Vixen SD103S Review
Vixen’s SD103S is the overlooked sibling, the Aberforth Dumbledore, of basic-but-premium 4” APOs. If that’s what you’re looking for, you’ll be thinking about a Takahashi FC-100D and wondering if the DZ is worth the extra over a DC.
You probably haven’t even considered a Vixen and certainly not the SD103S. But honestly that’s because it looks too much like a Skywatcher, but at twice the price, than because you know the Tak’ is a better performer. Or at least that’s the case for me.
So I thought I’d get hold of one to see how my prejudice holds up in the face of a proper review.
At A Glance
810mm w/o eyepiece holder
4.3 Kg incl. rings, plate, handle (no finder)
Data from Vixen/Me.
Alongside its forebear, the ED102S.
Design and Build
Whatever you think of the SD103S’ build, you can’t say it deviates from the family look – Vixen have been making small refractors in this style for a decade or more, since they ditched the Hammerite green and bold graphics of the late FL era.
So the SD103S looks exactly like the ED81S and the ED115S whose optical design it shares. But it’s outwardly not very different from a cheap A105M achromat or an expensive AX103S quadruplet either; and I think that’s a big marketing mistake, but what do I know?
The SD103S is the latest in a line of 102mm and 103mm ED doublet apochromats that replaced the FL-102 (presumably because that became too expensive to produce with its fluorite objective).
Laser test: ED102S left, SD103S right.
The original ED102 was split into two models – one with a 920mm focal length (F9), the other 650mm (F6.4) – but now there’s just this one model with an intermediate focal length of 795mm (F7.7) – similar to its obvious competitor, the basic Takahashi FC-100D.
Also like Takahashi’s basic F7.4 FC-100D, it’s a foil-spaced doublet, but with an ED crown in place of fluorite. This brings us up against the old question, ‘is fluorite better’. I’m still not sure. AP’s Roland Christen says ‘not much’, but when TEC wanted to improve the 140 they swapped from ED to FL and reduced the scope’s false colour.
I suspect that the truth depends as much on the (usually anonymous) mating element as ED vs FL for the crown. In this case, Vixen claim excellent correction – better than the old ED102S and little different from the current ED81S – with the graphs to prove it. In particular, the newer design appears to pull in the violet g-line: good news for imagers.
What about SD vs ED then? The plate on the OTA tells us that, to Vixen, ‘SD’ means ‘Super Extra Dispersion’. In other words the crown is made of the (once, before FPL-55 came out) best ED glass, Ohara’s FPL-53.
I expected the older ED102S to have a more basic ED glass, but based on the laser test above the difference does indeed seem to be in the flint – it scatters a little less in the case of the SD103S, suggesting it’s different, whilst the ED glass scatter in the same.
The cell and objective look just like an ED102 from twenty years ago and I assume they are still made by Vixen in Japan.
The cast cell doesn’t look as finely made as the machined Optron one in a Tak’ but more significantly doesn’t seem to have the fine ridge baffles or the temperature compensating expansion gap either, though the glass is secured by a threaded ring in the same way.
The outer coating looks dark, but reflections suggest that the inner element is uncoated.
All in all, the objective seems a step down in sophistication from Takahashi’s, but does that mean worse performance? Read on to find out…
The aluminium tube is finished in gloss off-white, same as a Takahashi. Finish appears flawless. But where the Tak’ has that embossed plate with a serial number, the SD103S makes do with a sticker on the tube (and – like all Vixens since the early FL era - no serial number).
Older Vixens had their focuser and/or objective cell attached with screws, but here they both attach more orthogonally on threads like a Takahashi.
The dew-shield slides over the cell on felt, a system that may feel less classy than thread-on, but is actually much more convenient if you regularly remove it to pack down for travel. Similarly, the push-in plastic lens cap looks less premium than a Takahashi FC-100D’s push-on metal cap but is functionally as good, a consistent theme in this review.
A key benefit of the competing FC-100DC is that with the focuser and dew-shield removed it’s just 19” in length and will easily fit in a carry-on case. The extra 55mm of focal length mean the SD103S won’t do that – with both dew-shield and focuser removed (something I didn’t try) the SD103S would be 580mm long.
Internally, the tube is sprayed in flat-black and has three knife edge baffles to trap stray light.
Focuser has plenty of travel, minimal image shift and a fluid action.
From the bushing tracks on the drawtube, the focuser on this example has clearly had lots of use (always a sign of a great scope, btw). Consequently, its action is a little free and needs a touch of the lock knob to avoid racking out with a camera. But despite this, it’s still very stable and precise, shows little image shift. And that lock knob is progressive and also creates minimal shift.
All in all, it may look basic, but this is an excellent, imaging-friendly focuser.
Light and compact, the SD103S is easy to mount and very stable on the SX2 you see in the photos. Most small equatorials would take it fine.
Vixen’s SX2 on CB90 heavy duty tripod makes an ideal imaging support for the SD103S, but a Vixen AP would take it fine for visual.
The SD103S ships with various accessories as standard, including a flip-mirror diagonal, a very nice illuminated 7x50 finder and the rings and handle you see in the pics. If you are comparing to a Tak’ it’s worthwhile considering that all those things will be pricey extras.
Vixen produce an SD flattener/reducer kit compatible with the SD103S and with the 81mm and 115mm models too. The flattener is inexpensive at under £300, but the complete kit looks better value at under £500 including the flattener, reducer and all the extensions required.
The flattener alone extends the focal length slightly to 811mm (F7.9), but gives a 44mm image circle.
With both the flattener and reducer attached, focal length is reduced by 0.79x, from 795mm (F7.7) to 624mm (F6.1) with a 44mm image circle and pinpoint stars to the edge of a full-frame image.
In Use – Daytime
The crown may be ‘just’ ED glass and not fluorite, but the SD103S has very low visual false colour by day.
My usual test of viewing branches at 100x in silhouette against a bright but cloudy sky yields only the merest, almost-imperceptible, hint of false colour out of focus: perhaps surprisingly to fluorite fans like me, indistinguishable from a Takahashi FC-100DC/DF and much better than, say, a TV-85 (an F7 ED doublet).
Every picture tells a story and in this case it’s a good ‘un, because the photo of branches shows excellent coverage, flatness, sharpness and minimal false colour – Vixen have designed this lens for photography.
100% crop of branches reveals excellent daytime correction.
In Use – Astrophotography
Vixen say the SD103S is optimised to avoid vignetting at full-frame and so it proves – coverage is good, but the field curvature means you’d need a flattener.
The curves and crossings suggest excellent false colour correction and that turns put to be true too – very similar to the FC-100D. The only demerit is for the spikes produced by the foil spacers (they intrude more into the lens than the FC-100D’s, hence the difference).
A snap of the full Moon is sharp and detailed. Comparison with an FL-102S, using the same exposure minutes apart, surprised with identical false colour fringing on the limb (I’d expected the fluorite F8.8 FL to show less).
As usual, images are unprocessed for easy comparison across reviews.
Pleiades: SD103S with Canon EOS 6D MkII 30s ISO3200.
Pleiades comparison: FC-100D left, SD103S right (exposure the same).
Lunar limb false colour: FL-102S left, SD103S right.
In Use – Observing the Night Sky
General Observing Notes
Even Rigel shows no significant false colour in the star test – pure brilliant white, at the risk of sounding like a Dulux ad’.
With a touch of extra tension on the black knob the focuser action was perfect – very precise with almost no image shift, even at high power.
I did think cooldown was a little slower than something like an FC-100DC. Having been left in the car to cool, it still needed a few minutes to stop showing tube currents in the star test.
The star test is pretty good, with near identical rings either side of focus and just a slightly more diffuse outer ring on one side at high power: better than the old ED102S I reviewed.
A 12 day gibbous Moon with Mare Humorum near the terminator has lots to see. I initially settle on one of my favourite craters, Gassendi, first at 113x with a 7mm T6 Nagler and then 159x with a 5mm T6. Moments of steady seeing had three or more craterlets pop into view and some fragments of criss-crossing rilles in the crater floor, crumpled lavas on the apron leading to Gassendi A.
Moving further north, bright Aristarchus shows its slumped and white-striped walls, the nearby plateau with its strange rille.
Tracking even further up the terminator I come across the Mons Gruithuisen (named after the guy who thought he’d spotted a lunar city) in Oceanus Procellarum, not far from Promontorium Heraclides. It’s an interesting area with a field of domes, ridges and craterlets.
I ended up doing lots of lunar viewing with the SD103S because it was a great view – brilliant, sharp, full of contrast and with loads to explore. Only a trace of purple false colour in-focus on the brilliant limb and a little more flare of white light into space distinguish this from the finest fluorite doublets.
I swapped back and forth comparing the older F9 ED102S I also had on loan. There was little difference between them, apart from mag’ for a given eyepiece, and the false colour level focusing through them limb was virtually the same.
High in the sky but only 8.6” across now, two months past opposition, Mars showed a perfectly sharp off-white gibbous disk with a hint of albedo markings at 159x with a 5mm T6 Nagler.
ED doublets often show a lot of red flare on Mars, but not the SD103S. There was just a trace of red ringing the blur inside of focus and perhaps the merest hint as the seeing fluctuated in focus, but much less than other ED doublets like a Sky-Watcher 120ED or a Tele Vue TV-85 for example.
However, the SD103S wouldn’t take much more than a 3.5mm eyepiece giving 227x on Mars; the 3mm setting of a T6 Nagler gave a slightly unsharp view with some flared light. By comparison, the FL-102S I was testing still showed a clean gibbous disk with the 3mm at 300x, as you’d expect from a scope designed for planets.
Castor was a huge and easy split as you’d expect, but Rigel which can trouble less than perfect optics, revealed its faint companion immediately and very clearly at 159x, thanks to the tight PSF of this fine lens.
The Pleiades with a 25mm Plössl were those sparkling diamonds on black velvet within a glow of blue nebulosity we all love. The stars were decently sharp across the whole field, with very minor astigmatism towards the edge with this simple eyepiece (a highly corrected Nagler was pinpoint to the field stop).
The SD103S suffers a bit from Toyota syndrome – you know it makes sense, but…
The corollary (sorry) is that, though it isn’t as glamorous as an FC-100DC it’s a great all-purpose scope with some real advantages.
Light and compact and easy to mount with its supplied handle, the SD103S has a very competent focuser designed with imaging in mind – still smooth, stable and precise, albeit a little loose on this used example.
The objective is very good indeed: with an excellent star test, it has (surprisingly) low false colour, great full-frame coverage and good flatness for an F7.7 doublet. I’d expect it to be a great imaging tool with Vixen’s flattener.
Visually, the SD103S impressed too, with fine views of the Moon and planets at high power and sparkly star fields. It gives away just a little to the very best 4” refractors at only the highest magnifications.
The basic appearance of the objective and cell don’t really play out in use, apart from slightly slow cool-down. The only downsides are the slightly basic build in places and the fact that it’s not as tear-down compact as an FC-100DC (though just as light weight).
Comparisons with the similarly priced Takahashi FC-100DC are slightly unfair because the SD103S’ focuser is more capable for imaging: it’s more comparable with the costlier FC-100DF. And with Tak’, all those accessories are expensive extras.
The SD103S’ workaday look conceals a really well designed general purpose refractor. It gets my highest recommendation.