Two Celestron DX Series 56mm Binoculars: Which is Better for You?

ads-medium
Celestron’s binocular line-up presents a choice of two 56mm models: a classic 8x Porro and a 10x roof prism model. Which is best? Credit: Alan Dyer

While 50mm binoculars (reviewed in abundance here at AstroGearToday) are the preferred models for astronomy, buyers are often attracted to the greater aperture of a 56mm. A 56mm glass can gather about 25 percent more light than a 50mm and, in theory, reveal targets about 0.25 magnitude fainter, a small but perhaps noticeable difference. Despite their larger lenses and greater weight, a 56mm glass can still be handheld for convenient sky scanning. 

Celestron offers two 56mm binoculars with hand-holdable magnifications of 10x and lower (any higher and tripod mounting is a must). I tested Celestron’s SkyMaster DX 8×56 (that uses Porro prisms) and Nature DX 10×56 (that uses roof prisms).  

Classic Porro vs. Roof Prism

The older Porro prism design (named for 19th century Italian optician Ignazio Porro) is the style often preferred by astronomers because of its high light transmission. Roof prisms (at least of the Schmidt-Pechan variety used in most binoculars made today) have a reflective surface where light can be lost unless high-reflectance coatings are applied.

The Celestron 56mm Duo: The SkyMaster (left) is a classic Porro prism binocular with the familiar zig-zag N-shape. The Nature DX (right) is a more modern roof prism design with its characteristic straight-thru shape. Credit: Alan Dyer

Porros are also cheaper to manufacture. I find amateur astronomers, unlike birders and hunters, often don’t want to spend a lot of money on binoculars, saving their cash for telescopes. Both models on test here retail for about $220, a bargain price for a 56mm glass. (For a look at the other end of the price scale, see my review of the 10×56 Maven B4.) 

Despite their advantages, old-style Porros have largely disappeared from the product lines of many binocular manufacturers, in favor of more compact roof prism models. However, Porros remain available in low-cost binoculars aimed at beginners and in models for mariners and astronomers. 

Binoculars with 56mm aperture lenses are specialized for low-light use. The SkyMaster 8×56 also has the advantage of offering a full 7mm exit pupil (calculated by dividing the 56mm aperture by the 8x power). In theory, this provides the brightest image possible because the cone of light exiting the binocular eyepiece matches the diameter of the dilated pupils of dark-adapted eyes.

However, the eyes of older observers (over age 50) can’t open that wide; a 7mm exit pupil is wasted as the eye effectively “stops down” the aperture of the binocular. An exit pupil of about 5mm is a better choice, and that’s what the 10x56mm Nature DX provides (56mm/10 = 5.6mm).

The 8×56 SkyMaster, as do most Porro prism binoculars, uses an external focusing mechanism that racks the eyepiece bridge back and forth. Credit: Alan Dyer

In addition, the Nature DX’s 10 power provides better resolution of star clusters and small galaxies. On the other hand, any 8x binocular is easier to hold steady. So, choosing between these two models involves more than just picking Porro vs. roof prism. 

Optical Comparison

The 8×56 SkyMaster DX has long been a popular model for astronomy. And rightly so. Images are sharp almost to the edge of the field. At a very generous 20mm, eye relief is excellent, making them comfortable to look through, either with or without eyeglasses. The same is true of the Nature DX with their 18mm of eye relief. 

The SkyMaster’s main drawback is the narrow apparent field of view of its eyepieces, just 46°. While this is what we were used to in binoculars years ago, most binoculars today of all designs offer a wider 60° to 65° apparent field. As it is, looking through the SkyMaster provides a tunnel-like impression. 

With their 8x magnification, they provide an actual field of view of 5.7°,  i.e., that’s how much sky they actually show. By comparison, a good 8×42 (see my review here) with 65° eyepieces can provide a field as wide as 8°.

The 10×56 Nature DX binoculars have eyepieces with a 60° apparent field for a more immersive viewing experience. Their actual field of view is 6°, slightly wider than the SkyMaster, despite the Nature DX’s higher 10 power.

The 10×56 Nature DX, as do all roof prism binoculars, uses an internal focusing mechanism that is more solid, and enables better weather sealing against dust and moisture. Credit: Alan Dyer

The downside is that stars are quite soft in the outer 40 percent of the field, bloating considerably with severe aberrations at the very edge of the field. However, both binoculars appeared equally sharp on-axis in the center of the field.

The objective lenses in the Nature DX do not use low-dispersion ED glass, leading to some false color fringing on bright stars and the limb of the Moon. But it is not objectionable. They do offer phase-corrected coatings on the prisms for better contrast, but not the high-reflectance dielectric coatings offered on premium roof prism binoculars. 

And yet, in practice with my aging eyes I saw little difference in image brightness between the two binoculars. Indeed, faint nebulosity like the Rosette Nebula was a bit more apparent in the 10x56s. The Porro SkyMaster DX did not appear brighter than the roof prism Nature DX; I did not see the “Porro advantage” in light transmission. 

However, observers more youthful than I might benefit from the wider exit pupils of the SkyMaster.

Mechanical Comparison

The eyepiece bridge in the SkyMaster was solid with no rocking or looseness that can cause focus differences between the two eyepieces. However, I found the Nature DX’s focusing wheel easier to reach, especially with gloves on, than with the wider body of SkyMaster. But I know some users prefer the wider grip of Porros. 

The prisms in the SkyMasters are glued in place; in the Nature DX they are held in cages for a more durable construction likely to maintain collimation better. Both are nitrogen filled and are said to be waterproof, a claim I did not test! 

Both models offer twist-up eyecups, making it easy to click them down for eyeglass use or click them up for “naked eye” viewing. There’s no fiddling with rolling eyecups up and down, only to have them pop off or split as they age. 

I did find that the focus travel of the SkyMaster was more limited, and just reached infinity focus with my myopic (near-sighted) eyes with glasses off. If you are very near-sighted you might need to use your glasses at all times with the SkyMaster. 

Both binoculars weigh about 1050 grams (37 oz), heavier than most 10x50s, an inevitable consequence of their larger aperture. Both have threaded 1/4-20 tripod sockets. The Nature DX will require a narrow tripod-mounting bracket to fit between their closely set barrels. 

Because both models sell for $220, the choice is not one of cost. I think the toss-up is between the higher power and wider but softer (at the edges) field of the Nature DX vs. the narrower but sharper field of the SkyMaster DX, and wider exit pupil for those who can take advantage of it. 

Plus: 8x SkyMaster DX has images sharp to the edge; 10x Nature DX has a wider apparent field of view.

Minus: 8x SkyMaster DX has a narrow field of view; 10x Nature DX has stars soft in the outer area of the field.

Celestron 8×56 SkyMaster DX 

Typical retail price at U.S. dealers: $220

Celestron 10×56 Nature DX 

Typical retail price at U.S. dealers: $220

Both available from Celestron dealers worldwide

 

 

About Alan Dyer

Alan Dyer is an astrophotographer and astronomy author based in Alberta, Canada. His website at www.amazingsky.com has galleries of his images, plus links to his product review blog posts, video tutorials, and ebooks on astrophotography.

Related posts