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 (a big thanks
to Justin for the loan of his scope).
At
A Glance
|
Telescope |
Vixen
SD103S |
|
Aperture |
103mm |
|
Focal
Length |
795mm |
|
Focal
Ratio |
F7.7 |
|
Length |
810mm w/o
eyepiece holder |
|
Weight |
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.
Optics
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…
Tube
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.
Focuser
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.
Mounting
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.
Accessories
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, with just a touch more violet
bloat on Alcyone (see below).
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). You can see this
effect in the side-by-side images of Alcyone below. Note:
the most recent version of the SD103S has rectified this with a ring spacer
between the elements. If you
want an SD103S for imaging, check before you buy.
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).
Alcyone: 30s
at ISO 3200, Canon EOS 6D MkII with Takahashi
FC-100DC (left) and Vixen SD103 (right) showing effect of foil spacers in light
path, slightly more violet bloat from SD103S.
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.
Cool
Down
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.
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.
The
Moon
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.
Mars
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.
Deep
Sky
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).
Summary
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.
