Refractor Breakthrough [Update 01/27/2008]
Written: Dec 14 '04 (Updated Jan 27 '08)
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Product Rating:
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Pros: Rugged construction, fantastic performance, large finderscope
Cons: Heavy, no piggyback mounting block
The Bottom Line: The Celestron 80ED combines extemely tough construction with excellent performance. It is a superb telescope.
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| Pirich's Full Review: Celestron 80ED Refractor (24x80 mm) |
The Celestron 80ED 80mm refractor can only be described as a breakthrough. The unassuming Celestron 80ED 80mm refractor marks the point when a $415 telescope incorporates design and performance of what was $1800 two years before. Its construction is more rugged than anything I have ever seen. Its optical performance leaves nothing to be desired. In short, this telescope has shattered the previous landscape of standards for quality and value in refractor telescopes. I will be revisiting and revising everything I have ever written about telescopes on epinions because of this instrument. Today, this refractor is a survivor of the test of time, and has become an even stronger bargain at $384. I have more general information in my article on Picking a Telescope
Background
I have written about the small refractor phenomenon in other reviews, but considering this telescope has shed much of this in a new light for me, so I am going to write a somewhat different treatment, here.
The refracting telescope is what Galileo used to discover Jupiter's moons. By watching them circle Jupiter, he was able to see conclusive evidence the Earth is not the center of the universe. The refractor is conceptually very simple- one can be made with two lenses- one with a large focal length and one with a short focal length. The large focal length lens acquires an image and focuses it while the short focal length lens will magnify it and produce an image to see.
But there is a hidden problem; Lenses bend light based on the color of the light, so they bend red, yellow, green, and blue to different focal points. In practice, this causes an image to have false color- essentially, anywhere you have sharp brightness contrast in an image, you will see a blue-violet fringe on the outside of objects.
There are three effective approaches to reducing the false color problem to have a decent image: (1) make a telescope with a long focal length so the glass is less curved and causes less false color. (2) Stop down the aperture so only the center of the lens is used. (3) make a multipart lens stack with lenses made from glasses with different indices of refraction to produce a more perfect light bending mechanism.
The first two approaches work by limiting the telescope's field of view, and in many ways, its utility. The path to greatest performance for these has been the most difficult to design- option 3. Two levels of performance have been achieved by making compound lenses. The earliest and most basic of these is called an "Achromatic" telescope, which has a two part lens made from plate glass and crystal (often referred to as crown glass and flint glass since the early makers knew them as that). The achromatic telescope narrows the size of the focus for different colors and produces a better image than any single lens could. However, this does have false color, which will become more visible the lower the focal ratio (the telescope definition of focal ratio is the focal length divided by diameter, so a 600mm focal length with 80mm diameter yields f/7.5, and you would use this just as you would to determine the exposure for a photo taken at f/7.5).
To overcome this, Apochromatic (APO) telescopes were developed over time using calcium fluorite crystal for its extreme index of refraction. And though some of the basic apochromatic designs were proposed as early as the 1830s, it wasn't until the late 1970s practical versions of these lens systems appeared in telescopes for non military users in large numbers. APO telescopes correct false color to the point where the entire visible spectrum is well focused enough to produce crisp images, and are able to out perform any other telescope type at their diameter.
There is, of course, a catch; they are more expensive per amount aperture than any other type of telescope. To give some idea, a 5" diameter schmidt cassegrain type spotting scope (which uses mirrors to defeat color effects) is around $550. A 5" apochromatic refractor is a cool $4500 or more.
The main makers have been Astro Physics, Tele Vue, and Meade in the US and Takahashi, Vixen, and Borg in Japan. Other available APO telescopes over the years have been from Pentax, Nikon, and Zeiss. None of these have a reputation for being cheap, but the APO refractor has become, for many birders, astronomers, and other naturalists that "Someday" telescope.
The first indication a price breakthrough may be coming was the Orion 80 ED telescope introduced in 2003. This telescope has an 80 mm apochromatic lens assembly and an f/7.5 prescription. The tube assembly is made up of components used in other Orion telescopes, so the new equipment is the lens. The breakthrough was this telescope is $499, while a 76mm Tele Vue is $1275. As a result, this telescope was quickly adopted after overcoming skepticism over how good Taiwanese APO optics would be, since frankly, nothing with that level of quality had been seen in this price range.
But the Orion 80ED showed things have changed across the Pacific. But it wasn't until this year until a fairly large and homely optical tube assembly was replaced to produce a more integrated design. That design is the Celestron 80ED.
Description
In the photo, the Celestron 80ED looks like it probably has a fair amount of plastic, and is lightly built. The scope appears to be fairly sleek, but what is not so apparent is it is a new design and is very solidly built. At this point, I find myself at a loss for anything to compare it to. The only plastic parts on this telescope's optical tube are in the len caps and part of the finder scope. Everything else can only be described as being built like military hardware. No other manufacturer's small refractors approach the heavy duty construction of this design. When I first took it out of the box, I was flat-out amazed at what I discovered- everything appears to have been designed for a lifetime of use. I had just been curious about this telescope and thought it might be a good wide field photography telescope when I bought it for $409 from Woodland Hills Camera, www.telescopes.net. It has already shown it will be a mainstay.
Although it has the same lens prescription as the Orion 80ED, the similarity stops there. The entire optical tube assembly is a clean sheet of paper design yielding a more compact optical tube (for example, the barrel is 90mm in diameter instead of the 100mm Orion barrel). Unlike the Orion scope, the celestron 80 ED comes with a tube ring assembly instead of a small fixed threaded block on the optical tube. This makes it easy to scoot the optical tube forward or back to balance on a tripod after you put a camera on the back. The 2" diameter focuser is a rack and pinion with helical gears to smoothly move in and out. This focuser is clearly built to take heavy equipment on the back end- without tightening the locking knob, the rack will stay put while the telescope is pointed straight up and a 2" diagonal and a heavy 2" wide field eyepiece are in it. The focuser knobs are two oversized aluminum wheels with rubber grip tread.
The tube ring assembly is also something new. It has both rings in one component, is all metal, and has a clamp mechanism like a hermetic sealing jar. In an unusual step, the tube ring has built-in ears which should allow it to lock on to any mount with a Vixen dovetail, including Celestron astronomical mounts as well as Vixen, Orion, and a few other makes. However, the length is non-standard, so it actually has a bit of trouble with most standard mounts, and it actually doesn't allow the tube to slide quite forward enough to balance. The solution is to bolt a Vixen type dovetail to the collar. Dovetail bars are available from Orion (www.telescope.com- search on Dovetail) and more detail is in my review of the Orion Dovetail. The other improvement I can see to this part would be to add a hard point for attaching piggyback cameras or finders.
The lens group is in a single machined block and pieces of cork are used to hold the lenses in place while metal foil spaces them apart. The lenses have an even forest green sheen from low reflection coatings. There was one flaw in the form of a large speck of white material on the inside. I removed this with a duster can after unscrewing the lens cell from the tube. The dust appeared to be some contamination from the threads, and since the rest of the optical tube was clean, I hope this was an anomaly. Note, if you are going to consider doing something like this, do not allow anything to touch the glass and prepare carefully before hand.
In the barrel, the telescope has two light baffles and a there is a third one in the focuser's draw tube. The dew shield is threaded on, and the lens cap plugs in to the end of this part (the front lens cap the least satisfying part the telescope came with). The dew shield is bell-shaped on the inside and is curved on the outside. The original paint inside is a matte black finish.
The back end of the focuser draw tube has a 1.25" adapter to take the 1.25" erecting image prism and 25mm eyepiece the telescope came with. The adapter has T-thread on the outside, so if you are going to use the telescope with a 35mm or digital SLR, the SLR to T-thread adapter will be all you need. The adapter allows you to reorient anything attached to the back end, and has a large conical capture feature to ensure there is no risk of it falling out.
Observations
The day the telescope came was overcast and gray- a phenomenon known as "The new scope curse." I was able to only do two things that evening- get a look at the star Capella through a small hole in the clouds, which showed it and other stars nearby were tiny focused points anywhere in the field. I then put it on some Christmas decorations down the street and had the first large surprise- no color around little lights but also incredible detail and clarity as though items in the foreground had been cut out with a razor blade. When my wife first saw the view through it, she gasped at the clarity, then asked, "How much did you say you paid for this?"
Since then, it has been out every clear night. The Celestron 80ED has shown the ability to bring in stars as pinpoints, and objects on the ground are remarkably crisp and clear even at 200X. Images have great depth and false color has proved to be extremely difficult to find. To see the false color in an image, put the telescope at high magnification while pointed at a white star. When at the center of the field of view, there is no significant false color, but if you move the star to the very edge of the field, it will be possible to see a small blue-violet plume shaped tail pointed towards the center.
The acid test for false color with any telescope is, of course, the moon. On the moon at magnifications from 18X to 75x, no false color could be seen. Where an older Tele Vue 85 showed a yellow-green band on the rim of a crescent moon, this telescope was true color with beautiful contrast between light and shadow. The blue of the reflected earth shine on the dark portion of the moon was also clear. All in all, a beautiful image. I have since found Comet Machholz with an 80% lit moon in the same sky. The comet and its tiny bright nucleus are visible quite clearly.
Magnifications above 150X do not appear to show further detail, and though objects such as Saturn look very crisp at lower magnifications, getting fine details such as the Cassini division require good seeing. So, this probably should be characterized as a telescope clearly in the apochromatic class, and capable of very good performance as a spotting scope or an astronomical telescope at magnifications from 12X to 150X.
Performance with various accessories:
2" Diagonal and 2" eyepieces:
The 2" focuser immediately accepts 2" size diagonals, which have a wider field of view capability than 1.25" size hardware. I installed a Celestron Mirror Diagonal, #93519, and a 2" barrel 32mm eyepiece, #93335. The resulting magnification is 18.75X and the true field of view is just under 3 degrees, or 156' at 1000 yards. The apparent brightness is just under 7 times the illumination of the unaided eye. This is the type of setup which has made APO telescopes popular since it is compact, gives a very nice image, and brightens up the large but dim objects in the sky. This setup also works well on the ground, but objects will be reversed left to right. In the sky, the entire Plieades is visible at one time, in color. Double Cluster shows up very brightly, and like the Plieades, the star are pinpoints across the entire field of view.
Celestron Binocular Viewer, #93690:
I originally posted a review indicating these don't work together. Not only was that incorrect, but it turned out to be hilariously easy to attach these two components together. If the binoviewer had a manual, it would have probably mentioned the outer barrel of the rotating portion is sized to fit perfectly into a 2" focuser, like the one on the 80ED. Not only does it fit, but it comes to focus with about 20mm of focus travel left. The images are extremely bright compared to the f/10 Schmidt Cassegrains. The only complication here is it isn't so good a combination for hight magnification since you would need a 2" size barlow, which is somewhat rare. But on the other hand, since it lets very large objects in the sky like the Orion Nebula and the Plieades become visible very brightly to both eyes, I really have trouble calling that a criticism. In fact, given this compatibility, I suggest if you own this binoviewer, that is grounds to get an 80ED.
Cameras:
Attaching a camera via the adapter or by par focal projection is easy to do- clearly this telescope is designed to do this. Remember the 2" to 1.25" adapter is also a T-adapter, so I recommend attaching directly to this since the deep notch on the bottom of the adapter assures the camera is captive. If you have old 1.25" size barrels from obsolete lenses/ diagonals, or such, these make it very easy to install filters to the adapter (compared to the usual T adapter, where this is a problem).
The current vogue in focusers for telescopes has been the Crayford design, which just uses a friction roller instead of a rack and pinion. A good example of this design is seen on the excellent Astronomy Technologies AT66ED, which even has a fine focus 10:1 reduction on the right hand knob. However, something to keep in mind with the older rack and pinion is the hard engagement of the gear with the rack means if you can feel the knob moving, you are feeling an exact proportion of how much the draw tube is moving. In the case of a Crayford, it is possble to lock the draw tube and the focuser will turn, with its wheel skidding on the tube instead of moving it. I personally like this engagement, expecially since it means you can be sure the focuser will lift heavy objects like cameras or binocular viewers.
Summary
The Celestron 80ED represents a huge change in the telescope world. Going on three years from its introduction, this scope still performs at a level unapproachable by any refractor in this price range. It is also available now bundled with a computer controlled equatorial mount, which is an obviously practical configuration for this telescope. And it works very well compared to telescopes at any price level. I am not sure if this is a telescope for a beginner since it would appear many of its features are geared for the serious naturalist, photographer, or amateur astronomer. But at the price, it certainly is in reach of any beginner who wanted to get something nice, and it would be a long term general purpose observing companion they would be sure to appreciate more with time. And after all, it's good to have nice things.
In comparison with other APO telescopes, the 80ED is a solid performer. In a way, it is troubling because a number of very high end telescopes made with great care now have this competition, and some will probably not survive. Already a 100 mm and 120 mm Taiwanese APOs have appeared from Orion, and a new 100ED and the beautiful Onyx 80ED are now being offered as an astronomical scope by Celestron (which has just been bought by Synta, the maker of this telescope) and larger ones have followed from William Optics. At the small end, the excellent Astronomy Technologies AT66ED f/6 refractor works well as both visually and photographically. In short, it looks like we are in the golden age of astronomical instruments. The Celestron 80ED is heavy duty and works well. At the current price, it is a very good choice for a family telescope, a grab-n-go travel scope, or an astrograph. If it has a fault, it is the rugged simplicity which makes it look austere in the face of its more polished cousins.
Recommended:
Yes
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Epinions.com ID:
Pirich
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 in Electronics |
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Member: Rich W.
Location: Tucson, AZ
Reviews written: 137
Trusted by: 40 members
About Me:
Dad, Engineer, Scientist, Astronomer, Traveler; order may vary.
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