REVIEW OF SUMERIAN OPTICS ALKAID 16” TRAVEL SCOPE
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Stellarvue 102 EDT and Tele Vue 85
Let me state it up front: the Stellarvue 102 EDT is a great scope - probably about as good as it gets for a short focal length (F6.1), achromatic ED refractor. I had recently received it after a 6 month wait, hoping to use it as a compact travel scope for both astro viewing and terrestrial spotting and had planned to replace my Tele Vue 85 with it. My reasoning was that I would get an additional 1.7 cm or about 2/3" of aperture in nearly the same length and weight tube with approximately the same crisp optics. I learned a lot in comparing these two scopes one night last Spring.
The SV102 EDT is well built and nicely finished. Its white enamel aluminum tube is sturdy and no doubt very durable, and with a 621mm focal length (the TV85 has a 600mm focal length), it is as I had expected very compact and portable. For travelers and eclipse chasers, it is ideal for airline carry-on. A CG5 equatorial mount is more than adequate to handle its 8.4 lbs and short 22" length (not including a diagonal). With a 2" star diagonal and eyepiece, however, it was a bit too much for my Bogen fluid head and 3011 tripod, which work well with the TV85.
It has a metal retractable dew shield, which (at least on the scope I used) was a little too loose causing it sometimes to bind when sliding it open or closed. It also had a tendency to sag when fully extended. The dew shield is covered with a heavy-duty plastic cap that fits nicely with thick felt; but unlike the TV85's machined aluminum cover, it doesn't lock the shield in place when retracted.
The scope comes with mount rings and the Stellarvue one-power
finder, but does not include a star diagonal. The LED red-dot style finder
is very good and has a large easy to use viewer. It is more robust and easier
to use than the smaller Tele Vue Quickpoint. The carrying case costs extra
and is disappointing. It does not fit the scope very well, since its depth
and length are a bit smaller than the scope itself and the width is almost
twice that of the scope. I had to take the mounting rings off to get the
scope in the case at all, and even then I had to put it in at a slight diagonal.
It might hold a few smaller eyepieces or a small diagonal, but it has no
cutout padding for accessories. I would recommend saving the $45 dollars
it costs (as of 6/02) and investing in a different case.
The most unique feature of the 102 EDT's hardware is its dual-speed focuser made by JMI. The dual speed allows coarse focus on the left-hand knob and fine on the right, making focusing very smooth and precise. Stellarvue claims that precision focus is crucial for wringing out as much color from bright objects as possible. The focuser can also be rotated 360 degrees by loosening a thumb screw at the base -- a big plus for photography and for dealing with the awkward positions an equatorial mount can place a refractor in.
Although there is plenty of out-focus length, I could not come to focus with a 2x Barlow and the 2" Tele Vue diagonal I was using - there was not enough in-travel. Specifically, I could not get any of the new Naglers, 16mm, 13mm, 9mm, or 5mm focused when using the Celestron Ultima Barlow. This may not be an issue with other diagonals, especially with a 1 ¼" diagonal, or with other eyepieces.
The optics on this scope were excellent -- well corrected, very little spherical aberration, nearly textbook airy disks on test stars. The coatings were also superior, so good in fact that the glass appears nearly invisible in the photo of it. These premium coatings combined with no less than five baffles make for high contrast views.
Moderately bright objects had a magenta halo inside focus
and a green halo outside. Adventitious color in focus was still always there.
On a very steady night, however, the fine focus feature might allow more
of the color to be removed. But the optics of this scope (and any good achromat)
are very sensitive to the quality of the seeing. The rapid, localized atmospheric
changes or transient "cells" that characterize moderate-to-poor
seeing (which is more often than not the case here in Wisconsin) cause frequent
though minute fluctuations in the focus, which in turn causes noticeable
color variations in the object viewed. This effect is virtually absent in
an apo refractor.
The color correction of the SV102 EDT is a step up from the Tele Vue Pronto and Ranger ED scopes, and that's saying a lot. I owned a Ranger for several years and loved it's small, grab-and-go friendliness, but I was always distracted by the spurious color. On the other hand, the SV102 is definitely not in the same league as the apochromatic Tele Vue 85. My TV85 gave a cleaner separation on Epsilon Bootis at a lower power than did the 102. This colorful double star appeared pale yellow with a cool green companion in the TV85, whereas in the ED in the unsteady sky on the night I observed with both scopes, there was moment-to-moment color variation making it difficult to judge the stars' "true" colors. As Lyra rose higher in the sky, I tested both scopes on the bench-mark Double-double. Again there was no real contest. The TV85 showed a cleaner separation and sharper stars. The stars in the SV102 were a bit mushier, and despite the theoretically better resolving ability of its larger aperture, it did not split the double-double or epsilon Bootes as easily as the TV85.
The difference between the two was especially apparent to me during daytime terrestrial viewing. Having used the apo for a couple of years as a spotting scope, I was spoiled by its crisp, clear views that allowed for very fine detail on birds. I had naively expected the optics of a top-of-the-line ED scope to give similar sharp true-color views. But this was not to be. The false color through the SV102 during day use was distracting and seemed to degrade definition. I felt like I was looking at birds through a thin magenta-blue gauze. I had experienced this to a worse degree in the Ranger, which is why I moved to an apo.
At night the 102's contrast was noticeably better than my C5 SCT, but not better than the TV85. All three scopes resolved at the same power M3, M5, and later M13 nearly equally well, but the views through the refractors to me were more pleasing. Even though at the time, I didn't pay much attention specifically to light gathering differences, my impression was that there wasn't much difference between the 102 and the 85, while the C5, despite its contrast handicap, seemed to show dimmer objects better.
Although in some ways not really "fair" (as one wag put it: like comparing Apos and oranges), this comparison gave me a much better idea about the characteristics and strengths and weaknesses of scope designs. The SV102 is obviously a fine telescope, but the buyer should be clear (as I was not) about the differences between an achromatic refractor, albeit one that is of high quality and uses extra-low dispersion glass, and an apochromatic refractor. Also, because the Stellarvue 102EDT comes without a diagonal, if one wanted to add a high-end 2" star-diagonal (such as the Tele Vue Everbrite), the total package would cost essentially the same as the Tele Vue 85, perhaps even a little more if you take into account the 20mm TV plossl and custom-made case that comes with the latter.
Both Stellarvue and Tele Vue have great reliable service. Vic Maris of Stellarvue has great integrity and seems to be in the business for all the right reasons. And he has a very loyal following as evident by the very active Stellarvue discussion group. If you know what you want and you know what you're getting, I don't think you will be disappointed with either scope.
There has been a certain amount of controversy generated by this "review" as expressed on SAA and the Stellarvue discussion group, that I'd like to briefly address.
It's true that I only spent a single day and night with the EDT, and it was not the best seeing (a 5-6 out of 10). But that is pretty typical here. It was the standard kind of sky we get in rural southern Wisconsin where I observe, typically a mag 6 sky with moderate to good transparency (6-7 on a 10 scale) and average seeing. In other words, if the EDT couldn't perform nearly as well as the Tele Vue 85 under this kind of sky, then it was not the scope for me. These are the skies I have to deal with more often than not. So I felt it would be good to tell others of my experience. I would have liked to have been similarly informed.
Moreover, daytime viewing for me is important, and the EDT just wasn't up to snuff. I could tell immediately that the color fringing was going to bug me, so I didn't see any reason to test the scope over many different days. And the same went for night viewing. The color and brightness might improve under steadier skies, but in my opinion, if a scope can't perform as well under less than ideal conditions then that's a limitation that the potential buyer needs to be aware of.
The bottom line: for me the extra 2/3" aperture just
doesn't make up for the non-apochromatic optics. Although my "review"
has its limitations, which I have not tried to hide, I believe that I was
fair. Take it for what it's worth. It is certainly not the last word on