f/5, a problem inherent in all paraboloids becomes ever more troubling. That problem is coma, which results in stars at the edge of the field looking fuzzy no matter how the telescope is focused.
There are workarounds. Using medium apparent-field-size eyepieces makes coma less obvious. But we all want Naglers and Ethoses and that “spacewalk” experience Al Nagler is always going on about, don’t we? The ultimate solution is an auxiliary lens/lens set before the eyepiece, a “coma corrector.” That adds expense and has its own tradeoffs. Me? I just pay more attention to the field center than the edge.
Then there is collimation, adjusting the aim of the Newtonian’s two mirrors so they are properly aligned with each other. Many beginners fear and obsess over collimation, and it is true that a miscollimated scope will yield images from the not-so-hot to the yucky depending on how far off it is. With the proper tools, though, Newtonian collimation is not difficult as long as you keep the big picture in mind and don’t overcomplicate the process.
Finally, there’s the body, the tube. Like the refractor, a Newtonian’s tube will need to be about as long as its focal length. This is maybe less a problem with the Newt than it is with the achromatic refractor, since inexpensive Newtonians are available in fast focal ratios that result in shorter focal lengths and shorter tubes. Be prepared to invest in a coma corrector if you go “below” f/5, of course.
One last thing: if you are intending to move on to imaging with your Newtonian, be prepared to do some modifying. The focuser of the Newt allows a relatively small focus range. All your eyepieces may come to perfect focus, but chances are you won’t have enough in-focus for your camera. Photography will often require you to move the mirror and its mount up the tube to a new location. If you want to go back to visual observing, you’ll have to add an extension tube to the focuser ahead of the eyepieces. Not that big a deal, maybe, but, still…
Dobsonians, by the way, have probably become the dominant type of Newtonian. Chinese telescope makers have done yeoman duty in reintroducing the equatorially mounted Newt to American amateurs, but most of us probably still want a Dob when we want a Newt.
It’s easy to see why. The Dobsonian is about as simple and steady (when properly made) as it’s possible to make a telescope. This is due to the intuitive up/down left right operation of its alt-azimuth mount, and the large bearing surfaces (made from Formica riding on Teflon pads), and the vibration-absorbing wood most are made of. It’s, frankly, quite possible there wouldn’t be many Newts around today, commercial ones, anyhow, if not for the Dobsonian revolution.
“Catadioptric” dern sure is a five-dollar word, but its meaning is easy to understand: a catadioptric is a telescope that uses both lenses and mirror to form images. Yeah, I know, refractor owners often add a mirror to their telescopes, the diagonal, but that is to make viewing more comfortable. In a “CAT” both lenses and mirrors are required for the telescope to produce good images.
Schmidt Cassegrain Telescopes
The other part of the Schmidt Cassegrain equation, the Cassegrain part, means these telescopes display real advantages for you and me, too. In addition to the primary and corrector, there’s a third part of the SCT optical system, the secondary mirror. This is a convex mirror that does two things: it directs the light/images from the primary mirror back down the tube an out through a central hole in the primary (the hole is in the shadow of the secondary, so no light gathering power is lost) for enlargement by an eyepiece. The convex, and thus magnifying, secondary (usually 5x) also slows down the fast focal ratio, usually f/2, primary to something more useful, usually f/10.
As you can see in the diagram above, the SCT’s light path is folded. When you combine the magnifying power of the secondary mirror with this folded light path, the tube of an 8-inch SCT, which would normally have to be over six feet long, is less than two feet in length. That makes the telescope very portable and very easy to mount and very steady on any even marginally adequate mounting.
There’s one other big draw, the SCT’s focusing method. Both Schmidt Cassegrain makers focus their scopes not by moving the eyepiece in and out, but by moving the primary mirror back and forth. Not only is anything you want to mount on the scope, a camera, a big eyepiece, a spectrograph, whatever, very sturdily mounted on the rear cell of the telescope and free of flexure since it does not have to move to focus, the moving mirror yields a huge focus range; almost any camera or anything else can be brought to focus on an SCT.
Finally, the rear eyepiece position makes the telescope easy to balance and comfortable to use. You can sit and observe with almost any SCT setup, just like a refractor, meaning you see more. If you are not standing and swaying at the top of a ladder, it’s easier to see everything the scope can show.
Course it ain’t all hearts and flowers. As the SCT’s critics will point out, all this goodness comes at a cost. The corrector is easy to make in an industrial setting, but maybe not so easy to make perfectly. Usually, images will be just slightly better in a comparable aperture (mirror size) Newtonian.
There’s also that wonderful secondary mirror of the SCT. In the designs essayed by Meade and Celestron, it must be fairly large in diameter. Almost always slightly larger than the secondary of a Newtonian. If the Newt secondary reduces contrast, the SCT secondary reduces it a bit more.
There’s also the SCT focus method. Theoretically, there’s no reason it can’t work perfectly. However, in order to keep costs down, Meade’s and Celestron’s tolerances are fairly large. That causes the mirror to move slightly as it is pushed up and down to focus. Which causes images to move, to shift, slightly in the eyepiece. If the telescope changes attitude radically, as when crossing the Local Meridian, the mirror can move, flop, a little bit, also causing images to move in the field, which can be a problem for imagers. Neither mirror flop or focus shift is a show stopper, but they are annoying.
Finally, there’s cost. Yes, Meade and Celestron have managed to keep their scopes amazingly inexpensive, but the least expensive SCT will always be more expensive than the least expensive (comparable quality) Newtonian.
Collimation woes? Some users are fearful of attempting to collimate their SCTs, but it is a remarkably easy procedure—at least when compared to collimating a Newtonian. The SCT only has one user-adjustable optic, the secondary mirror.
Maksutov Cassegrain Telescopes
Downchecks? The main one is price. The “deep dish” MCT corrector requires a sizeable blank of very good glass, much thicker and heavier than the blank for the SCT’s window-glass thin lens. That is not a huge problem until you go larger than six - seven inches. When you do, the price of the glass and the price of the MCT skyrockets.
The optimized MCT is also a slow system. Most Maks come in at a final focal ratio of around f/15. That is fine, and is just as good for observing the deep sky as any other design—as long as the targets to be observed are not too large. Even with focal reducers, wide field eyepieces, and long eyepiece focal lengths, there is a limit to how wide you can go with a Maksutov Cassegrain.
Commercially made MCTs rarely—if ever—require collimation. “Gregory” style MCTs, anyway. A Gregory design Mak has a secondary that is not separate from the corrector; it is a silvered (aluminized, actually) spot on that lens. These telescopes can often be collimated by adjusting the tilt of the primary mirror, but that is really a job for an optician with an optical bench. There is another variant of the MCT, the “Rumak,” that has a separate secondary mirror in a corrector-mounted holder, just like an SCT. These telescopes are collimated just like SCTs.
Maksutov Newtonian Telescopes
The MNT design has been around for a while and was quite popular in the 1990s. It offers the advantage of medium-fast focal ratios and a small secondary obstruction. Since there is no magnifying secondary, what you see is what you get. The final focal ratio of the scope is the native focal ratio of the primary mirror. Which is usually about f/6. That gives the MCT a nice, medium-wide field, and also allows the secondary to be kept small, ensuring good contrast.
Tradeoffs? The small secondary used in most MNTs doesn’t allow anything approaching 100% field illumination, and while that is not a problem for visual use, it can be a problem for imaging, with light-falloff giving the effect of looking through a porthole. That can be fixed in digital processing, to some extent anyway, but it is a consideration. Anything else? The long tubes—compared to SCTs and MCTs—of the MNTs make for large, heavy telescopes. The presence of the corrector means the MNT will always be heavier than a comparable Newtonian, and harder to mount adequately. These things are likely the reason for the MNT’s lessened popularity in recent years, though it is still prized by some planetary observers.
Like other CATs, the MNT can need collimation, which is achieved by adjustment of both the primary and secondary mirrors. Most MNTs, like most MCTs, rarely require attention.
Schmidt Newtonian Telescopes
“Maybe,” because to my knowledge nobody is doing an upscale SNT. The only commercial Schmidt Newtonian I know of is the Meade LXD-75, which is a bargain instrument, and probably does not demonstrate everything the design is capable of.
In collimation, an SNT—as built by Meade—is, like the MNT, a combo of an SCT and a Newt, with both secondary and primary requiring adjustment. Doable, certainly, but a little more difficult than SCT/MCT collimation.
So there you have it, all the readily available commercial telescope designs. Well, all those likely to be of interest to beginners focused mostly on visual observing. Which should you choose? Only you can decide, and a bigger help than reading the cotton picking Internet and looking at magazine advertisements is getting out and trying a few. At your club’s observing field. Pick the brains of your fellow astronomy club members, too. What have they favored most over their years of observing? You ain’t a member of your local club? Fix that. I insist.
You still want to know which one I think you should pick? OK. But don’t complain if you don’t like my answers. If you intend to start out and maybe stay a purely visual observer, and especially if you don’t want to spend a whole lot of money at first, get yourself a Dob-Newtonian. Maybe one of Orion’s scopes. How big? All things being equal, more aperture is always better—“aperture always wins,” some of us like to say. But all things are not always equal. Don’t convince yourself to get a scope that’s so large you will be reluctant to take it out for a weeknight 30-minute look at the Moon. That said, also do your best not to go below 8-inches.
You think you might want to do more than just look at the Moon? Maybe take pictures of it or even the neighborhood galaxies some day? Or do other interesting things like put a deep sky video camera on the scope? Think “SCT.” The advantages of the SCT are its portability and versatility. It may not be the best at any one thing, but it is very, very good at many, many things.
Don’t agonize too much over your decision. Keep in mind that any telescope is better than none. You would be gobsmacked at the wonders I’ve seen—and continue to see—in my humble 4-inch StarBlast Newt. A telescope that gets used regularly always shows far more than a big, expensive “hangar queen.”
Next Time: I have received a set of the Sky and Telescope DVDs. What, you’ve been living under a rock and don’t know what the hell Unk is a-talking about? Sky and ‘Scope has put nearly seven decades of their wonderful magazine on disk. And you can dang well be sure I am excited about it and am gonna tell y’all all about it next Sunday.