Sky-Watcher Evostar 100ED DS-Pro Review

The first of the larger Sky-Watcher FPL-53 doublets I tested – the Equinox 120 ED – is F7.5, but the same basic lens and cell design has long been available in a 100mm F9. Now F9 is quite a slow focal ratio for an APO, but I’ve become a bit of a believer in slower optics, finding out what others have always known: that most aberrations ease at longer focal lengths and, due to the shallower curves on the glass, the optical quality is often higher too.

Perhaps less ideal for imaging (and so less popular as the result), such longer focal lengths are often found in specialist planetary scopes, like the APM/TMB 130/1200 and Takahashi’s limited edition FC-100DL. But in the case of the Sky-Watcher F9 100ED, the slower f-ratio offers the possibility of a really good general purpose APO for an indecently low price (taking bundled accessories into account, about half that of the 120ED).

The same Synta 100ED lens is available in various tube designs from various brands – including an Equinox similar to the 120ED I tested before – but by far the most common is the Sky-Watcher ED DS Pro, so that’s the one I decided to look at here.

At a Glance

Telescope	Sky-Watcher 100ED DS-Pro
Type	ED Doublet Apochromat Refractor
Aperture	100mm
Focal Length	900mm
Focal Ratio	F 9
Length (incl 2” visual back)	87cm
Weight (OTA + Rings, NOT finder)	~3.5 Kg

Please Note: weight and length are my own measurements, as I couldn’t find official ones!

Design and Build

Optics

The objective lens boasts premium glasses: Schott for the negative element and FPL-53 for the positive.

All the Sky Watcher ED Pro and Equinox APOs are ED doublets, but have different focal ratios, varying from F6 to F9. The 100ED has been around the longest and is the slowest of the lot at F9. Nonetheless, SW boast of Ohara FPL-53 ED high-fluoride glass as the positive crown element and some unspecified Schott glass (no whisky in this one) for the negative flint element. Ohara’s FPL-53 is one of the best ED glasses available and is used in many premium refractors.

The management summary is that, given its long f-ratio and premium glasses, this 100ED should be a proper apochromat with minimal chromatic aberration (false colour fringing). But note that earlier versions of this lens may have used cheaper glasses, so beware of extrapolating my findings in this review to earlier versions!

The lens has the usual ‘China Green’ coatings: perhaps not the very best, but good nonetheless.

The doublet sits in a basic, non-adjustable cell, but this saves weight and keeps the tube slim. The cell appears to thread onto the tube, locked in placed by black cement.

There were reports that the Equinox 120ED achieved its excellent performance in small part by a bit of sneaky vignetting. In other words, the baffling stopped down the aperture to perhaps 114mm/F7.9. My (admittedly crude) exit pupil measurements reveal no vignetting on this 100ED.

Tube

Sky-Watcher’s ubiquitous Black-Diamond livery.

This version of the ED DS Pro is finished in the current ‘Black Diamond’ finish, shared with numerous other SW products. It’s a combination of shiny metallic black enamel for the tube and knobbly white powder coat for the cast parts. The OTA is just like the SW Evostar achromats, lens and focuser aside.

The more expensive ‘Equinox’ version of the 100ED shares optics with the ED DS Pro reviewed here, but differs in a number of areas:

· CNC-machined tube.

· CNC rings.

· CNC focuser.

· Sliding dew-shield (the Pro version is fixed but can be unscrewed).

· The Equinox all threads together; the ED Pro uses push-fit-with-screws to hold the focuser in.

· Different finish in gloss piano black and chrome.

Mostly these differences between ED DS Pro and Equinox comes down to the focuser and cosmetics, unless you plan on using the scope on a GOTO mount, in which case the ED DS Pro won’t be orthogonal enough to get good pointing accuracy and certainly not enough to align the mount! (Orthogonality is the extent to which the optical axis of the scope aligns with its physical axis and varies with position and focuser etc. Generally only scopes with CNC’d tubes, rings and focuser are highly orthogonal).

All in all, the ED DS Pro has a more budget feel than higher-end Sky-Watchers, with cast parts and rings, rather than CNC. There is an upside, though: the DS Pro OTA is lighter than the Equinox (it is one of the lightest 4” APOs) and will mount more readily on a smaller mounts, at least for visual use.

The overall build quality is still very good with flawless paint and powder-coat. The interior is flat-black painted and has a just pair of knife-edge baffles for stray-light, though these show some rather crude shaping and cementing and a bit of light leakage at the tube join you wouldn’t find in a premium APO.

Focuser

Dual-speed Crayford focuser on ED100 DS Pro.

Again, the focuser is different from the Equinox. It is a Crayford type, with a cast body that has a finely-machined dual-speed insert. As with other dual-speed designs, the outer knob (black on recent models, gold on older ones) on the right side is for fine focusing. The draw-tube is 55mm diameter and has ridge-baffles machined in and is properly flat-black painted to reduce stray light.

The visual back is a 2” dual set-screw type with a lock-ring on a thread that can serve as a camera rotator: unlock, turn visual back, re-lock. The standard VB can be replaced by the reducer and a 1.25” adapter is provided.

The draw-tube has commendably long-travel: I measured about 14cm. I didn’t need an extension for imaging with a DSLR, with or without the reducer.

The focuser is generally excellent, with no appreciable image shift and very smooth and precise fine focusing: a big step-up from low-end rack-and-pinion focusers in terms of precision and feel. It can handle heavy 2” eyepieces and diagonals, but take care to avoid rack-out by using the lock screw!

My only criticism is that it can rack out of its own accord with very heavy eyepieces/cameras and unfortunately the lock-screw underneath changes focus position and so doesn’t really work as intended. Like all Crayfords, which work by friction and have no rack-and-pinion, it can also slip with heavy loads. I had a frightening moment with a camera attached – the drawtube suddenly slid out when I was messing about with the camera.

If you’re a keen astrophotographer you’ll want to replace the focuser eventually; for visual use it’s absolutely fine, but keep a bit of tension on the lock screw with heavy eyepieces.

Mounting

The 100ED comes with a Vixen-pattern dovetail. In terms of weight, it easily mounts on a Vixen GP or EQ5, but the long tube means you get more persistent vibrations at high power than with shorter OTAs. You would need a heavier mount for serious imaging and longer exposures.

I also mounted it piggyback on my big mount for imaging/tracking.

Sky-Watcher 100ED Pro mounted atop my AP1200 rig for testing.

Accessories

The Sky-Watcher 100ED DS-Pro comes with a hard case and accessories.

One area where the ED DS Pro wins decisively over the Equinox version is accessories. Currently, SW will sell you a bundle containing:

· OTA

· Rings

· Vixen-pattern plate

· 9x50 Finder

· 2” Diagonal

· 28mm Eyepiece

· 0.85x Focal reducer

· Aluminium carry case

Let’s take a quick look at each of these accessories.

The 9x50 finder is the standard Sky-Watcher unit used on most of their telescopes. It has a narrower field and less eye relief than the best, but it is of good optical quality and is very usable, with a sharp bright field.

Sky-Watcher’s bundled 28mm LET eyepiece is surprisingly good.

The 28mm LET eyepiece looks a bit cheesy, in that it’s styled to look like a Pentax XW, though it clearly isn’t one. But … big surprise when you look through it – this is an excellent eyepiece. It’s a simple three element design (modified Kellner), but is very sharp across 80% of the field and commendably free from chromatic aberration. As you’d expect from a simple design, it’s bright too. With a wide 31mm field stop, top-notch coatings and a comfortable level of eye relief and no blackout problems, this is a very useful basic eyepiece.

The supplied 2” diagonal has a dielectric multi-layer mirror like expensive diagonals from Tele Vue and AP. Why does this matter? Dielectric mirrors are supposed to reflect more light as well as being more resistant to cleaning. The only disadvantage with this SW diagonal is that the body is not milled from one block, so the thread-on barrel could unscrew with a heavy eyepiece.

The aluminium carry case is the same pattern as other Sky-Watchers and is a good quality hard case.

ED100 0.85x Reducer

I got the ED100 0.85x reducer as part of a bundle deal, but it’s widely available after market if you don’t. The reducer flattens the field (not that it suffers much curvature to start with) and speeds up the F-ratio to F7.65 to give imagers shorter exposure times; it’s a dedicated unit for the 100ED - the 120ED reducer is specific to its faster (F7.5) f-ratio.

One thing to note is that you will need a special adapter ring (M48 to M42) to connect the reducer to a T-mount.

I’ve put together a sequence of images to show you how to attach the reducer and a DSLR, in case (like me) you aren’t clear on where the adapter fits in:

1: Remove the visual back

2: Thread on reducer and tighten lock-ring

3: Fit the M48 – M42 T2 adapter (likely NOT included with the reducer).

4: Thread on the T-ring specific to your camera

5: Attach DSLR

In Use – Daytime

The 100ED is much too big to be a crossover daytime spotter, but daytime views can be revealing. The 100ED showed absolutely crisp views at 150x with a 6mm Ethos during the day, with minimal chromatic aberration and a very flat field.

My usual photo of branches, taken in silhouette against a bright sky, shows virtually no in-focus chromatic aberration and very little either side. But, as we will see, this is partially the result of the 100ED’s objective having been tuned for middling (typical daytime) wavelengths – false colour for imaging isn’t as good as the daytime image might suggest.

Branches silhouetted against a bright sky are sharp and show minimal CA.

In Use – The Night Sky

General Observing Notes

The focuser takes the weight of a 2” diagonal and a heavy eyepiece well and the focusing sweet spot is long due to the long F-ratio. The micro-focuser is thus not essential, but has a smooth precise action; the focuser generally seems very smooth and accurate. It has a little slop with the drawtube racked out, but so do most focusers.

The 100ED makes a viable grab-n-go on a lightweight mount (it’s just light enough to balance with the smallest counter-weight on my Vixen GP which helps make for a portable setup).

Cool Down

Cool-down is very rapid for a 4” refractor, due to the doublet lens and low tube mass.

Star Test

The star test was not quite perfect, a little less so than the 120ED I reviewed. The diffraction rings were evenly illuminated, but not quite identical either side of focus, though the in-focus Airy disc looked text-book perfect. I would estimate good diffraction limited optical figure, but not much more on this example. This doesn’t limit its practical in-focus performance one bit, as we will see.

The Moon

I always think a good 4” refractor is a great way to view the Moon and the ED Pro is no exception. On a waning half-moon at 69x with a 13mm Ethos, there is no discernible false colour. The field is wide and flat enough to enjoy the whole Moon hanging in space, but doesn’t push enough light through to dazzle too seriously. It’s a fantastic view.

At 150x with a 6mm Ethos, lots of Lunar detail is visible and the view is still very sharp. A trace of chromatic aberration is visible when focusing through the limb, but none in-focus. This combination gives that ‘Lunar Module porthole’ effect that smaller apertures can’t quite achieve.

I noticed that Rimae Hippalus is shown to excellent effect, as are the wrinkle ridges in Mare Humorum. The dome field to W of Copernicus is interesting to explore and for a while I forget I am testing, a sure sign of a fine telescope.

Other notable detail includes the central crater in Timocharis and subtle banding on the lava plain on the floor of Archimedes. Rima Hyginus is easily resolved.

Upping the power further, to about the maximum I would use, the view is still crisp and very detailed at 257x with a 3.5mm Nagler in good seeing, with Copernicus showing its central peaks and details of slumped walls and lots of surrounding craterlets and crater-chains.

Mars

On Mars, the 100ED exhibits some red blur around the planet in focus, like the 120ED, but I was nonetheless able to get a crisp dawn view at 257x with the Nagler T6 3.5mm. Mars was still small, but a polar cap was visible as was some albedo detail.

Jupiter

The 100ED gives a fine view of Jupiter, with plenty of detail and good focus snap at 150x with an Ethos 6mm, including fine banding in the polar hoods. I was also able to see a shadow transit.

Perfect focus was easy to obtain at high power with the smooth micro-focuser. There was some minor red-green fringing on Jupiter either side of focus, but minimal chromatic aberration in focus.

Deep Sky

Doubles

The ED100 split Rigel without difficulty even when low and easily split 2.3” Epsilon Lyrae at 150x, just as you would expect.

M42

My first view of the Orion Nebula with the ED100 came as a surprise. I had been testing a couple of smaller scopes and had forgotten just how big and bright and beautiful M42 looks through a good 4”-class refractor: whorls and dark lanes in the nebulosity, the dense ‘cloud-bank’ arcing away to one side, the Trapezium glittering in the middle and numerous pinpoint stars to the field-stop of a 6mm Ethos at 150x. M42 never looks this good in smaller refractors and reminded me what a step-up a 100mm refractor is from an 80mm Unscientifically, I always feel the next 20mm increment, to 120mm, is less dramatic.

M37

Clusters give a diamond dust effect in a 13mm Ethos, with pin-point stars. I notice that the field is flat and coma is very well-controlled.

The Pleiades

The Pleaides – M45 – really sparkle through the ED100, more brilliant than in a smaller APO, but with plenty of field width to fit the whole cluster.

Globular Clusters

M15, M3, M13 and M69 in the autumn sky all start to resolve into their component mass of stars and again the extra performance of the larger aperture really wins here and is just another reason why a 4” refractor like this ED100 is much more versatile than a smaller refractor.

In Use - Astrophotography

Yin and Yang: that slow F9 focal ratio makes for longer exposures, but means the field is natively very flat, making for especially nice wide-field images even without the reducer.

Below I’ve included a few basic frames straight from my budget APS-C cameras (a Nikon 5100 and a Fuji XM-1). The photo of the Moon is especially impressive with lots of detail and superb sharpness limb to limb thanks to that well-corrected field. Incidentally, this is an area where an apochromatic refractor really wins over an SCT.

With the reducer I can’t detect much difference in terms of off-axis aberrations, because there wasn’t much problem to start with on an APS-C size sensor, but it obviously speeds things up. The reducer might be necessary for full-frame. However, the image of M42 shows some blue-bloating on Nair Al Saif that is a product of the objective not the reducer (you still get it without the reducer).

Moon through SW ED100 at F9: 1/500s at ISO 200 with Nikon D5100 DSLR: sharp and detailed.

M1 through SW ED100 at native F9: single un-processed frame at ISO 3200 and 75s with Fuji XM-1.

M42 through SW ED100 at F7.5 with 0.85x 100ED reducer: single un-processed frame at ISO 3200 and 45s with Fuji XM-1.

Summary

Takahashi recently released a limited edition 100mm F9 doublet, the FC-100DL. The Sky-Watcher ED100 Pro has the same specs, but just a little more chromatic aberration and doesn’t deliver quite the supreme contrast. A bright lunar limb or planet produces just a bit more flare of unfocussed light. Images show a bit more blue-bloat on bright O-A stars. But these differences are quite subtle. Reports of earlier Sky-Watcher 100ED models suggested semi-APO performance, but I would rate this 100ED as a full-apochromat, if not quite equal to a premium fluorite doublet, even the basic (F7.5) FC-100D.

Nonetheless, the Sky-Watcher 100ED is a quality ED doublet, doing all the things you would expect from a good 4” apochromat at half the price of an FC-100D. It gives great high-power views of the Moon and Planets, but of DSOs and star fields too and takes excellent images with nice tight stars and well-controlled off axis aberrations. It is also light weight and so easy to mount and cools quickly.

Build quality is generally very good for the price-point, with care taken where it’s not obvious as well as where it is, but differences from a premium APO are more noticeable.

Downsides are few for the price. It is a long scope and so vibes more on a small mount than a shorter OTA would, despite its very modest weight. The long OTA and case are not portable in the way a 3” refractor, or a compact 4” is. The focuser is smooth and precise, but can rack out or slip under heavy loads and has a useless lock.

But overall, any criticisms are nit-picking. The ED100 Pro is a superb general-purpose telescope and an absolute bargain. It doesn’t have any fancy features, or make any pretence at ultra-portability, it is just a well-designed and made basic doublet APO. It is typical of what I love about a 4” apochromat: easy to use, mount and deploy, yet delivering effortless views on all kinds of objects and with good imaging potential to boot.

The 100ED DS Pro is a very competent 4” APO and another quality telescope from Sky Watcher; I highly recommend it. It’s amazing value compared with just about any other refractor in its class. If you want to try a really good 4” APO for a sensible cost, this is the perfect choice.

The Sky-Watcher ED100 DS-Pro is an easy ScopeViews best buy.