Explore Scientific ED127 apo refractor

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Magazine Verdict: 
Explore Scientific ED127 apo refractor

Vital Stats

127mm (5 inches)
Focal Length: 
952mm (f7.5)
Telescope House
01342 837610
A lightweight and portable 5-inch refractor with great optics at a sensible price, the ED127’s air-spaced triplet is ideally equipped for bringing the best out of faint deep-sky objects

Refractors such as the Explore Scientific ED127 apochromat make great all-round instruments, suitable for both visual observing and astrophotography. Not all refractors are the same though, and it’s the quality and design of the main objective lens that is key to their success or failure.

A lens works by refracting (bending) the light that passes through it to produce a focused image of a distant object. Single lenses suffer from aberrations: they focus different wavelengths of light at slightly different positions – blue focuses slightly closer to the lens than red.

A doublet uses two lenses to minimise aberrations and brings red and blue light to the same focus. A triplet uses three, bringing red, green and blue to the same focus to eliminate colour fringing. The term apochromat, or apo, is used to describe a lens that’s designed to bring these three wavelengths to the same focus.

The ED127’s air-spaced triplet is ideally equipped for bringing the best out of faint deep-sky objects and for teasing subtle details out of the planets and the Moon.

Keep it light

As refractors get large they can become heavy and expensive, but despite the generous 5-inch objective found here, the fact that the tube is made from carbon fibre helps to keep the weight down. The result is an instrument that’s extremely manageable and should work reasonably well on mid-range mounts. The lack of serious weight also means that it is an easy telescope to transport.

The scope has a focal length of 952mm, giving it a focal ratio of f/7.5. In terms of suitability for deep-sky or Solar System objects, the ED127 is pretty good for both. If you’re looking for a starter instrument but aren’t sure what you want to look at yet, this one is certainly worth considering.

A good test star for small scopes is Epsilon Lyrae, the Double Double. Initially appearing as a fairly easy wide double, good optics and high magnification should reveal that each component is actually a very tight double in its own right. The tighter doubles are separated by 2.6 and 2.3 arcseconds.

The supplied 25mm eyepiece, offering 38x magnification, showed both pairs as single stars only. Using our own high-quality 5mm eyepiece increased the magnification to 190x, enough to show the pairs as two bright discs, each surrounded by a faint diffraction ring – an almost perfect rendition of what a telescopic star should look like at high magnification.

The low brightness of the overlapping rings gave the impression of a dark separating space between the stars. In a centrally obstructed scope such as a reflector or a Schmidt-Cassegrain, more energy goes into the first diffraction ring, making it brighter and bringing less distinction to the separating space.

The colour rendition of the ED127 is excellent too – the beautiful yellow and blue tones of the double star Albireo in Cygnus looked vibrant. Stars were delivered crisp and sharp, helping to bring open clusters such as the Wild Duck Cluster, M11 in Scutum, alive. The Ring Nebula, M57 in Lyra, appeared with a definite central dark ‘hole’ at 38x magnification.

Focal points

While focusing, we found the 10:1 dual focuser was adequate for visual use but less impressive photographically. With a reasonably heavy camera attached and the telescope at high angles, fine focus movement suffered and we had to tighten the locks to prevent slippage. Imaging tests showed good consistency across a non-full frame DSLR sensor at prime focus, with only minor star shape changes in the extreme frame corners.

The ED127 also proved its worth as a great, medium-resolution planetary imaging instrument and delivered some finely detailed shots of the Moon when we paired it up with our high frame rate camera. To test colour correction, we imaged the lunar surface at prime focus through a red, green and blue filter without re-focusing between filters. The result was very impressive, and combining individually filtered results didn’t introduce any noticeable colour fringing.

Generally we were very impressed by this instrument, although it’s let down slightly by the focuser and the supplied 25mm eyepiece, which was fairly average. However, it has great light grasp, excellent optics for the money and is capable of delivering some stunning results.

Triplet objective

The headline act for this 5-inch apo refractor is the triplet objective, which is made from low-dispersion Hoya FDC-1 glass. For lens glass aficionados, FDC-1 has similar properties to FPL-51, a highly regarded alternative to the optical excellence of fluorite glass. Allowed to cool, the triplet really comes into its own and functions beautifully, producing excellent colour-corrected and high-contrast images. The slight let-down is that the supplied 25mm wide-angle eyepiece introduces some colour fringing of its own, visible at the extremes of field.

As well as being delighted with the crisp, colour-rich stars the objective delivered, we liked the scope’s performance for high frame rate Solar System imaging. The objective lens delivered a decent resolution for picking out fine lunar and solar detail.

The objective cell has three hex key-adjusted collimation points around the main lens housing. This allows the scope to be fine-tuned for optimum results. Unlike a reflector or Schmidt-Cassegrain, once collimated properly it will rarely need to be adjusted again.

8x50 finder - The 8x50 right-angled finder has six positional adjusters – three on each ring. Focus can be adjusted by turning a rubber-coated ring close to the eyepiece. A red LED illuminator screws into the side of the eyepiece unit to provide an illuminated sight reticule across the field of view.

Carbon-fibre tube - The use of carbon fibre as the main material for the tube results in a telescope that is light and also has excellent thermal qualities. A metal tube has the tendency to expand and contract as temperature varies through the night, and focus may change as a result. This effect is minimised by using carbon fibre.

Focuser - The supplied dual-speed (10:1) Crayford-style focuser works well for visual focusing, but struggles a bit with a heavy camera attached and the telescope at a high angle. Three tensioner/locking thumbscrews are provided. The whole focuser can be rotated by unscrewing two of these.

Tube rings - Tube rings clamp around the telescope, providing a means to connect to a mount. These ones have a Vixen-style dovetail plate fitted. The ring clamps are flip-clipped, which makes adjusting the tube position for balancing very simple, and the rings themselves have a handle to assist portability.

Flight case - The Explore Scientific ED127 offers a good balance between instrument size and portability – it can be lifted on and off a mount with one hand. To emphasise the portability and to provide extra protection, Explore Scientific also includes a robust flight case packed with high-density foam. 

This review originally appeared in the September 2013 issue of BBC Sky at Night Magazine. Prices correct at time of going to press.

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