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Sky-Watcher Evostar 100ED DS-Pro Review

Title

I’ve often said that a 4” APO is one of my favourite telescope formats: light and portable, a 4” APO can show a lot more than APOs in the 3” class, from planets to DSOs and can be good for imaging too. As usual with small scopes, there’s a caveat: only if it’s a good one.

When I first tested one of the larger Sky-Watcher FPL-53 doublets – the 120 Equinox – I was amazed at the quality they had achieved for the price. Since then I’ve seen a lot of Sky-Watcher optics and they have all been good to excellent.

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. That Equinox 120 I tested was F7.5, but the same basic lens and cell design has long been available in a 100mm F9, quite a slow focal ratio for an APO and one found on planetary refractors like the TMB 130mm F9. Perhaps less ideal for imaging (and so less popular as the result), it offers the possibility of a really good general purpose APO for an indecently low price (taking bundled accessories into account, currently about half that of the 120ED).

The 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 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

SWED100Lens

Sky-Watcher’s 100mm F9 ED Doublet.

All the Sky Watcher ED Pro and Equinox APOs are ED doublets, but have different focal ratios. The 100ED has been around the longest and has a conservative (for a 4” APO) F9 focal ratio. Nonetheless, SW boast Ohara FPL-53 ED high-fluoride glass as the positive crown element and some unspecified Schott for the negative flint element. Ohara’s FPL-53 is one of the best ED glasses available and is used in many premium refractors.

SWED100Schott

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

The management summary is that, given its long f-ratio and premium glasses, this 100ED should be a proper APO with minimal chromatic aberration (false colour fringing). I suspect 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, but in this case they look very dark: perhaps not the very best, but excellent 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

SWBlackDiamond

Sky-Watcher’s Black-Diamond livery

This version of the ED 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 Pro, 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 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 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 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 Pro OTA is lighter than the Equinox (it is one of the lightest 4” APOs) and will mount more readily on a smaller, cheaper mounts like my Vixen GP, 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

SWED100_Focuser

Dual-speed Crayford focuser on ED100 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.

ED100AP1200

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

Accessories              

DSC_2298

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

One area where the ED Pro version wins decisively over both the Equinox version and its bigger 120 ED sibling 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

This is a great deal at current prices. Let’s take a quick look at the accessories in turn.

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.

SWEP

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 2” diagonal is a very decent unit that’s supposed to employ a dielectric multi-layer mirror coating like expensive diagonals from the likes of TeleVue 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 is that the diagonal 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.

Reducer

Reducer

The bundled reducer is a limited offer and may not be available when you read this, but it’s a deal whilst it is. Normally £170, it 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 though, the 120ED’s seems to be different to work with its native F7.5.

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

Redfit_1

Redfit_2

2: Thread on reducer and tighten lock-ring

Redfit_3

3: Fit the M48 – M42 T2 adapter

Redfit_4

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

Redfit_5

5: Attach DSLR

Redfit_6

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.

For a doublet, the 100ED has outstanding performance in the daytime, a sure sign of a good optic.

SWED100Branches

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

In Use – The Night Sky

I notice the focuser manages 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.

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

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.

 

Astrophotography

Exposure times are proportional to the square of the focal ratio, so expect exposures 40% longer with the basic 100ED than the 120ED due to that F9 focal ratio. Having said that, even without its reducer the 100ED makes an excellent astrograph for the following reasons:

·         Optical quality is very high. Not essential for wide-field, but certainly for lunar and planetary imaging.

·         Chromatic aberration is very well controlled for a doublet.

·         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); as always, no stacking, sharpening or post-processing of any kind, so you can compare like with like, at least across my reviews. The photo of the Moon is especially impressive with lots of detail and superb sharpness. The photo of M1 at F9 demonstrates how flat the field is, no extra glass required.

SW100ED_Moon

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

SW100ED_M1F9

M1 through SW ED100 at F9: 75s at ISO 3200 with Fuji XM-1: no stacking or processing.

SW100ED_M42F75

M42 through SW ED100 at F7.5 with reducer: 45s at ISO 3200 with Fuji XM-1: no stacking or processing.

With the reducer I can’t detect much difference in terms of field flatness and off-axis coma, because there wasn’t much to start with on an APS-C size sensor, but it obviously speeds things up. The reducer might be necessary for a full-frame sensor.

 

Summary

The ED100 Pro is not – quite - a Takahashi FS-102 (my favourite 4” doublet): it has just a little more chromatic aberration, particularly at the red end on Mars and doesn’t deliver quite the supreme contrast. A bright lunar limb or planet produces just a bit more flare of unfocussed light. But these differences in optical quality are very subtle indeed.

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.

So the 100ED is a very good ED doublet, doing all the things you would expect from a quality 4” APO at a third the price of Tak’s current 4” doublet, the FC-100. 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.

Reports of earlier 100ED models suggested semi-APO performance, but I would easily rate this 100ED as a full-APO. Chromatic aberration is very well controlled, though some red-blurring does occur around Mars (it’s probably been tweaked to perform best at shorter wavelengths for CCD imagers). Older version may have used a cheaper crown glass, whereas the FPL-53 used here (combined with modest aperture and longish focal ratio) yield a proper apochromat, if not quite the equivalent of a fluorite doublet.

It’s worth repeating that a 100mm (4”) APO is a surprisingly big step up from a 76mm or 80mm in terms of what it will show you. The ED100 easily betters any 80mm refractor (but is also significantly larger).

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” APO : easy to use, mount and deploy, yet delivering effortless views on all kinds of objects and with good imaging potential to boot. If you want a single, fast cooling, easy-mounting, general-purpose scope that gives detailed views of the Moon and planets, whilst giving wide-field views and images as well, this 4” APO remains a better bet than an 8” SCT because it is so much more versatile; amazingly, it’s cheaper too.

The 100ED 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.

For a 4” APO, the Sky-Watcher ED100 DS-Pro is an easy ScopeViews best buy.

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