Celestron SkyMaster 25 × 100 binoculars

The Celestron SkyMaster 25 × 100 binoculars will be one of the largest binoculars that you will have used for astronomy, with 99mm apertures and weighing just shy of four kilograms. Image: Celestron.

As any experienced comet hunter, asteroid chaser, supernova spotter, variable-star observer or satellite tracker will tell you, a quality pair of large-aperture binoculars may be all you need for these fields of astronomical observation. Indeed, binoculars will always trump a telescope when a wide, bright field of view at a relatively low magnification is required.

Why both eyes are best

Nature endowed us with binocular vision not merely out of redundancy should we lose sight in one eye, but because a stereoscopic view of the world gives us far more than a sense of depth perception. The brain analyses the two visual channels in very subtle ways, using neural summation to reduce random noise and exploit the best characteristics of each eye, all of which leads to a heightened level of perception.

The fit and finish of the SkyMaster 25 × 100 is excellent for a binocular in its price bracket

Compared to a telescope of the same aperture and magnification, a pair of binoculars enables you to detect stars up to 40 per cent fainter, and possibly more for extended sources such as nebulae. Colour perception is also enhanced by binocular vision, making subtle star hues stand out more. Simply put, everything appears brighter, more colourful and more nuanced when using binoculars.

For most observers, 7 × 50 (the first number is the magnification, the second is the size of the front lenses in millimetres) or 10 × 50 binoculars may be all that we aspire to for casual astronomical viewing, but the sheer comfort and visual enhancement associated with using both eyes simultaneously could get some of you smitten with a larger binocular as your primary instrument.

While it’s easy to become fixated on acquiring large-aperture binoculars, do be mindful of the practicalities of their use. A 10 × 50 instrument is about the limit of what most people can use handheld without image shake, so any binocular of higher magnification or larger aperture should be mounted for prolonged viewing sessions.

And don’t think that you must have an expensive instrument employing ED (Extra-low Dispersion) glass, since a well-engineered achromatic model can turn in a good performance if it uses prisms made of BAK-4 glass and its optical surfaces are multi-coated to reduce light loss. An attractive specification for its price led me to the 25 × 100 instrument appraised here, kindly provided for review by Wex Photo Video.

The SkyMaster binoculars come with a black vinyl case measuring 44 × 27 × 15 centimetres, and a robust, adjustable shoulder strap. Image: Celestron.

Introducing the SkyMaster range

Wex carries six binoculars from Celestron’s Chinese-made SkyMaster range, ranging in aperture from 70mm to 100mm and magnifications of 15× to 25×. All of the instruments are of the Porro design with BAK-4 prisms and multi-coated optics, but there are two Pro models – 15 × 70 and 20 × 80 – that employ Celestron’s enhanced XLT coatings in addition to being waterproof, nitrogen-purged and rubber armoured for extra protection. For the purposes of this review we’ll take a look at the largest model of the range, the SkyMaster 25 × 100.

Once you dispense with the cardboard shipping boxes, the eye-catching soft carry case bodes well for what lies within. Made of a black vinyl with quality stitching and a robust adjustable webbing shoulder strap mounted on the sides, the outer case measures 44 × 27 × 15 centimetres. It features a zipped 25 × 14-centimetre front pocket and a Velcro-fastened rain-guard sporting the Celestron logo overlying the case’s main zip. If you don’t feel secure carrying its 5.1-kilogram mass over one shoulder, the rear of the case has three triangular attaching points for wearing it securely on your back like a rucksack.

Upon unzipping the outer case, you will find a detachable padded liner that lies within. While the liner performs its intended function of protecting the instrument, it’s not easy to extract the binocular and manoeuvre it back into the case after use, particularly in the dark. Also in the case you will find a microfibre cleaning cloth, a small multilingual user’s guide and a neck strap. The last made me smile, since the SkyMaster 25 × 100 tips the scales a few grams short of four kilograms and only those sporting the physique of a young Arnold Schwarzenegger would carry it this way!

The SkyMaster 25 × 100’s objective lenses have a clear aperture of 99mm, and appear to be multi-coated with a characteristic green tint. Image: Ade Ashford.
The integral tripod adaptor has 16 centimetres of travel on the central mounting bar and lies just forward of the mid-point when used terrestrially in a largely horizontal configuration. Image: Ade Ashford.

First impressions

The fit and finish of the SkyMaster 25 × 100 is excellent for a binocular in its price bracket. The instrument exudes an air of quality that certainly stands up to close inspection. The 30-centimetrelong central mounting bar and robust hinges ensure that the lengthy objective-lens assemblies maintain collimation and provide a means to alter the inter-pupillary distance. While the Porro prism body has a textured rubber finish, the centre of mass lies about 15 centimetres behind the objective-lens cells.

The two 11.5-centimetre-internal-diameter rubberised objective caps fit snugly over the front lenses, and a combination cover protects the eyepieces. The outer elements of the objectives are recessed by about 24mm, which affords them some protection, while their cells provide a clear aperture of 99mm. Close scrutiny of the optical train as seen from the front of the binocular under a strong inspection light reveals no defects or blemishes to the anti-reflection coatings throughout, or any internal dust whatsoever. While the internal prisms and eyepieces do appear to be anti-reflection coated throughout, likely with magnesium fluoride (MgF2 ), it is only the objective lenses that appear to be multi-coated, with a characteristic green tint. The instrument was also in perfect collimation.

I found that two-thirds of the binocular’s central field of view could be considered sharp and aberration-free, with minor degradation towards the remaining field edge

Binocular operation

Unlike smaller, centre-focus binoculars, the SkyMaster 25 × 100 has individual eyepiece focusing. While this is of no consequence for astronomical use, they don’t lend themselves to continual adjustment from near to far for daytime use. The chunky knurled grips on the barrel of each ocular does mean that it is feasible to alter their focus while wearing gloves. The motion is positive with enough friction not to accidentally move them from the optimal position. While Celestron’s specification states that the SkyMaster 25 × 100’s interpupilliary distance is adjustable between 56 and 72mm, the review instrument’s measured range was a generous 56 to 75mm.

Regarding the eyepieces themselves, I found that the 10mm-high rubber eyecups provided adequate shielding from extraneous light to use the binocular comfortably, even in daylight. However, the rear lens elements of the eyepieces are recessed by just 2mm, so to make the most of the stated 15mm eye relief for spectacle wearers such as myself, the eyecups are required to be turned down. Both eyepieces appear to offer a sixdioptre adjustment according to the scales, but in reality there is a wider range than this.

The SkyMaster 25 × 100 has an integral tripod adaptor so that the instrument can be used hands-free. The 11.5-centimetre-high adaptor has 16 centimetres of travel along the central mounting bar, and once you’ve achieved balance it can then be secured by a robust, knurled lock knob. The foot of the adaptor has a textured rubber pad to prevent movement on your mounting block, and a standard ¼-20 tripod thread to ensure a firm purchase. The binocular is largely horizontal when used terrestrially and the balance point lies almost midway along the central mounting bar, as one would expect.

Your target can potentially lie overhead for astronomical observation, so a parallelogram or fork binocular mount, and a reclining chair, is the best way to avoid a cricked neck. However, if your subject has an elevation of 60 degrees or less, then a tall, sturdy photographic tripod with a robust ball head will suffice. You will need to position the integral tripod adaptor as close to the rear of the central mounting bar as you can for high-altitude targets, but it works, and this was how I used the binocular for tests on the night sky.

To make the most of the stated 15mm eye relief and see the full three-degree field of view, spectacle wearers will need to turn down the 10mm-high rubber eyecups (right). Image: Ade Ashford.

Optical tests: terrestrial

Celestron’s specification gives a close focus distance of 24 metres, which immediately informs you that the SkyMaster 25 × 100 is not suitable for birdwatching in a typical suburban back garden and comes into its own for subjects 40 metres or farther away. I’m fortunate to have a number of birds of prey and waterfowl living around the nearby water meadows and I viewed them in stunning detail through this instrument.

The field edge is well defined, and close inspection of brickwork on the wall of a distant house revealed minimal amounts of pincushion distortion. Intense specular reflections of sunlight from curved mirrored surfaces are always a good test of an instrument’s suppression of chromatic aberration, and the 25 × 100s performed well. I was impressed with the SkyMaster’s contrasty views on a wide range of terrestrial subjects commensurate with a binocular of this aperture, magnification and price bracket.

The SkyMaster has individual-eyepiece focusing over a six-dioptre range on the dial, though in practice it’s much larger. Image: Ade Ashford.

Optical tests: celestial

The most stringent test object is a bright star or the Moon, particularly when viewed off-axis. The SkyMaster 25 × 100 delivers a 4mm exit pupil, so your eye can accommodate the full light-cone emerging from the instrument in twilight to dark skies, irrespective of your age (your dilated pupil size gets smaller as you get older). In tests conducted during March and April 2021, I found that two-thirds of the binocular’s central field of view could be considered sharp and aberration free, with minor degradation towards the remaining field edge.

The SkyMaster has an integral tripod mount with an 11.5 centimetre-high adaptor that has a knurled lock-knob and a ¼-20 thread to ensure a firm purchase on your tripod head. Image: Ade Ashford.

The SkyMaster 25 × 100 proves that a well-designed achromatic binocular can deliver an attractive opticalperformance-to-cost ratio

The early evening of 23 March 2021 produced some of the steadiest seeing conditions I can recall in the past two years, so I divided my time observing the nine-day-old waxing gibbous Moon between my 120ED refractor and the SkyMaster 25 × 100. The binoculars revealed the Sinus Iridum bisected by shadow with the ‘Moon Maiden’ (Promontorium Heraclides) jutting out from the lunar night like an island, with fine wrinkle ridges over Mare Imbrium and crisp detail in the Riphaeus Mountains.

Impact craters Copernicus and Tycho displayed extensive ray systems, and the southern highlands provided crisp, eye-catching distractions. Despite my initial misgivings about the instrument not possessing fully multi-coated optics, there was minimal glare within the optical system when the Moon lay just outside the field of view. Some chromatic aberration was noticeable on the Moon’s limb when positioned near the field edge, but hardly perceptible in the centre of the field of view. The SkyMaster 25 × 100 showed its deep-sky prowess once astronomical twilight faded to dark on 1 May, when I chose to view from my favourite Bortle 3 (bordering on Bortle 2 when the transparency is good) rural dark-sky retreat in north Norfolk before moonrise. Having dark adapted for half an hour, I turned the SkyMaster 25 × 100 towards the Leo Triplet of galaxies (M65, M66 and NGC 3628). I was surprised at how well the three galaxies stood out from the residual skyglow, and their morphologies were clearly different. The same was true of Ursa Major’s prominent galaxies M81 and M82, which were obviously different in size, texture and shape.

The SkyMaster 25 × 100 delivers exit pupils of 4mm in diameter, so your eyes can accommodate the full light cone. Image: Ade Ashford.

Arcturus did appear to have a weak halo, which I attributed to very slight light scatter within the instrument arising from the lack of full multicoatings, but the colour of the northern celestial hemisphere’s brightest star was rendered in faithful hues without false rays or flares. Globular clusters seen from a dark-sky site are glorious in this pair of binoculars too; M13, the Great Hercules Cluster, was larger and brighter than its globular cluster compatriot M3, almost two spans of an outstretched hand away in Canes Venatici. I realise that a 25× instrument is not going to resolve either globular, but it felt like they were almost granular in texture – perhaps it was just a superlative night!


The SkyMaster 25 × 100 proves that a welldesigned achromatic binocular can deliver an attractive optical-performance-to-cost ratio. To experience a three-degree field at 25× from 100mm objectives will be worth the cost of admission for many observers. However, don’t lose sight of the fact that this 41 × 25 × 14 centimetre binocular tips the scales at just under four kilograms, so you will need to factor in the price of a suitable mount. My Manfrotto 055 tripod and its heavy-duty ball head were certainly up to the task for casual viewing, but a costlier parallelogram mount would make prolonged observing more comfortable and productive.

At a glance

Specification: Binocular type: Porro

Prism type: BAK-4 glass

Anti-reflection coatings: Multi-coated

Focusing: Individual eyepiece

Clear aperture: 99mm (measured)

Magnification: 25×

Field of view: 3 degrees

Exit pupil: 4mm

Eye relief: 15mm

Interpupilliary distance: 56–75mm (measured)

Dioptre range: 6 (±3)

Close focus: 24 metres

Special features: Integral tripod adaptor with ¼-20 thread

Included items: Lens caps, neck strap, lens cloth, instructions

Weight: 3.99kg net (5.1kg with case and accessories)

Price: £319

Website: wexphotovideo.com

Ade Ashford has travelled the globe writing about astronomy and telescopes, serving on the staff of astronomy magazines on both sides of the Atlantic.