Every Astronomer should have at least one low power, one medium power and one high power eyepiece in their collection, although the higher powered EPs will only get used when the atmosphere is at it’s steadiest. Enter the Williams Optics SPL 3mm, one of the finest high power EPs in the medial price bracket (around £70).
Although the magnification yielded by the eyepiece is wholly reliant on the focal length of your telescope the short 3mm focal length of the eyepiece ensures that however long your telescope, it will be pushed to it’s practical limits.
For example the two telescopes I use this EP with have focal lengths of 1000mm and 1500mm respectively, in the 1000mm refractor I get a magnification of 333x which is only just over the useful magnification recommended by the manufacturer of 283x. The 1500mm Dobsonian gives me a magnification of 500x which is well within the practical limits of the telescopes 600x useful magnification.
It is true that the eyepiece will push 90% of scopes to their natural threshold, so to get the most out of the EP it is integral that you use it only when the atmosphere is at it’s least turbulent or to put it in a more astronomical way when the “seeing” is good. There are more complicated and effective methods of determining seeing but a quick and easy way is to simply observe the stars with your bare eyes, the more they twinkle the more the atmosphere is undulating and the less likely it is that a high power eyepiece will be useful for that particular evening.
If you’ve checked the sky and you’re good to go what can you expect from the WO SPL 3mm? Well first off this is a BIG eyepiece, it dwarfs the other eyepieces in my collection by a factor of at least two. It is just over four inches long and while the end is 1.25 inches in diameter (so it can be slotted in almost all standard focuser barrels) it bulges to almost two inches at it’s center. It’s also a heavy EP at 200g (or 0.4 lb if you prefer),
Which can cause balancing problems with refractors when swapping between this beast and it’s lighter cousins but this takes only a few seconds to correct so cannot be considered a major failing of this EP.
It is a heavy EP but where does this extra weight come from? Williams Optics have used a 7 element design (that is to say they’ve used 7 lenses) with the SPL series, grouped in 3 lots of 2 and a single lens. One may think that the extra 3 elements over the traditional 4 element design is a risky move by WO, after all the more glass you put between you’re eye and the target your viewing the more chance there is for light scattering or image degradation, WO have avoided these potential pitfalls by using a highly effective coating on it’s lenses that keeps these problems to a bare minimum.
In fact it is the 7 element design that allows the SPL to stand above the competition because SPL stands for “Super Planetary Long eye relief”. Usually the higher magnification the smaller the eye relief (eye relief being the distance in millimetres your eye must be positioned in order to see the whole field of view), but the SPL series give excellent magnification while still retaining a long eye-relief due entirely to the use of extra lens elements.
Practically speaking this means you don’t have to push your eye up right against the lens in order to see properly, it makes for a more relaxing experience which shouldn’t be overlooked after all what use are beautiful views of the heavens if you have to give yourself eyestrain in the process. A long eye relief is also beneficial to anyone who wears glasses, enabling the eye to be further away from the EP lens.
It’s all well and good discussing the technicalities of the SPL but how does it actually perform, what can you expect to see? Of course this will depend on your telescope but I have tested the eyepiece on two common types of telescope, a small refractor (120mm) and a mid to large reflector (300mm), this will give you a basic idea of what to expect through your own scope.
Through the small refractor at a magnification of 333x the moon was simply breath taking, tiny craters pocked the lunar surface, my favourite crater “Tycho” located near the moons southern pole looked the best I’ve ever seen it, the tiny prominences in the craters centre were clear and defined. After the moon it was onto Saturn, the jewel of the solar system. The planet was so crisp and clear in the eyepiece, it looked like I could have reached out and touched it. The rings were razor sharp and the usually bland body of the planet was a cornucopia of detail, equatorial bands were clearly visible and it had a warm yellow colouring. In moments of excellent seeing I could even see banding towards the planets northerly pole, although subtle it was definitely there.
Lastly I gazed upon the Jovian giant Jupiter, the planet was a sea of swirling detail, four bands were clearly seen and the great red spot stuck out like a sore thumb.
On the big Reflector the moon was marvellous but the high 500x magnification meant craters and rilles whizzed through the field of view too quickly, in order for this EP to be useful on longer focal lengths you may need to invest in a motorised tracker for your telescope, this will follow objects across the sky stopping them flying out of your fov in 2 seconds flat.
When viewing Saturn and Jupiter I really noticed the atmospheric turbulence and as such for 90% of the time what I was looking at was a bland bright disk that appeared to be boiling. During the 10% of steady air the views were amazing but again a motorised drive would have been very useful as you’re constantly worrying about re-centering the planet and this does detract from the enjoyment of observing.
My advice is to use this EP in shorter focal lengthened telescopes, ideally a short refractor, you get a good high magnification but the wider field of views afforded by the shorter lengths mean the object will stay in the EP for longer cutting out any frustrations and leaving you simply with stunningly clear views.
