The viewing provisions of an eyepiece are expressed in two ways. Firstly, the circle of light that is presented if you hold an eyepiece up to the eye is called the apparent field. This is a function of eyepiece design.
The second is what is called the true field. This is the area of actual sky that is presented to the viewer when the eyepiece is used in a telescope. This true field is easily calculable by dividing the apparent field by the resulting magnification that it produces at the telescope.
As you increase the apparent field of an eyepiece, so the window on the real sky also increases, which is why those eyepieces of very large apparent fields are becoming increasingly popular – including the range of which this reviewed eyepiece is a new member.
APM’s line-up of extreme-wide-angle eyepieces (sometimes called hyperwide) is impressive, but until recently a teammate has been missing. Following delayed production and eager anticipation (by not a few, myself included!) the HDC XWA 7mm, 100-degree X-treme has finally made its debut. I snapped it up as soon as the first tranche of deliveries became available and was keen to discover how well it complemented the other focal lengths in the range.
The missing link
Offering apparent fields of 100 to 110 degrees, APM’s line-up of extremely wide-field eyepieces has hitherto comprised focal lengths of 3.5, 5, 9, 13 and 20mm. At first glance, this range might seem comprehensive, but as the focal lengths of eyepieces become shorter, the difference in the magnifications between them becomes proportionately larger.
Consider a telescope of 2,000mm focal length. While a 20mm eyepiece will provide a magnification of 100×, the 13mm will yield 154×, providing a step up of just 54×. Nearer the highermagnification eyepieces, the 9mm gives 222×, yet the 5mm increases this to 400× –a difference now of 178× (three times the lower magnifications gap). Such a divide may, by prevailing seeing conditions, span the space between a power that could be pushed a little more, and the next that the seeing conditions would not support.
So, the introduction of this 7mm eyepiece has filled a sorely needed vacancy in the APM XWA range and, so far as my own needs are concerned, meets a critical optimum position where further magnification is normally thwarted by average seeing conditions.
Like other eyepieces in this range, this is not a small eyepiece. With a length of 157mm and girth of 58mm, the nine-element eyepiece weighs in at 414g. Yet it is far lighter than some of the largest I have seen, but with sufficient mass to perhaps require compensating adjustment of a small telescope on its mount.
The barrel (which has a conical design to accommodate binoviewer use) is kitted out with a rubberised grip and a fold-down eye guard that shields its 13mm eye relief. This, plus an eye lens fully 30mm in diameter, means that despite the eyepiece’s relatively short focal length, there is no eyeball-grazing ‘pinhole’ discomfort here. As a bonus, the rubber eye guard is removable to expose an M44.5 × 0.75 male thread for cameras.
Despite its size, the eyepiece sports a 1.25-inch fitting, but also includes an adaptor for deployment on a two-inch focuser – with both nosepieces threaded for filters. As befits an eyepiece containing nine glass elements (in six groups), all of the optics are edge-blackened and fully multicoated.
The focal length designation is clearly inscribed, but (and I make a general heartfelt criticism of all eyepiece manufacturers here) not particularly visible in the dark, which by virtue of its purpose is when you need it. Why are eyepiece focal lengths not luminous, fluorescent, signified by raised dots, speak their focal lengths when handled or whatever? Either the manufacturers are missing something obvious or I am. If someone takes up one of these ideas, please don’t forget my commission!
This is a beautifully constructed eyepiece, which hits the sweet spot of yielding a respectably high magnification without pushing further than the conditions would normally permit
An astronomical guinea pig
For the purposes of this review, I used two telescopes. The first is my mainstay 356mm, f/10 Meade ACF. But used at f/10, most eyepieces should be unchallenged and perform well, so I accompanied my assessment with a small, rich-field 114mm, f/4 Newtonian, which is entirely unfair in the opposite direction.
Preferring to assess equipment on objects that I am familiar with (deviations from the norm are more easily detected), I chose the Andromeda Galaxy (M31) as my initial guinea pig. I began my review using the small Newtonian. The irony that there was probably more glass inside the eyepiece than in the telescope was not lost on me. This telescope has only a 1.25-inch focuser, but this was no problem for the eyepiece as it took only a moment to unscrew its two-inch adaptor.
The view was vast. On a telescope of only 450mm focal length, the resulting magnification for a 7mm eyepiece is already just 64×. But with an apparent field of 100 degrees, the true field delivered by this eyepiece was 1.6 degrees, which is able to encompass more than three Moons edge to edge!
The bright core of the galaxy was obvious, with the fainter outer ellipse well seen too. As with all hyper-wide-angle eyepieces, what was also delivered was context. The large field allows surrounding stars to be included with the object under observation, and it is this aspect that gives such eyepieces their wow power. To see this classic galaxy together with a sprinkling of ‘foreground’ stars gave this commonplace target added depth.
Transferring the eyepiece to the larger instrument, the Meade’s 3.5 metres of focal length resulted in a magnification of 508×. This is at the higher end of this telescope’s usability under normal conditions. Yet with this eyepiece, it nevertheless still delivered a true field of 12 arcminutes. The larger aperture, of course, delivered a core of Andromeda that was brighter, but what impressed me most was the impact on light pollution that a wide-field eyepiece achieves.
Remember that it has always been a useful ploy to increase magnification in order to enhance contrast – the background light pollution is spread more thinly and consequently darkened. But doing so with conventional eyepieces also reduces the size of the true field. With the 7mm, 100-degree eyepiece, I was able to enjoy its enhanced contrast of a darkened sky without the impact of a reduced field size. My view of Andromeda demonstrated this principle astoundingly well.
Indeed, I subsequently found that (when using the small Newtonian), I could see more of the Andromeda Galaxy in the 7mm than I could through a 40mm eyepiece!
Beating the drift of death
Of course, the undisputed master of our latesummer sky this year was Jupiter, having come to opposition in mid-August. The planet’s presence offered a number of teases for this new eyepiece.
Beginning with the small Newtonian, I carefully centred Jupiter and found that even at only 64×, quite a lot of detail in Jupiter’s belts could be discerned. To the east of its disc, all four Galilean moons were strung out like beads on a necklace – and in their orbital sequence. This pleasing line-up, 8.5 arcminutes in extent, could be enjoyed in its majesty because they swam in a field about ten times as large. Again, context of the system’s surroundings enhanced the view. And, of course, this largess means that if used on a telescope that has no sidereal drive, a target can be kept in view with minimal effort.
I tracked the planet slowly towards the periphery of the field, in what I like to term the ‘drift of death’, to discover how near to the edge I could get the planet before its image quality started to stress.
Commendably, the image remained tack sharp until Jupiter was within about 15 per cent of the field stop. Considering the demanding f/4 focal ratio, without use of a coma corrector, I was impressed – and it performed significantly better than similar eyepieces on the same test. I also noted that there was no ghosting or internal reflection, despite placing Jupiter just within and just outside of the field.
Staying with the Jovian system, I then deployed the 7mm eyepiece on the 356mm, f/10 Meade. I was not expecting too much as the seeing conditions were far from superior. In fact, what the weather agencies these days choose to call clear nights are increasingly blighted by a high haze that fuels light pollution. Yet the thinning of background sky illumination that the eyepiece achieves provided excellent contrast that yielded Jovian detail that would otherwise have been washed out.
There were colours, but not as a result of chromatic aberration, which is entirely absent in this eyepiece. Detail in Jupiter’s belts was nicely depicted in ruddy-browns and blue-greys. Very pleasing.
Even at 508×, the true field of 12 arcseconds that the eyepiece delivered meant that despite the relatively high magnification, I could still encompass both planet and Galilean moons with room to spare. It was as if I were hitching a ride on a Voyager spacecraft while approaching on a fly-by.
Again, I walked Jupiter through the drift of death. The planet sailed through the barrier of the field stop without a backward glance – as clear and undistorted as it was when dead centre. There was not even a glimmer of chromatic aberration as it passed through. Of course, in the 356mm, the Galilean moons were clearly discs – and that is the way they remained as they left the field, one after the other, in Jupiter’s wake.
The devil in me slewed the 356mm to repeat this test on nearby Saturn, lower and to the west of Jupiter. The reduced altitude was fairly detrimental to the quality of the image, but a finely depicted Cassini Division offered itself as detail to be scrutinised. Its curving pencil line barely flinched as I forced it through the edge of the eyepiece field.
This is a beautifully constructed eyepiece, which for me hits the sweet spot of yielding a respectably high magnification (on my 356mm) without pushing further than the conditions would normally permit. Glare and internal reflection countermeasures, together with its expansive field, result in great quality, highcontrast viewing. Pulling targets closer with enhanced contrast is a job this eyepiece excels at. I have enjoyed my time with the eyepiece during this review and fully expect that to continue.
Unfortunately, the Moon was decidedly shy during the review period, being around when it was cloudy and not when it was clear. However, my calculations indicate that used on the 356mm, I should be able to view the entire Imbrium basin area, encompassing the lunar Apennines, Copernicus, Aristarchus and Plato, all at 508×. I can’t wait.
At a glance
Focal length: 7mm
Apparent field: 100 degrees
Length, diameter: 157mm, 58mm
Optical construction: 9 elements in six groups – all FMC Waterproof to IPX5 standard
Price: €193.28 excluding VAT (teleskop-express.de)
Price: £230 including VAT (astroshop.eu)
Price: €269 including VAT (apm-telescopes.de)