It's a Wee Little Thing, But It Works Well
Written: Mar 10 '07 (Updated Apr 08 '07)
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Product Rating:
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Pros: Solid Construction, beautiful fit and finish, Lots of Features, Good optics, Economical
Cons: Back-focus limits, short visual back adapter.
The Bottom Line: The AT-66 ED is a very capable telescope in small scale with beautiful fit and finish and fantastic performance for a very reasonable price.
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| Pirich's Full Review: Astronomy Technologies Astro-Tech AT66ED 66mm f/6 ... |
OK, so the really big scopes do a lot. So, how far can you go in the other direction and find a useful telescope? The fact something small and light weight is easy to take along and use makes it a compelling question. And in the past, the answer was singularly bizarre- 60mm toy telescopes or $850 Takahashis and Tele Vues. But now Astronomy Technologies has taken a serious step towards finding a happy middle ground. I bought the AT-66ED thinking it would be a good travel scope, and perhaps it could do some astrophotography. It has ended up surpassing all of my expectations.
Background
We've all heard bigger is better, right? How about aperture wins? And then again, is the business about there actually being a reason for small scopes in the city just an astronomical urban legend?
In all seriousness, the performance of a telescope is a sort of envelope between how much it tries to amplify light versus how much it tries to magnify the image. The ratio of its focal length (which is the measure of magnification) divided by its diameter (the measure of how much light is coming in) gives the basic information on where the telescope has its middle ground. So, any telescope with an F/8 prescription, for example, will produce the same brightness image with a given eyepiece, whether it is 70mm in diameter or 120mm. Short focal lengths yield lower focal ratios and brighter images, but are only obtained in refractors with more curvature to their lenses, which causes them to have false color around objects. This is because like in a prism, light going through a lens is bent different amounts based on its color, so red, green, and blue will come to focus at different points. Because of this effect, shorter focal ratios to produce bright images are difficult to make in a refractor. In comparison, longer focal ratios, with less curvature to the glass, are far easier to make, but at the price of narrow fields of view and dimmer images.
F/12 will be one of the long thin 60mm scopes we are used to seeing in stores- in an attempt to control false color from inexpensive optics, they try to do a lot of magnification, and images are not very bright. Since even modest eyepieces cause these to shoot up to large magnifications, they have been frequently touted as "450X Telescope" when, in fact, even something as bright as the moon looks like a brownish blur seen through it at that magnification. At F/8, the telescope has brighter images, and strikes more of a balance between and low and high magnifications, though refractors with simple achromatic lenses in front will show some false color when it is relatively well controlled at F/12 to F/15. In recent years, a set of very economical short tube achromatic telescopes, such as the Celestron Wide View 80, have been made where they operate at F/5, and control the apparent chromatic abberation by centering their spectrum in yellow rather than green. This means the color isn't quite right since most objects are missing much of their blue component, but at least they are usable and the field of view is bigger. High resolution photography is possible through these scopes only if you are using a filter which only lets one part of the spectrum through, so, for example, hydrogen alpha red can be brought to focus with a filter such as the Orion Hydrogen Alpha Filter. The rest of the spectrum would be at poor focus, resulting in a rainbow of colors around objects, but is not visible with the filter blocking them. And, with the filter, there is not enough light for the human eye to see, either, so this is a photo-only trick.
What has been missing in this picture have been economical refractors with good color correction. These have been available as Apochromatic telescopes from companies such as Tele Vue, Takahashi, and Astro Physics at prices which are the most extreme for any telescope. The first hint of a change to this situation came with the Orion and Celestron 80ED f/7.5 Refractors. Although these came with very good glass, the mechanics of the telescopes were nice and rugged, but otherwise strictly no frills with fixed glare/dew shields, non-rotating focusers, and single-speed focusing. And more to the point, this telescope is close to two feet long with a diagonal in place, so it is only vehicle portable. The high end scopes were still the exclusive domain of features like retracting dew shields (for portability), rotating focusers (for convenience and framing photos), and dual speed crayford focusers with fine focus (smoother and the ability to get a precision focus).
Now, since the something like the excellent Celestron C5 (if I had to pick one scope to have with me on a desert island, it would be one of those) is available, which is a 127mm telescope with excellent performance and mechanics about the size of a coffee can, the need for a well corrected short focal length refractor isn't based on absolute performance or portability; it is really a question of how to get to the range between camera telephoto lenses and telescopes and the ability to see wide areas of the sky.
An apochromatic reftactor in the 60mm to 70mm range with a focal ratio between F/5 and F/6.5 has the following roles nothing else would fulfill:
(1) Extreme portability- at this scale, the telescope becomes small enough to go with you on a hiking trip since it is about the volume and weight of a set of 10X50 binoculars.
(2) Very wide fields of view: With a focal length of about 400mm, typical eyepieces will go from 10X to 100X, with a field of arounf 4.5 degrees at low power to .5 degrees, the width of a full moon, at high power.
(3) Rugged construction- Simply put, the smaller scope can be a bit overdesigned without the absurd weight penalties which have showed up in telescopes like the Thomas M Back 100mm refractor, which weighs close to 30 lbs (compared to the C5, which is around 5 and a half pounds with a 127mm diameter).
(4) Extremely good color correction- due to absolute dimensions in the optics, smaller diameter should come with a tighter focus zone, and thus better color correction.
The AT66ED suddenly appeared from a company brand new to the telescope community: Astronomy Technologies. Their initial offerings included the small 66mm AT66ED apochromatic scope at F/6, as well as two 80mm telescopes, an achromatic version and a more expensive apochromatic telescope. I had been looking for something compact with better color correction than the Short Tube 80 to piggyback on the Celestron NexStar8GPS, to use for wide fields of view, and maybe photography. At $329, a modest weight of about 3.5 lbs, and a focal length about 1/3 of the NexStar 8, even with the focal reducer, it looked like it was nearly ideal, and though Astronomy Technologies didn't have a record, the dealer, www.astronomics.com, is one of the best and most responsive in the business, so I decided to take the step.
Description
The AT66ED shows up in a matching aluminum hard case with a die cut foam liner with room for a diagonal, a few eyepieces, and other basic accessories. The telescope is available in an unusual variety of colors including basic white, anodized black, red, green, blue, and orange, as well as brass and chrome plated. I thought about it and concluded chrome was likely to be a durable finish, though it would have a higher bling factor than anything else I own. The finish is, in a word, magnificent- it is a true mirror finish.
The telescope has a slide-out dew shield with a metal lens cap on the end. The cap is stiff and requires some force to remove it, so it will not be lost accidentally, and more interestingly, it is more force than is needed to extend the glare and dew shield, so taking off the lens cap extends the glare/ dew shield at the same time, so when the cap comes off, the shield is extended, which makes it far less likely fingers will accidentally touch the lenses (compared to the Takahashis with a set screw on the glare/dew shield, this is the best design in the business).
The bottom of the optical tube has a large dovetail shaped foot with two 1/4"-20 camera tripod thread holes in the bottom for mounting it. The dovetail is close to the size of a Vixen Dovetail, but is slightly less tapered, so it WILL NOT go directly into a Vixen Dovetail mount- you will need to bolt it to a 1/4"-20 camera adapter, or make your own from a piece of dovetail stock. Dovetail bars are available from Orion (www.telescope.com- search on Dovetail) and more detail is in my review of the Orion Dovetail. This dovetail will go into some of the popular Unistar series point mounts from Universal Astronomics, but this is the only clamp I know of able to accept this dovetail directly.
The focuser is gloss black, which makes a nice contrast with the chrome barrel. The telescope has a stepped ring around it, which when loosened, allows the entire focuser to rotate around the optical tube. The focuser itself is a Crayford type, which uses smooth bearings and friction to move the draw tube, which is smoother than the rack and pinion type, which comes with some ratcheting from the finite size of the gear teeth. the knobs are covered with rubber grips and the right hand knob has a fine focus 11:1 reduction knob outside of it. The focuser draw tube has a milimeter focus scale on top, so once you have worked out a configuration, you can just nore where in the 60mm focus range you were when focused, and directly go to that point and be in the ballpark (a very simple but nice feature).
Finally, the end of the focuser is in Schmidt Cassegrain thread, the same as on the Celestron, Meade, and Orion telescopes, so devices made to thread on to them will effortlessly adapt to this telescope. To attach 1.25" diagonals and other accessories, it comes with a very well made 1.25"visual back with a compression ring rather than a set screw. The only complaint with this part is it is shorter than the standard SCT visual backs, and this causes a couple of problems discussed under Usage. This telescope does not come with a diagonal or eyepieces.
Usage
I first received my AT-66ED in October of 2006, and one of the first experiments was to attach it to the simple Unisar mount for some trials. The slightly oversized dovetail on the telescope immediately picked up silver witness marks where the mount's dovetail rubbed the flat black anodized finish on this part. Since this was a small amount of material rubbed off of the jaws on the mount's dovetail clamp, it doesn't mean the telescope is damaged, but it is still annoying that the finish is rough enough to do this. So, I have bolted on a piece of Vixen dovetail for most mounting which stays on, but this has a complication where the telescope will no longer go into its matching hard case while this is attached. So, while the shoe does have a nice cork non-skid feature on the bottom, I have to say this really doesn't work that well as a dovetail, and if Astronomy Technologies is considering features for a Mk 2 version of this scope, this should be on the list of things to change.
The focuser moves very smoothly, and the fine focus enables very small fine adjustments to be made relatively easily. As delivered, the focuser was just tight enough to hold on to a light mirror diagonal and eyepiece at the zenith, but the draw tube would slide to the bottom stop if you let go of the knob. The scope does have a locking screw for this, but it was missing on my copy- The crew at Astronomics dropped one in the mail to me and I had the scope intact a few days later. The optics are multicoated to a degree where they are difficult to see unless you get the lighting right (this is very good). The optical tube has a series oof 10 light baffles inside- they weren't kidding about attacking stray light with this tube.
In daylight, I put in a mirror diagonal and a 25mm Plossl eyepiece to put the telescope at a moderate 16X with a field of view a little more than 3 degrees across and pointed it at the Santa Catalina mountains (approx 12 miles away) from my back yard. The view was nothing short of eye-popping in its crisp and bright detail. I tried out some further experiments such as looking at black telephone wires against the bright sky, where very often false color will show up- just not in this telescope.
At this point, the next real optical tests would happen under a night sky, but the telescope has already showed it works very well in daylight as a spotting scope. The acid tests come when trying to differentiate objects in the night sky. In a moment of the unusual, the new scope curse, where the first night is cloudy, skipped this telescope. I took the opportunity to do a series of tests to see how it would behave. Since Epsilon Lyra, the famous formation of four stars arranged as a pair of pairs a.k.a. the "Double Double" near Vega, was in the sky, I started with it as a test of resolving power. Although the four stars seemed fairly well distinguishable at 80X, I put in a barlow and went to 120X to see if the circles of the stars (since stars are so far away, they end up being an image of your telescope's resolution, not their size- this image is called the Airy disk, after astronomer Sir George Airy). In the little AT66ED, the four Airy disks of the Double Double do not touch. In comparison, the larger diameter Celestron C90, a very capable telescope, has a diffraction ring around its Airy disk caused by its secondary mirror (a feature of all reflecting telescopes with a small central mirror in the front), and does not split these stars as cleanly as a result. The next test was to put it on the blue-white star Vega, which if this telescope had false color, would have a vivid purple halo; it did not.
The last test was to put it on an object near the bright moon to see if it would have its image washed out by glare. As it turned out, it is a bit difficult to even point the telescope at the moon since the usaul visual cue I use in the eyepiece- the rising glare as you get near the moon, was practically absent until the moon started to enter the field of view. All in all, a perfect result for ANY telescope. Star clusters like the Plieades and the beehive are effortlessly encompased in one field of view with true color and pinpoint stars. The optics are good enough to be very tempted to push all the way to 200X, though in truth, the sweet spot really is at lower magnifications.
When mated to the NexStar 8 GPS using the piggyback adapter, the focuser on the AT66ED can make a full 360 degree circle without bumping the optical tube. At this point, I realized I was looking at an ideal photographic setup, so I decided to link this scope up with the Minolta Maxxum 7D and try some photography. This is where I hit a few snags.
First, the visual back is short, since the telescope was apparently designed to have a diagonal attached. The problem here is the camera, when mounted with a 1.25" T adapter will not reach focus, even when the focuser is extended to full length. It will reach focus if the visual back is removed and replaced with one from a Celestron telescope, but since these do not have the nice compression ring, this is disappointing (AT, this goes on the Mk2 list). The other problem is with the Celestron visual back attached, the scope has to press into the foam in its case to fit. Note, you must counterweight this scope to do this- either with the counterweight kit, or by something similar to what I did with a Vixen rail on the bottom and making a weight sled. Balance for this configuration takes about 5 lb. near the nose of the large scope, and prevents harmful loads from being applied to the clutches and the drive.
The second problem was the focuser had to be tightened significantly to be able to move something as heavy as the camera. The good news is the focus is a very stable location, so once you are there, you can lock it and forget it. This isn't such a big deal unless you discover it on a polar aligned telescope at 11PM without a full toolkit nearby, the way I did.
With that said, this is the first large photo series of the Orion Nebula and the Running Man taken through this telescope: http://www.buytelescopes.com/gallery/view_photo.asp?pid=10915&c=35913.
Please note the following:
The stars are true-color, so the color correction can be observed.
The only visible optical flaw is in the corners of the image, where if you look carefully, you can see the stars are a little egg shaped in a direction pointing from the center of the image.
One interesting thing is the chrome finish appears to thermally stabilize this scope, so temperature change hasn't resulted in a warm up or cool-down delay, thus far. I will say the main problem with this finish is it shows ALL fingerprints as clear as day, so I find I mount the scope, then carefully wipe off my prints, then carefully make sure all I touch is the lens cap itself when I am taking the cap off and extending the glare/dew shield.
Conclusions
The AT66ED is great as a high quality small telescope. For the actual size of the telescope, its performance is superb, and especially if someone is interested in visual observing, this telescope is a good fit for spotting on the ground. Its performance as an astronomical photography telescope is just too good to be ignored, given the large angular size of many of the more interesting objects in the sky. The scope has a few minor flaws, but what it does it does so well, I can't find it in me to hold them against it, and at the end of the day, that is the mark of real value.
Recommended:
Yes
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Epinions.com ID:
Pirich
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 in Electronics |
 - Top 500 |
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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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