ASTRONOMY TECHNOLOGY TODAY
Volume 2 - Issue 2

By Josh Walawender 

Stellarvue recently released a new low cost 4-inch ED refractor which is garnering a lot of attention. I had the chance to put one through its paces and compare it to a much more expensive doublet apo refractor to see how it stacks up. I've always preferred comparative reviews as they help give a better sense of the absolute performance of a telescope instead of a very cheerleading statement tot he effect that "I just spent $$$ on the AcmeUltraSuperScope and I love it!!!" In this review, I will be comparing the performance of the Stellarvue SV102ED tothe Stellarvue SV102ABV. No review is without bias, of course, and I certainly expected (and wanted) the ABV to win in this comparison. The ABV cost roughly twice as much, so one would expect it to outperform the ED.  

The focus of this review is the ED and the ABV serves primarily as a performance benchmark. What I hope you'll get out oft his is where the ED sits in terms of both absolute performance and value relative to a high performance apochromatic doublet of the same aperture abd made by the same manufacturer.

The SV102ED that I am testing is on loan from Stellarvue. Vic Maris of Stellarvue had been contacted by Astronomy Technology Today about a review and asked me to do a comparison with my SV102ABV. I am a repeat Stellarvue customer, but have no financial interest in the company. I asked Vic to review the two paragraphs below describing the scopes to ensure their accuracy, but other than that, any biases, omissions, or other errors are entirely my own.  

The Players
The Stellarvue SV102ED is a new telescope introduced by Stellarvue in 2007. It is a 102­mm aperture 710­mm focal length (f/6.95) doublet which uses a low dispersion element to reduce false color. The model I tested (the 102ED) has an imported dual speed Crayford focuser and costs $1095 new. Another model (the 102ED2) which uses the Feather Touch dual speed Crayford focuser made by Starlight Instruments is

 The reference scope is a Stellarvue SV102ABV which is no longer in production. The ABV is a 102­mm f/8 doublet apo. It uses a low­dispersion primary lens element and a lanthanum secondary element providing excellent correction of chromatic and other aberrations. While Stellarvue does not advertise the ED as apochromatic, the ABV was advertised as an apo. The ABV has the additional feature of being a dual tube with a removable section for bino-viewing (hence the BV part of the moniker). In the configuration I am using (dual tube with Feather Touch focuser), the ABV used to retail at $2195.00, almost exactly twice what the ED configuration I am comparing with costs today

Mechanics ­Tube and Finish
Both scopes are clearly quality instruments, constructed with machined aluminum tubes. The ABV has a beautiful "Stardust Blue" powder coated finish with silver highlights (photos don't do it justice). The ED has a "Pearl White" powder coated finish which is still nice looking, but lacking the silver highlights of the Stardust Blue. Aesthetically I prefer the blue finish. Colors are a subjective matter though and you will find others who would prefer the more traditional white of the ED.

Both scopes have very nice aluminum, threaded lens covers which are a vast improvement over push on caps. This won't fall off at inopportune times and will protect your lens against almost any conceivable abuse while in place. For such a minor feature, I really appreciate these caps; they exude a sense of quality and attention to detail.

Both scopes come with sliding dew shields; the ABV's is a bit larger and uses a lock screw to secure the shield in place. The ED's dew shield is a bit smaller in diameter, making for a lighter weight scope. It also lacks the lock screw of the ABV, but is a tighter fit. The motion is stiffer in the ED, but is very solid with no droop or sag at all. The dew shield stays in place even through significant temperature changes. Given the slightly more svelte look of the ED dew shield and its ideal slide­it­out­and­forget­it behavior, I actually prefer the dew shield of the ED to the ABV. Stellarvue provides instructions on adjusting the stiffness of their dew shields by adding small bits of felt, so that it can be adjusted to taste. I did not find that necessary on either scope.

Clamshell vs. Rings
The ED comes with a machined aluminum clamshell to hold the scope (traditional heavy duty rings are also available). The clamshell has a short 2­inch wide, Vixen­style dovetail built in and one can easily attach a 3­inch wide, Losmandy-­style dovetail to the bottom of that. The scope is held securely by the clamshell, though those interested in astrophotography might prefer rings which can be spaced further apart for enhanced stability and for their larger surface area at the top for mounting large accessories such as a guide scope or a piggybacked camera. The clamshell is smaller and behaves like an integral part of the scope; it will slip more easily into cases than the rings

Focusers
The ED has a dual­speed Crayford focuser while my ABV uses a 2­inch Feather Touch made by Starlight Instruments. The ED's focuser is a step down from the Feather Touch(but almost every other focuser is as well). My first night out with the ED, I immediately put a 2­inch diagonal and 31­mm Nagler eyepiece in.This combination required a quick tensioning adjustment before the ED's focuser would lift all that mass. With that tension, the focuser felt a bit tight on the coarse focus knob. I also noticed that the coarse focus always felt a bit rough regardless of the tension. The fine focus was smooth and easy to use even with the tension tightened. In comparison, the Feather Touch is buttery smooth and I've never needed to adjust the tension on it regardless of temperature or load.

Focusing at high powers was easier with the Feather Touch, but the ED's dual­speed Crayford was a huge step up over other single­speed Crayfords I've used ­ both imports and high end models. Unless you've already been spoiled by using the Feather Touch (or other comparable focuser) on another scope, you will probably find the ED's focuser more than satisfying for visual use. As for me, even though the ED's focuser performs adequately, I'd opt for the Feather Touch option on the ED (or on any scope for that matter). You won't regret it.

The ED's focuser (either the base focuser or the Feather Touch) is rotatable with a collet style locking system which ensures that the focuser remains aligned to the optical axis. Don't underestimate the importance of maintaining the alignment of the focuser. You're paying for a telescope that has been tested and properly aligned; you don't want to lose that. The collet works well, though I must admit that I didn't use it much. My ABV does not have a rotating focuser, so I'm used to simply rotating the diagonal and that's what I did.

Portability
The ED can be made airline carryon size by unthreading the focuser from the tube. The threads are very fine and precision machined to maintain alignment. I flew from the Astro Imaging Conference in California back to Hawaii with the scope and had no problems. There are two optional cases available, one for air­line carryon and one longer case.  I would recommend getting the longer case for the ED unless you plan on flying a lot. If you save the step of unthreading and stowing the focuser each night, it saves time during setup and takedown.

Testing Setup
Most of the testing was performed with the scopes mounted side­by­side on a Losmandy G­11 mount. Most testing occurred during four observing sessions like that pictured at the Visitor Information Station(VIS) on Mauna Kea on the Big Island of Hawaii at about 9,000 feet above sea level. I also spent two nights with the ED riding solo on the G­11 while observing from Mauna Loa. The G­11 carried the two scopes well and I could run at high powers with negligible image shake.

I live in Hilo near sea level, so for each observing run the scopes were transported from warm humid conditions (roughly 70 degrees F and 60­70% humidity) to cold, dry conditions at altitude (usually 30­40 degrees F and <20% humidity). Neither scope had any trouble cooling down under these conditions even though they were warm to the touch when coming out of their cases. Both scopes handled temperature changes well as the night cooled from 40 to 30 degrees F over the first couple hours.  One night at Mauna Kea was very humid; we were near the top of the inversion layer, so we were still contaminated by humid air from below. Neither scope had trouble with dew on the objective while two SCTs set up nearby were completely out of commission due to moisture on their corrector plates

On Sky Performance
I will try throughout this review to make both sharpness and contrast comparisons. These are two different attributes of an optical system. Refractors tend to have very good contrast as their unobstructed aperture places the maximum amount of light into the center spot of the Airy disk and the minimum amount into the surrounding diffraction rings. (See "A Closer Look at High Magnification" by Gordon, R.W. in The Best of Amateur Telescope Making Journal Vol. 1 pg. 229. ed. Cook, W. J. pub. Willmann­Bell or Telescope Optics A Comprehensive Manual for Amateur Astronomers by Rutten, H. and vanVenrooij, M.pg. 218 pub. Willmann­Bell)Those diffraction rings have the effect of moving light from one area of the image into a neighboring area. This reduces contrast, for example between the ice caps and dark markings on the surface of Mars. I sometimes find that people respond immediately to differences in brightness between two scopes (i.e. of different apertures), but look past the contrast transfer of the optical system which is also a component of seeing fine detail in extended objects be they planetary surfaces or nebulae. Seeing the contrast between the dark nebulae around the Swan Nebula and the surrounding sky is what originally sold me on my ABV refractor

Moon over Mauna Loa. All Photos by Josh Walawender.

Bright Stars
The ED shows a modest violet halo around bright objects, but it is a very deep violet color, not the bright blue that is thrown up by inexpensive achromats. I haven't done a side­by­side with the following scopes, but to my memory, the halo is substantially fainter and deeper violet than all of the following scopes that I have looked through: a 152­mm f/8 inexpensive achromat, a Stellarvue 80­mm f/6 AT1010 achromat (aka the Nighthawk), and a Stellarvue 80­mm f/9.4 SV80/9D achromat. The diffraction rings are well defined in the ED and the scope appears well collimated.

The ABV shows truly color free stars in focus, with the only color visible in the out of focus images. The ABV would appear to meet any reasonable definition of apochromat over the wavelength sensitivity of my dark adapted eye. It also shows nicely symmetric diffraction rings indicating good alignment.

For all high power observations(of bright stars, the moon, and double stars), I was able to start the comparison by dialing in almost exactly 200x in both scopes using a 2­4­mm Nagler zoom eyepiece (at 3.5 mm in the ED and 4 mm in the ABV). At 200x the views were nearly indistinguishable aside from the presence of the faint violet halo on bright objects in the ED.

The Moon
The ED images are nice and sharp, but the violet fringe around the edge of the moon is, of course, visible. When compared with the ABV, it was clear that the dark shadows behind mountains or in craters were slightly bluish in the ED due to the slight chromatic aberration. As a result, the ABV appears more contrasty with nice deep, dark, black lunar shadows. This is a subtle effect however and another observer with me did not notice it until I described it to him.

Beware which eyepieces you use to evaluate color correction in refractors. When using some wide field units, I initially saw more color on the limb of the moon, but that went away when the limb was centered in the eyepiece field of view indicating that it stemmed from lateral color in the eyepiece and not the objective.

Double & Multiple Stars
I spent a couple nights hunting down multiple stars with both scopes. There was never a pair which could be split in one scope, but not the other, so the sharpness of these scopes appears very similar. On the very closest doubles, the views in the ABV showed a black separation between the two stars a bit more often than the ED, I suspect that this is due in part to the small amount of chromatic aberration on the ED lessening the contrast of that gap. These differences were only visible at high magnifications, those at the higher power settings on the 2­4­mm Nagler zoom (roughly 250­400x), so both scopes were performing well beyond the 50x per inch rule of thumb that is often put forward as a reasonable expectation for a given size scope. On color contrast doubles, if the companion was a faint blue star, it was brighter and more vivid in the ABV .

 Planets
I examined Mars when it was high in the sky under good seeing conditions. The ABV showed a bit more contrast on dark surface markings, but that was subtle. When I saw something in the ABV, I, making detection easier and giving a more pleasing visual appearance and color contrast I could always switch to the ED and find the same feature, even if it was a bit less contrasty. With both scopes, I tried a range of magnifications using the 2­4­mm zoom and usually found that although the seeing supported high magnification, the best views of low­contrast details were at more modest powers (around 200­250x, rather than 300­400x).

I took a look at Saturn one morning and found essentially the same results as with Mars as far as the contrast apparent in both scopes(edge to the ABV).Unlike Mars, Saturn supported the higher magnifications since its features are higher contrast. I spent most of my time on Saturn at the 2.5­mms etting on the zoom (280x on the ED and 320x on the ABV).

Deep Sky Objects
I viewed a range of deep sky objects including the bright showpieces of M42, M81, and M82, but also examined both scopes' performance on fainter objects such as M78. Observations at high powers were easier to compare between the two scopes since I could begin by dialing in 200x on both scopes to get a good baseline comparison. This wasn't possible at moderate to low powers with my eyepiece selection, so detailed comparisons on deep sky objects were more difficult. I ended up doing most of the comparisons by switching the same eyepiece back and forth between the two scopes. I used Pentax XWs between 5 mm and 20mm.

A side from the obvious difference in magnifications (about14% more in the ABV) the views were indistinguishable. When there was a slight noticeable difference in the views it was probably attributable to the magnification difference rather than scope performance. The preference always went to the view at slightly higher power whether with the same eyepiece (advantage ABV) or with a shorter focal length eyepiece in the ED to compensate for its shorter focal length (advantage to the ED).

Summary
Being familiar with Stellarvue's reputation, I was expecting the SV102ED to perform well. I thought it would fit nicely in the gap between the 80­mm achromats I'd used and the 102­mm true apo ABV (which is also where it falls interms of price). I was surprised to see, however, that its performance was much closer to the apo than to the achromats. The ABV is a spectacular performer. I've compared it to another high end apo doublet which uses a more conventional secondary element and the ABV had a slight edge. Given that, I expected the ED to fall behind quickly, but it didn't.

Yes,the ED shows a noticeable blue halo around bright objects, but it is a deeper blue or violet than traditional achromats. At high powers, in excess of 50x per inch, this led to somewhat decreased contrast in the ED, the sharpness was nearly equal, but there were hints in good seeing that the ABV may have had a very slight edge in sharpness. For many observers this will be a negligible effect and not worth paying substantially more for the true apo performance.

The Stellarvue SV102ED is an excellent choice for someone looking for a reasonably priced, high performance 4­inch refractor The optical performance is excellent and the mechanics are both functional and aesthetically pleasing. Given Stellarvue's reputation for good quality control, I would expect very consistent performance from all SV102ED scopes. It is portable and cools down quickly, and thus would make a good first telescope that will not be outgrown. It will also nicely complement a large light bucket and so should appeal to beginners and experienced observers alike.

Reprinted with permission of Astronomy Technology Today. To subscribe to this magazine, click here.

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