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Astro-Tech AT102ED


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The AT-102ED, the Search for the Perfect Comprise in the $1000 Price Range


The Short Story: I like this Telescope. The Astro-Tech 102ED is a handsome 102mm F/7 refractor. It is compact, well made with a rotating two speed Crayford focuser, a sliding dewshield and nice overall finish. Optically it is quite good, pushed hard on difficult targets like Venus, a faint purple fringe is visible.


The Long Story: I like refractors. I really enjoy spending an evening with a ~600mm Focal length refractor on an ALT-AZ mount, looking at the big sky with a 3 degree field of view, zooming in on double stars and planets, catching clusters, nebula and maybe even a galaxy or two. With the coming of the ED-80, budget-minded astronomers like myself were given a chance at some wonderful viewing experiences. Over the last few years I have had some good times with an ED-80 and now with a William Optics 80 mm Megrez II FD. I love the pristine views these telescopes provide. The next generation after the ED-80, the William Optics Megrez II, it is also well-made and simply beautiful to look at. Still, as clean and clear as the views are, these scopes leave me wanting a bit more aperture, a bit more light, a bit more resolution. So, when I first heard of this new batch of 102mm F/7 ED refractors hitting the market at the $1000 price point, I was more than excited and wrangled a chance to try out the AT-102ED. This is the story of what I found.


Expectations: A $1000 is a lot of money for many of us but in the world of 4 inch APO refractors, it is a small sum, just barely enough to get started. It will buy an Orion ED-100 with it’s Spartan but effective mechanicals and very good FPL-53 F/9objective. For many that it is good choice. But it’s a relatively long scope (36inches) and not well suited if one is looking for the short focal length ease of use and portability. The AT-102ED scope offers a much more compact package with much nicer mechanicals but it has an H-FK61 based objective. This ED glass has an Abbe Number (Vd) of 81.61, very similar to FPL-51, so I did not expect the quite the color correction I had come to expect in these other affordable APO’s.


Much of this story involves a comparison between the AT-102ED and the 80mm Megrez II FD because the question first and foremost in my mind was simply whether adding the 102ED to my collection was worthwhile, was there enough added capability to the 102ED to justify yet another telescope?


Here’s the Story:


Mechanicals: The AstroTech AT-102ED is a nice looking, made telescope typical of the better telescopes being manufactured in China today. The fit and finish are superb by my standards, very similar to the William Optics 80mm Megrez II FD I am so familiar with. The included Rings were nicely finished. In comparison to the 80mm F/7, it is a much bigger scope, longer, heavier and bulkier. Regardless of the added size and weight, it does fit nicely on a Vixen Portamount.




Monks Monkulus comparing the AT-102ED with the smaller WO 80mm Mergrez II FD and the WO Zenith Star 66 SD




There were two mechanical issues with this particular telescope. It should be noted that this particular telescope was an early arrival. The focuser showed a certain “notchiness” that resulted in difficulty achieving focus under some situations. While some focusers showed no issues, there certainly was a problem here. As a result, AT has announced that future AT-102EDs will include a Feathertouch focuser as standard equipment. I disassembled the focuser in order to resolve the problem. Investigation determined that the fine focus planetary ball system was the cause of the “notchiness” and that careful cleaning and re-assembly resulted in a smoother action. I was impressed with the quality of the design and manufacture of the focuser. The Crayford shaft rides in ball bearings in a machined slot that allows the preload to be adjusted. Nice.


The second problem was that the felt under the sliding dewshield did not provide enough friction so that the shield would not stay in place at higher altitudes. As I had experienced this problem previously with the Megrez II FD, I knew that I could adjust the felt to provide more friction and after that simple repair, there was no further problem.


Using the AT-102 ED....


This is a nice telescope, no doubt about that. The AT-102ED combined with a Vixen Portamount make an easy to use, reasonably stable, light and portable combination. It goes out the door easily and is ready to observe in but a moment. I enjoyed observing a great variety of targets with the AT-102ED and spent several nights comparing it directly with the William Optics Megrez II FD. This what I saw:


The Planets: In late October, I was just able to catch Jupiter at sunset, low on southwestern horizon. Tough conditions to see anything but I was pleased to be able to see multiple cloud bands, including hints of more than the two largest. Even under these poor conditions, I am confident I would have seen a shadow transit. Later, as Mars was nearing opposition, this scope showed that it was able to provide some nice views of the surface markings. In particular, on one January evening with sub-arcsecond seeing, I was quite pleased with the multiple markings I was seeing at 200x and greater, this is what I was hoping for. In comparison to the 80mm Megrez II, the image was brighter and showed the detail more clearly This single view was sufficient to convince me of the value of this scope.



Double Stars: I enjoy viewing double stars, both splitting tight doubles, pushing a scope to the limit as well as just looking at beautiful pairs. For favorites like Albireo and Almach (Gamma Andromedae), the color rendition was good and the views refractor-perfect.


I did spend some hours splitting doubles and triples with the AT-102ED. Refractors are good at splitting doubles and will generally handle doubles right up to the Dawes Limit, 1.15 arc-seconds for a 4 inch telescope. More difficult are unequal doubles as they challenge the optics to produce a tight image of the bright primary.


First off during any fall evening is Epsilon Lyrae, the famous “double-double.” Not a challenge for 102mm scope, it’s really just a check on the seeing and an appreciation of an old favorite. Still, with the good seeing my San Diego backyard offers, on one occasion I was able to convince myself of a clean split at 47x with a 15mm TV Widefield. For me, this was a record low magnification. Other doubles like Rigel (29x one night!!) and Alnitak (Zeta Orionis) were nicely split but again no real challenge for a 4 inch refractor.


Challenge Doubles:


-Epsilon Arietis: magnitude 5.1-5.6 pair at 1.4 arc-seconds which the AT-102 ED handled nicely. At the limit for the smaller 80mm F/7, it merely showed elongation. This required 200x.


- STF 749: About 3 arcminutes from Mars in mid-January, this 1.16 arc-second binary consists of a 6.5-6.5magnitude pair. On a night of exceptional seeing, I was able to convince myself that I could see a split. This is at the Dawes limit and I was pleased. This required 200x.


- 36 Andromedae: magnitude 6.0-6.4 pair at 1.0 arc-seconds, elongation seen but it is beyond the limit. This split nicely in a 10 inch Newtonian.



- Delta Cygni: a magnitude 2.9-6.5 pair at 2.6 arc-seconds. This is sometimes considered a challenge double for a 4 inch. On a good night with good seeing, the WO 80mm Megrez II FD handles this nicely. The AT-102ED also did a good job but at the 160x magnifications necessary, the primary star was not as clean as in the 80mm.


Deep Sky: Due to the San Diego Fires that closed the back country and an unexpected personal physical difficulty, I was unable to travel to a dark site with the AT-102ED. However I am avid backyard DSO observer/nut and I spend a good deal of time looking at the faint fuzzies. On the best nights, my urban backyard allows me to see magnitude 4+ stars and I enjoy the challenge of finding and viewing fainter DSOs.


- Clusters: The Pleiades is a show case object in any widefield telescope regardless of aperture. The AT-102 with my venerable TV 32mm WF is a stunning target. Fainter clusters such as M103 in Cassiopeia and M38 in Auriga definitely show more stars than in the smaller 80mm telescope. M103 in particular came alive with the added aperture of the 102mm. At 100x in a Nagler 7mm M103 was nicely populated. I missed my favorites M7 and NGC 6231 (Table of Scorpius) but that will be a treat next summer. Globular clusters are brighter and somewhat more resolved in the 4 inch than in the 80mm but I find it takes 8 or 10 inch scope to do justice to even a bright globular like M13.


Nebula and Galaxies: In general, outside of a few brighter nebulae and galaxies, the fare of the smaller scope is limited from an urban backyard. Oh, M42 is wonderful in any telescope at any magnification and the AT-102 is no different, it is simply wonderful.... Small planetaries like the Blinking Nebula and the Blue Snowball are brighter and more apparent in the AT-102ED than the 80mm Megrez II FD.


Challenge DSO: M76. Over the years I have developed a fondness for M-76, the “little dumbbell,” and most nights will spend some time detecting it and observing it if possible. In my 12.5 inch Newtonian, it is detectable with the moon full. When the moon is not wrecking havoc and the night is clear, I have seen it in a 70mm TV Pronto. I used M-76 as a gage to compare the performance of the AT-102ED vs the WO 80mm FD. There were nights after the first quarter moon when I could see M76 in the larger scope when I could not see it in the 80mm. And when I could see it in both, it was easier and showed more of its double personality in the 102ED. The AT-102ED definitely passed the M-76 Test.



Chromatic Aberration: For a number of reasons I am interested in the level of chromatic aberration displayed by the Astro-Tech 102ED. I see no chromatic aberration on any target with the 80mm F/7 FPL-53 doublets like the ED-80 and the WO Megrez II FD, I consider them visual APOs and I think the consensus agrees. But color correction is a function of both aperture and focal ratio so a FPL-53 doublet (a similar design to the ED-80) would need to be F/9 to retain the ED-80’s level of color correction. In addition, this new wave of 102EDF/7’s such as the AT-102ED uses the less potent, less costly H-FK61 which does not allow for the same level of color correction. (Of course, it's the combination of glasses as well as the design that provide the color correction, but it seems the trend in mass produced scopes has led us to a general rule of thumb concerning glass types and color correction. While perhaps not always correct, it does help an amateur sort through the various offerings.) I expected to see some false color, the question in my mind was how much and would it seriously affect the views.


I

Noted S.A.A. regular Monks inspecting the coatings and the baffles

looked hard for in focus chromatic aberration and I found it, it is there to be seen when this telescope is pushed. In my experience, Venus is one of the biggest challenges for a refractor, if there is purple to be seen, Venus, in her glory, will show it. At 202x, indeed I was able to see the slight bit of a purple fringe surrounding Venus. It is the sort of thing one has to look for and it is likely that a casual look would miss it. Some observers are less sensitive to color fringing, some are more sensitive. I suspect Tom Trusock can see color fringing when I cannot. On the other hand, I setup a 60mm F/13.3 Asahi-Pentax Achromat for comparison purposes. At F/13.3 this scope is beyond the 5x criteria for minimal false color in an achromat. By comparison, I was able to see a purple haze at 167x viewing Venus with the 60mm. The color correction in a 60mm F/13.3 should be the equivalent of a 102mm F/23 achromat and the AT-102ED is better than that. Conclusion, it ain’t perfect but it’s pretty darn good.


Other targets like a rising Rigel showed a tinge/flash of false color as well. However Vega, high in sky, seemed free of any false color at 150x.... In my experience one of the toughest tests for chromatic aberration is a daylight test of a dark object with a bright sky behind it, a telephone pole in the shadow of a bright sun shows color fringing in a scope if anything will. With the 80m F/7 at 150x, I see no color fringes, with the AT-102ED at 150x, there is some detectable color fringing.


I did look at the color inside and outside of focus. On a bright star such as Rigel, inside of focus the purple is apparent outside the fresnel rings while outside of focus the purple is apparent inside the fresnell rings. I suspect this causes the AT-102ED to be more sensitive to seeing as the twinkling of the star causes the image to go in and out of focus with a resulting twinge of purple.


Other Optical Issues: At times, I noticed some flaring on bright stars at high magnifications, somewhat like the thermals one sees in an almost cooled Newtonian. I also observed that when far out of focus, there were 4 little bumps/ valleys at the edge of the defocused ring that corresponded to the position of the collimation screws. This same issue has been observed on the very similar SV-102ED, as well as the WO 102F7. One way to address this problem successfully is to slightly loosen the collimation screws. This is indicative of a slight pinch, but I'd like to note that extreme care should be taken in loosening the collimation screws. Too much will result in an out of collimation telescope! The issue is apparently worse in cold weather, and may not be a problem for many observers.


The End of the Story:


The Astro-Tech 102ED is a good scope, it's portable, small and lightweight. It's pleasing to the eye when looking at or through it. Mounted on a Vixen Portamount, I consider it a Grab-and-Go rig that offers a sufficient increase in capabilities over the 80mm APO to make it a worthwhile step up. The increased aperture over the 80mm is enough to make serious planetary viewing possible, as well as allowing an observer to split tighter doubles and reach deeper into the universe from your backyard. It is a nice combination of aperture, portability and compactness.


Mechanically it should be the equal of the APOs like the 80mm Megrez II FD. Optically, the smaller FPL-53 doublet offers cleaner images at high magnifications but the defects in the AT-102ED are small. Given the level of false color, the AT-102ED is definitely not an APO in the strict sense. But the color correction is far better than that of 102mm F/7 Achromat and for visual purposes other than star testing, the level of chromatic aberration is probably not significant. From a visual standpoint, I suggest an appropriate term for scopes like the AT-102ED would be Virtual-Apochromat or Quasi-Apochromat.


If you are a perfectionist, this and the other similar 102mm F/7 ED telescopes are not good choices. But if you are someone who accepts compromise as part of the equation and does not expect an Astro-Physics Traveler on an ED-100 budget, then this could be the scope for you.


Does this scope provide enough added capability to justify ownership? After using this telescope for about twenty evenings, for me the answer is yes. I am in the process of negotiating its purchase. When it is mine, it will replace the 80mm Megrez II FD as my first choice Easy Viewing Backyard Telescope as well as the primary companion to the larger reflectors on the trips to the dark skies. It would also be my choice for long car camping trips when there is not time or space for a large DOB. The WO 80mm Megrez II FD will remain primarily for birding and digiscoping.


Jon Isaacs, January 2008



My thanks to Monks Monkulus for help in evaluating the AT-102ED, to Astronomics for the opportunity to test this telescope, to Warren Bitters for providing an Excel worksheet that computes the current separation of binary stars and to Tom Trusock for aiding with this review.


Optics used:


2 inch TeleVue Everbright Diagonal


Eyepieces


42mm GSO SuperView (Finder)

32mm TeleVue Widefield

24mm TeleVue WideField

16mm Nagler II

15mm TeleVue Widefield

13mm Orion Stratus

9mm Synta Widefield

7mm Nagler I

4.8mm Nagler I

2X Barlow Celestron Shorty

2X Barlow Celestron Ultima


Mounts


Vixen Portamount

Orion EQ-3


The AT-102ED comes an aluminum carrying case, tube rings, a dovetail and a multi-reticle finder which was not tested.


Jon Isaacs is a budget minded astronomer who enjoys observing every clear night from his urban San Diego backyard. Jon is something of a “telescope junky” with a collection of a couple of dozen telescopes.. He is a moderator for both the CloudyNights and Astromart Beginners Forums among others.


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