Skywatcher ED-100 pt1

By Antony McEwan

Or, ‘A Budget APO that doesn’t bring the blues.’

4 Inch Apo. There’s something about that term that makes people sit up and take notice. Apo refers to the term ‘apochromatic’, which is a definition of how well a lens system will correct for chromatic aberration, and yield colour free images. Oh dear. Only two sentences written and I suspect some of you are switching off already… Right, I’ll go back to basics for a minute.

A traditional ‘achromatic’ refractor uses lenses at its front end to ‘refract’ the incoming light to a focal point at the back end of the telescope, where an eyepiece is used to view the image. Typically, refractors use a pair of lenses cemented together, and this combination of two slightly different lenses reduces a nasty side effect of refractors- chromatic aberration. Different wavelengths of light (which we perceive as different colours) refract by different amounts, so if a white light source shines into the front (objective) lenses in a telescope, the image seen at the eyepiece will not have all the wavelengths come to the same focal point. This results in different focal points for the different colours, and so when viewing a bright object like Vega or a planet, the colour definition will not be true and there will be a ring of ‘false colour’ around the target caused by the blue wavelengths that don’t quite come to focus at the same point as the others. The amount of false colour displayed varies according to the focal ratio and aperture of the refractor, so that high focal ratio refractors generally show less than low focal ratio ones, and smaller apertures are also generally more forgiving. It is more noticeable at high magnifications too, so will show up more when looking for detail on the planets, for example.

Enter the Apochromatic refractor. The Apo, as it’s known, uses specially produced glass types (typically fluorite or some other exotic material) in one of its lenses, which so changes the way that light is refracted through the lens that it corrects for the difference in wavelengths to a much higher degree than an achromatic telescope, bringing them all to focus much closer together, and so significantly reducing (and sometimes completely removing) any trace of the false colour produced in an achromat. The price of this quality is, well…the price! The glass used is expensive, and the manufacturing process time consuming, so that naturally puts a premium price on Apo ‘scopes. But it’s not only the glass in an Apo that is of high quality. With a superb set of lenses in the tube, manufacturers such as TeleVue, Stellarvue, Takahashi and others typically make sure that all the other components in the telescope are also top notch, and so the retail price continues its climb, until at the end of the process a 4” Apo can typically cost about £2000 or higher. Compare that with £250 for a standard achromatic refractor.

So, all 4” Apo’s are expensive then? Well, not anymore! Synta is a well-known Chinese telescope manufacturer that produces not only complete telescopes, but also components and optics for other telescope manufacturers. Their products are typically good quality and robust, but lacking a certain finesse found in more expensive brands. Their own brand label is ‘Skywatcher’, and they have been on the market for many years now, with offerings of reflectors, achromatic refractors and Maksutovs. Well, they have now branched into apochromatic refractors; first with an 80mm model, then more recently with a 100mm model. Could I resist? Not for long…

Opening the Box

Having read much about the Skywatcher ED-100 on the internet, and having heard about the joys their owners were experiencing, I decided to keep an eye out for one at a good price. About that time a new model was introduced- the ‘ED-100 Pro’. The Pro model had the same basic design as the original model: 100mm aperture, focal length 900mm so a focal ratio of 9. Aluminium tube with a 2” Crayford focuser, and usually supplied with a pair of tube rings to attach to most mounts. The 2” focuser allows use of wide angle 2” eyepieces, allowing one to take in large swathes of sky at low magnification- great for viewing large extended objects such as the Andromeda Galaxy or the Veil Nebula. The only visual difference was that the Pro was gold coloured with white trim, while the older original model was a nice sparkly Skywatcher-Blue with black trim. The Pro was also about £100 more expensive, despite having exactly the same optics! Naturally I started looking for a nice older one, but found them to be very rare indeed- people didn’t seem to sell them on after using them! Gold tubes were taking over, but eventually I found a very good deal at Pulsar Optical, where I obtained a showroom demo Pro for the price of the older original model. I decided I could put up with the 'bling' colour scheme and paid the money.

The telescope arrived a few days later, in perfect condition. It looked brand new, never mind demo model! The objective was clean and shiny, with superbly applied deep green coatings, and the tube was unmarked. There was a small amount of surface dust present on the lens, but this really was no problem and you simply can’t prevent this from accumulating so I was not worried about it. I was interested in the Crayford focuser, having never used one before. It is different from a Rack & Pinion focuser, in that it uses a roller to apply pressure to the underside of the drawtube, and this roller moves the drawtube in and out. A rack & pinion obviously depends on gears and teeth to do the same job, and if the two don’t mesh exactly the result can be a slight choppiness to the movement. Not so with a Crayford. I found the movement to be very smooth, much more so than the rack and pinion focusers on my two reflectors. My TeleVue 85 has a rack and pinion too, but it is exceptionally smooth- about the same as the ED-100 Crayford. There is a tension adjustment knob for increasing the friction against the drawtube, and this is recommended if you are using a heavy diagonal or eyepiece and want to prevent the drawtube slipping when the telescope is pointing high in the sky. Even with the telescope mounted indoors on my alt-azimuth Giro II mount, with the tube pointing straight up I noticed that this would be a problem, and realised that I’d be using that friction adjustment knob quite often.

The focuser also has a built in shoe for attaching a standard Synta type finder bracket. I happened to already have a few of these, so decided to dedicate my right-angled 9x50 to this ‘scope. I also ordered a high quality 99% reflective dielectric coated 2” diagonal, to get the best possible views through the eyepiece from my new baby.

Expectations

What did I want my new telescope to achieve? What did I want it to do that my 6” and 8” reflectors and 85mm Apo couldn’t do? Why had I bought it?

What I love about my TeleVue85 is the fact that it can go from being a wide-angle vista-viewer one minute to a zoomed-in detail-deliverer the next. It excels in both fields, and is portable, lightweight and easy to transport. I wanted the ED-100 to do the same job, but with a little more light-grasp to show a tad more detail. The reflectors show much more detail than small refractors, but at the expense of bigger tubes and increased bulk. Ultimately though, there was an element of curiosity too- could a ‘budget Apo’ deliver serious performance at an affordable price?

First (and other) Lights

First light was on a dark autumn night, at a completely dark site. I mounted the ED100 on my driven Vixen GP mount, and even when I was polar aligning the mount I was impressed by the view of Polaris and its neighbouring stars, with Polaris’s 9th magnitude companion star showing itself easily even at 42x magnification. The GP mount handled the tube assembly with ease. The ED-100 is really quite lightweight at 7lb, a smidgen less even than my TeleVue 85! A diagonal, finder-scope and eyepiece add one or two pounds to that, and as all of those weights are added to the back end of the tube, the balance point moves towards the rear of the tube and the rings that attach it to the mount can then be placed quite close to the eyepiece end. This is a subtle, but important, factor. It means that when slewing the telescope all over the sky, the eyepiece will not actually move as far as if the telescope were mounted lower on the mount. Balancing any telescope tube assembly on a mount is something that quite often gets neglected, and by having the tube balanced you are less likely to suffer sudden swings and dangerous dips when loosening an axis locking knob, for example, which could result in a nasty (and expensive) meeting of optic and tripod.

I toured some easy double stars, and just revelled in the views of Albireo and Mizar. The ‘Double Double’ in Lyra showed both pairs very easily split at 85x, and the views were overall colour-free when the ‘scope was in perfect focus. I noticed it was important to have the focus ‘spot-on’ though, as a touch too much or too little would result in some false colour showing around bright stars, though even then not much. But then, you always want the ‘scope to be properly focused anyway. (As an aside, did you know there is an easier way of achieving perfect focus and a harder way? The harder way is by getting close to the sweet spot, then racking the focuser in and out and in and out, until you eventually hit the spot. The easier way is to approach the sweet spot by moving the focuser ‘inwards’ only, and if you overshoot simply back it out to a point well away from focus, and then use very small movements inwards again, until you reach focus. The reason for this has to do with your eye’s ‘accommodation’ - to put it simply, the eye is more relaxed and natural if you use the in-travel method, and has to strain more if you are racking the focuser in and out.)

For me the highlight of that night was seeing the Veil Nebula in Cygnus. This is an elusive object to catch in a 4” telescope, and yet the view was really quite bright for the limited aperture. I was using a 30mm 2” eyepiece for 30x and a field of view of just over 2 degrees, and slowly panning back and forth across the separate strands of nebulosity that make up the nebula was a real joy. More importantly, it showed that the ‘scope had high light transmission, and would excel in showing faint M-objects as well as planetary detail.

The next time I used the ‘scope, I was baffled by something. I set it up this time in my back garden, which is unfortunately enveloped in the glow of two sodium street lights. Try as I might, I could not get rid of what I thought was a strange yellow tint to all the stars I looked at, no matter what spectral type they were. Even white stars showed an orangey hue. Of course it should have been obvious, but with a new ‘scope in my hands I was perhaps a bit panicky, and wondered if something was wrong with the optics…

Third light was at Culloden in November 2005, on one of the Mars viewing sessions organised to let members of the public see Mars in all its glory as it approached opposition. That particular night we were discouraged from setting up the Society’s 10” reflector by persistent cloud, but we set up our portable ‘scopes in the car park to use in the fleeting clear periods. Lining up the ED-100 on Vega this night, I could see it in its true colour- bright blue-white, with no orange or yellow tint at all. The same went for other white stars. The detail shown on Mars was similarly excellent, with no hint of discolouration. I went home happy that night, taking with me the conclusion that the orange glow I was detecting from my back garden was some interaction of the sodium lighting with my ED-100’s optics.

Click here to go to part 2.