Pirich's Full Review: Orion ShortTube™ 24688 (189 x 80mm) Telescope
The Orion Short Tube 80 mm wide field spotting scope is a unique adaptation of a small Chinese made refractor originally sold in the US by Orion (www.telescope.com). This version has been put together to serve as an astronomical telescope. In the time since these telescopes first appeared for around $230 in 1998, they have steadily improved in performance as various manufacturing flaws, such as having screw ends protruding into the light path, have long since been eliminated. Currently, it appears to be everything a first telescope should be and can be had with an equatorial mount for $300. The optical tube has appeared in many guises from Orion and Celestron, including on a computerized mount on the Celestron NexStar 80GT for about $250. I have more general information on picking telescopes in my article on Choosing a Telescope.I am organizing this review in the following sections:
Background Description Observations Summary
Background
The short tube telescopes are essentially a complete reversal of conventional thinking on how to build a refracting telescope. These are an attempt to break all of the rules and make a nifty little inexpensive telescope do a decent job at just about anything a telescope can do.
Refractors were the first telescopes. Until Sir Isaac Newton invented the mirror based telescope bearing his name, they were it. They have had an unusual history since their designs are difficult to derive, the optical materials to make them from are expensive, and they have a host of peculiar optical defects. And yet, a good refractor will yield a superb crisp image like nothing else.
The refracting telescope has lenses to bend light and concentrate it to make a higher brightness and magnification image of distant objects. What makes this hard to do is the way light moves through transparent materials. We usually think of light as going through transparent materials. In reality it is sort of like shooting the cue ball into a group of balls on a pool table and causing one to fly out the other side. This means different colors of light, which have more energy per photon as you go from red to green to blue, will all go through transparent materials at different angles, where blue can be thought of as a fast ball, and red a slow one. This is why prisms split light into colors. It also means a simple lens like the one in a magnifying glass will cause different colors of light to come to focus at different locations from red to yellow to green to violet. The result is when the image appears to be in the best focus, it will have a colored fringe on the edge caused by part of the spectrum not coming to focus.
This color based focus can make it nearly impossible to interpret the image, and is called chromatic aberration. Historically, four things have worked to diminish this effect and get a crisper image. First, The effect is less extreme if the glass is less curved since less bending happens to the light, so the focal points are closer. This results in long focal lengths, or telescopes with intrinsically high magnification. Second, a refractor with stopped-down aperture, meaning only the center of the lens is used, will have less color since the most extremely curved parts of the lens with the greatest light bending don't contribute to the image. This has the bad side effect of reducing resolution and brightness since what this has done is reduce the size of the telescope. Third, filtering the light coming out the back to look at specific parts of the spectrum. For example, using a yellow filter, red filter, and blue filter to look at different areas of the spectrum with their own focus separately. The fourth and most sophisticated method is to use several different kinds of glass with different optical properties to produce a compound lens with reduced color effects.
The fourth approach is what is used in modern refractors, with the basic version called an "Achromatic" telescope and the most sophisticated called an "Apochromatic" telescope. Basic achromatic telescopes like the 80 mm short tube have a double lens in the front with one lens out of common plate glass (often called "crown glass") and a second lens made from the glass used in lead crystal (also called "Flint glass"). These telescopes diminish the effects of chromatic aberration, and if combined with other approaches like making the telescope with a long focal length, the false color around objects is largely diminished.
Starting in the late 1970s, several manufacturers became serious about trying to make a sort of "Dream scope" with a short focal length, ideal color correction, and the ability to take high magnifications in one small package. One first-rank answer to this was the Schmidt Cassegrain Telescope (SCT) which had converged to designs with a 10:1 focal length to diameter ratio, or f/10, which was faster than the f/14 achromatic refractors common at the time, but slower than the f/5 to f/8 newtonian reflectors then available. To break out of this set of instruments, the apochromatic refractor has a more complex lens set than an achromatic telescope using exotic glasses such as calcium fluorite (some refer to these as "Fluorite" telescopes as a result). These more extreme glasses have indices of refraction further away from the difference between plate glass and crystal. As a result, it is possible to get them to bring a wider area of the spectrum to a single focus, and thus diminish false color. There is, of course, a catch: The apochromat is, per unit of diameter, six times more expensive than any other design. Or, to put it another way, $2200 will buy a 4" diameter apochromatic telescope's optical tube and nothing else with it. $2000 will buy an 8" SCT on a GPS capable self-guiding computerized mount, which will outperform the 4" scope, but is not as portable and has a longer focal length.
Despite this, the short focal length refractors were so nice to use they soon developed a following and Tele Vue, Astro Physics, TMB, and Takahashi are the Ferraris of the telescope world simply referred to as an APOs. It must have been one of those moments of magic and willing suspension of disbelief behind all creativity when someone said, "Why not make an inexpensive achromatic scope do this?"
The answer was what was called the Short Tube 80, an 80 mm diameter 400 mm focal length (f/5) achromatic telescope Orion (www.telescope.com) started selling in 1998. They were made in China, and appeared to violate all rational thought by going to an optical prescription which must generate incredible amounts of false color- after all, it wasn't an APO, it had a short focal length, and heck, it had come from China, and since when did any decent optics come from there?
But they really were inexpensive at $230, so people tried them out anyway and made a startling discovery- they worked surprisingly well. The Chinese company Synta had produced a simple optical tube with all of the classic elements in an achromatic refractor. The change made to produce the short focal length was a surprisingly subtle one employing a refined understanding of light and optics. Simply put- they didn't make the achromatic the way modern refractors are most often made.
Typically, an achromatic refractor has its color correction centered in the visible light spectrum. This includes light from long wavelength red and orange to intermediate wavelength yellow and green, to short wavelength blue and violet. This would, of course, give the best average compensation for a visual image. What isn't so obvious is the actual weight of light arriving at different colors in our environment isn't the same. Violet light represents a far minority of the light energy visible to us. This telescope is compensated on a narrower part of the visible spectrum from red to yellow to green, and has violet light so poorly compensated it is not a halo around objects; it is smeared across the entire field of view.
The result is a telescope with an extremely short focal length and achromatic color correction which produces sharp images of dim objects. If you look at something brighter with a significant blue component, such as the Pleiades star cluster, the stars appear sharp and numerous, but they appear to be yellow-white and the background looks like indigo velvet. These stars are newly formed and still burning blue-white hot. In comparison, the image in a telescope with no color issues such as an SCT like a C5 will show these stars as brilliant blue-white. More recently, this telescopes' APO brother, the Celestron 80ED #52280 offers performance with no color issues. They lose some of their diamond like quality in this 80 mm scope, but it does produce a focused image, which otherwise would be practically impossible for a refractor like this unless it was has previously been a prohibitively expensive APO. In fact, the APO equivalents to this telescope were $1800 when it was introduced. But now that an 80mm APO is $408-$499, this constraint is not as important.
Description
The telescope tube is gloss white with Orion written on each side of the dew cap in black. The telescope has a lens cap with a secondary cap in the center to be able to stop the telescope down to about 42 mm (more about what this does later). Previous versions of this telescope have had a plastic threaded block on the bottom of the tube for attaching it to a mount. This version of the optical tube has the single mount deleted and instead comes with a set of tube rings with a tripod adapter bolted on the bottom. Although this looks busy compared to the other versions of this telescope, it actually is the source of an incredible amount of flexibility.
The fact the scope is on tube rings lets you mount it any way you want to. The bottom of the adapter block has three 1/4"-20 threaded inserts to attach at different balance points. This allows the telescope to be balanced in its installation since the T-threaded focuser makes it possible to attach a camera straight to it and radically change its balance. The tube rings also make it possible to rebalance the scope by moving it forwards or back in the rings. The rings also let the scope attach directly to an equatorial astronomical mount instead of a camera tripod, as this version comes with. It also makes it easy to piggyback this telescope on another one, which is the inevitable fate of many of these scopes.
The finder scope for it is a little 6X30mm scope on a short arm. To be honest, with a focal length at 400 mm, I am not sure this is a necessary component since the telescope has a field of view over 3 degrees wide (six full moons across) with a 25 mm Plossl eyepiece, so just pointing it in the direction of what you want to look at is very likely to put it in the field of view. In fact, when I took mine and mounted it on a wooden mount I built, I left the finder scope off and have yet to miss it.
The focuser is a rack and pinion with a plastic knob on each side. The barrel of the focuser is silver colored, so there is a pleasing contrast with the rest of the dark metallic gray focuser body which is also useful since it means you can see where you are in the focus travel even under a dark sky. On top of the focuser is a large knob for adjusting friction. The back of the focuser has a 1 1/4" eyepiece holder threaded into the focuser with a locking ring (it is hard to see it has it until you unscrew the focuser while holding on to where it joins the draw tube). This lets you choose which way the end of the focuser is turned so you can choose where the set screw for eyepieces is (top, bottom, somewhere else), or if you have threaded a camera's T adapter to it, you can turn it so the camera is right-side up.
All in all, this telescope is sturdily constructed and has a fit and finish superior to the first examples I saw several years ago. They certainly are up to the task of surviving being a first telescope, a bang-around travel telescope, or a child's telescope (9 or older, though I seem to be getting away with letting a 3 year old use one).
The mount is a minimalist German equatorial mount. It does have a worm gear for matching earth's rotation, but the declination axis is just a screw jack with limited range. With the advent of inexpensive computer driven mounts, there are other options for what to attach this scope to. However, even without a motor, an equatorial mount can make it fairly simple to keep an object in the eyepiece by noting how far one turn on the knob moves the image, and then timing how long it takes an object to move that far in the image. With that said, the mount is fairly basic, and is one component where there really isn't much there to re-use on a future telescope, since the load carrying capacity is very low, and the quality of the mount just doesn't warrant building an observing rig around it. Observations
The most important thing to understand about this telescope is how to get good optical performance out of it. Because of its unusual optical design, it behaves very differently in different applications. The designers had a plan in mind for it, and it will do a lot, but there are certain limitations this scope simply will not yield to.
The optical performance between day and night is interesting. The telescope does have false color, but this is less extreme in several circumstances. For example, low magnification subdues false color since any fringe is small compared to the image. In daylight, it is actually difficult to see at any magnification unless you look at certain types of situations. For example, looking at a tree's leaves gives a light spectrum mostly in green, so no false color is visible. To make false color easy to see in daylight, look at something with black and white lines on it- a street sign, for example. At the line between black and white, the false color and colored halos will become visible as they blur across the boundary.
In the night sky, false color only occurs on certain types of objects. Dim nebulas, galaxies, and star clusters do not show this effect. False color will appear on bright objects. For example, the moon will have blue around it and will have a greenish tint on the interior side of the moon's edge (or limb). Planets such as Mars and Jupiter both have blue halos which also degrade the contrast in their images. Thanks to its amber-beige color, the false color on Saturn does not appear so severe. Several stars, such as Vega will show bright false color halos, as most would expect. What isn't so obvious is dimmer stars such as the double Albiero will also have significant halos.
The telescope comes with an answer to this- the lens cap with a cap in the center. The idea here is to stop down the telescope so the most sharply curved parts of the lenses are not used. In essence, the telescope goes from being an 80 mm f/5 telescope to being a 42 mm f/9.6 scope. This greatly reduces the apparent false color to a negligible level. On the Moon, the false color becomes a small amount of blue fringe and on Mars it disappears altogether. On Jupiter, the stripes on the planet are nearly impossible to make out without using the stopped down lens cap. In a way, that appears to be true for every aspect of this telescope's design; the designers were honest with themselves and the user about their instrument's limitations, and planned accordingly.
Now, in the stopped-down mode, this telescope tops out in useful magnification around 90X on bright objects only. In its full aperture mode, the telescope is fairly clearly a low-power instrument in any case. And this is where the difference between this telescope and an 80 mm f/10 achromatic refractor or and f/6 apochromatic refractor becomes clear. This telescope can't soak up the full magnification its 80 mm aperture suggests is possible, but it is able to go to very low magnifications. This is a compromise to be certain, but it is a reasonable one. Further experimenting with custom aperture masks made of donuts of hobby foam at different diameters could show a little more flexibility, but after trying this out, I have found you will need to try them out on specific objects to see if you are getting a benefit. For example, a 60 mm aperture mask would turn this telescope into a 60 mm f/6.7 which would be a little better at the false color problem and would be a bit brighter image than the 42 mm aperture, but is only really useful if there wasn't much false color to begin with.
To get some idea of what is going on here, I looked at Mars to see what could be seen with and without the aperture mask in place. At 80 mm, the aperture allowed for a nice bright object at 90X, but the image had a vivid blue halo which made it extremely difficult to resolve any details on the disk. The aperture mask dimmed the image and the orange-red color became much more obvious. The vivid halo disappeared, and the planet revealed its ice cap and some of the dark features on the disk. This was even clearer on the double star Albiero, where the two parts were impossible to resolve with the full aperture, while they were easy to separate and the color of the bright yellow star and its blue companion were easy to pick out. To try this on something bigger, I put it on the moon when it was a sharply curved crescent after new moon. The telescope showed a very colorful image with a blue blur around the image of the moon and a strange sort of white-blurred appearance on its surface caused by the overlapping image elements. The edge of the moon itself appeared to be a yellowish green. Stopping down the image dimmed the moon, which greatly helps with eye strain from looking at such a bright object. The features on the surface look very crisp with the mask cap in place and sharply snap into focus.
In daylight, the aperture mask dims the image and makes it look sharper. This is from the depth of field dropping along with the reduction in chromatic aberration. If you are in well lit conditions, say full sun, then this is a fine way to use the telescope. Near dawn and dusk, or with an overcast sky, the full aperture is better for illuminating the image, especially if the telescope is used at lower power. I can see this being a great birding or hunting scope as a result. In essence, this is one optical instrument operating between a conventional pair of binoculars and a larger telescope. The field of view is wide for a telescope, but narrow for a pair of binoculars, while the full aperture capability is massive compared to most binoculars. The ability to shoot a camera straight through it makes it an economical 400 mm telephoto lens.
The other solution to the problem is a modern violet light reduction and contrast boosting filter such as the Baader Contrast Booster,http://www.epinions.com/content_136272449156. I have been using this filter for a while, now, and have found although it does give everything seen in this telescope a mild yellow tint, it is even more effective than stopping the telescope down, so I have left the filter in the diagonal at all times. This is my single most highly recommended accessory for one of these telescopes.
Summary
I have had to give a lot of thought on how to rate this telescope. It has optics capable of a wide range of uses. It has been well designed to make the most of what has been considered to be a practical impossibility. On the other hand, the design is mature and has shown it has real staying power. There are a few hoops to jump through to get it to do what it can. That leaves a real question of pragmatism- what does this instrument do compared to what else is available for similar cost. There really isn't anything more flexible available.
The Orion 80 mm Short Tube telescope is a very capable telescope in a lot of ways, and fills a niche nothing else really appears to. If compared to a pair of binoculars, they will be better at low power wide field viewing. If compared to other telescopes, most have less aperture in this price range, and the complete flexibility with interchangeable eyepieces this telescope offers makes it incredibly versatile. The complete flexibility for mounting it only adds to its utility. In comparison, small telescopes such as the Bushnell Spacemaster are limited in performance, difficult to employ, and awkward to mount.
The simple equatorial mount with this telescope makes it possible to basic astronomical observing. At the cost, even a robust photo tripod is more expensive, so it is hard to knock this. They really are very good first telescopes. If you want something better than this telescope, spotting scopes such as a C90, 80ED, or a Celestron C5 spotting scope have beautiful images as either a spotting scope or an astronomical telescope. For wide fields of view, binoculars do a wonderful job. If you really are constrained on price, this is a very useful answer.
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