Explore Scientific 9mm 120° eyepiece
A medium-power eyepiece with an astounding field of view, the quality of the field is excellent on axis and for a large portion of the main field of view
This 9mm, 120° eyepiece is the latest in Explore Scientific’s (ES’s) line of wide-field eyepieces. The purpose of a telescope eyepiece is to magnify the image produced by the main telescope optics. The overall magnification is easily calculated by dividing the telescope’s focal length by that of the eyepiece. For example, a telescope with a 1,000mm focal length used with a 10mm eyepiece gives 100x magnification, or power.
It’s a good idea to have at least one low-, medium- and high-power eyepiece in your collection. As a general rule, a low-power eyepiece will give a bright, wide field of view, while higher powers show reduced contrast and narrower views. For this reason, higher-power eyepieces tend to be less comfortable to use than lower-power ones.
This 9mm eyepiece is ostensibly medium-power, but rather than having the typical restrictive view it offers an immense 120° apparent field of view. The result is an eyepiece that delivers more of an observing experience than a simple vista.
It’s important to understand what is meant when referring to a field of view. When you look though an eyepiece, the area of sky you can see depends on the nature of the scope being used. This is the true field of view and gets smaller as a telescope’s focal length increases.
The apparent field of view describes how large the view looks to your eye. In the case of the ES 9mm eyepiece on test, this measures a whopping 120°. Get your eye in the optimal position, 12.5mm from the lens surface, and you can let your gaze roam in all directions around the view. In fact, so wide is this apparent view that you can even look sideways into the eyepiece to take in the extreme edge of the field. The true field of view, or sky area you’re looking at, can be calculated by dividing the apparent field of view by the magnification.
A wealth of sky
The first target we observed with this eyepiece was the fabulous Ring Nebula, M57 in Lyra. We’ve seen this object many times before using a 4-inch (918mm focal length) refractor, but this eyepiece really brought it to life. With a magnification of 102x (918mm/9mm), the nebula had a decent size and was quite obvious. But it was the extent of the star field surrounding it that really set the scene, with over
1º of sky to wander your gaze around.
M13, the Great Globular Cluster in Hercules, was the next test target. With a large apparent diameter, it looked amazing through the eyepiece. The field was wide enough to show the globular and the two mag. +7.0 stars that flank it in one view. These are 30 arcminutes apart.
Many cluster stars can be sharply resolved by carefully focusing the eyepiece. The quality of the field is excellent on axis and for a large portion of the main field of view. Looking at the Double-Double, Epsilon Lyrae, we had no problem resolving both close pairs of stars at 102x. The resolution and contrast of the eyepiece is fantastic. Some chromatic aberrations did creep towards the extreme edges, but this didn’t detract much from the overall experience.
It’s all about the experience
The final observing target was a last quarter Moon. Again, the view was spectacular and it was amazing to be able to see the whole Moon at a reasonable magnification. It’s the sort of view that will have you staying at the eyepiece for a long time to take everything in.
One point to consider is that the eyepiece is quite large and heavy, weighing in at 0.6kg – you can see how big it is compared to an average hand at the top of this page. This has the potential to cause balance issues, especially if you’re swapping between eyepieces. Despite that, the ES 9mm, 120° eyepiece is a great piece of kit to add to your telescope. It’s expensive, but the wonderfully wide and comfortable views it delivers will no doubt mean it ends up sitting in your scope’s focuser for long periods, allowing you to really experience your telescope rather than just look through it.
Quite a sight
The outstanding views this eyepiece delivers mark it out from the crowd. Constructed from 12 lens elements arranged in eight groups, the eyepiece offers such a wide apparent field of view that your eye can really relax into it – it’s a pleasure to use. In addition, the medium-power magnification granted by its 9mm focal length still allows you to get in close to your target.
When using it to observe the vast Andromeda Galaxy, M31, through a 4-inch refractor, the view feels like one you’d get using a low-power eyepiece with a much larger scope. One hidden advantage of being able to look around the field of view is that it’s also easier to use the averted vision technique, looking to
the side of an object to place its delicate light on a more sensitive part of your retina.
Each lens element is multicoated with blackened edges. This results in an exquisite high-contrast view
that really is a joy. It’s truly an amazing experience using this eyepiece.
Moisture countermeasures - The 12 lens elements are housed in a waterproof, fogproof, argon-purged and sealed body. This means that there’s no way for moisture to get inside the eyepiece – whether it’s from natural sources or excessive cleaning. This is a very welcome feature, especially when using the eyepiece outside on cold, damp UK nights.
Filter thread - As you’d expect from a premium eyepiece, the barrel is threaded for 2-inch screw-in filters. This means you can attach your favourite filter and continue viewing with the same impressive field of view. Such filters would typically offer enhanced ultra high contrast or even narrowband views of objects such as nebulae.
Rubber eyecup - The rubber eyecup creates a secure seal around your eye to prevent light seeping in and interfering with your view. It also prevents spectacles from touching the hard body of the eyepiece, which could cause scratches. Placing your eye against the rubber cup with a gentle pressure also positions it for the perfect view.
Tapered 2-inch barrel - The double-tapered barrel has two purposes. The first section is designed for easy insertion into a telescope’s eyepiece holder. The second taper, closest to the eyepiece, is precautionary. If the setscrew holding the eyepiece in place becomes loose, this should prevent the eyepiece slipping out and falling to the floor.
Finish - The build quality of the eyepiece is excellent and its body is nicely rounded off with a textured rubberised section around the middle. This section increases grip in cold weather but is subtle enough to allow the eyepiece to keep its attractive, sleek appearance.
This review first appeared in the November 2013 issue of BBC Sky at Night Magazine. Prices correct at time of going to press.