Whether a stargazing novice or an experienced amateur astronomer, you’ve probably been tempted to buy a monocular. As photographers move to say, the best camera is the one you have with you, and that also applies to astronomy. Throwing a monocular in a backpack will always be easier than carefully packing a pair of the best binoculars or one of the best telescopes.
A monocular weighs half that of a pair of binoculars and a fraction of a telescope. Can a monocular handle the night sky and show you its treasures as easily as a pair of binoculars or a telescope? What features should you look for in a monocular for stargazing?
Here’s everything you need to know about what to look for in a monocular for stargazing so you can choose the right optical instrument for you.
Optical materials
Monoculars are nothing without quality optics, but knowing what’s important and what’s not on each product’s specifications sheet can be tricky. A major issue with monoculars, particularly more affordable models, is blurry around the edges of the field of view.
You can get close to avoiding that by choosing high-density BaK-4 glass (barium crown glass) prisms over the cheaper, blurrier BK-7 glass. For the best light transmission, choose ED (extra low dispersion) glass objective lenses, which will mean sharper-looking objects and better images in low light. Meanwhile, multi-coated lenses will reduce glare and allow light to penetrate the glass more easily.
Tracking night sky objects
If you’re buying a large monocular for stargazing – and since it should be at least 10×42 or 10×50, it won’t be small – it’s likely to conclude a tripod thread. You can consider mounting it on a tripod if you’re looking at the moon but expect to have to nudge it every few minutes to keep it in the field of view (to account for Earth’s rotation). If you already have a star tracker mount, you could attach a large monocular, though typically they’re used for small telescopes, which offer much more magnification.
High-end monoculars are available that offer high-end image stabilization, such as the Bushnell Equinox Z2 3X30 ($265/£299) or Bresser 16×30 with Image Stabilizer ($779/£558).
Still, these are aimed more at birding and lack the aperture for stargazing. The best way to hold monoculars steady is to use two hands – one in front of the other – and use gravity when observing something at the zenith, allowing your eye socket to gently support their weight.
Objective lenses and magnification
Like a pair of binoculars, every monocular comes with two numbers, such as 8×42 or 15×50. What do they mean? You really need to know. The first number is magnification. For example, 10×42 and 10×50 monoculars have the same magnification, such as the Bushnell Legend 10X42 and Celestron Outland X 10×50.
As high a magnification as possible may seem logical for looking at tiny objects in the night sky, but beware. While monoculars are available at 8x and 10x magnification, and some go up to 15x and beyond, the bigger that number, the more zoomed-in you’ll be. That means a narrower field of view, making it hard to locate objects. It also means a heavier monocular that’s slightly harder to hold still, meaning a likely shakier view.
What you will look at in the night sky is also important. If it’s deep sky objects, consider a high magnification, such as the high-end 15x of the Vortex Recon R/T 15×50 (thought’s designed more for wildlife). You’re fine with 8x or 10x if it’s just the moon.
The second number quoted for a monocular refers to the size of its objective lens size – the end that faces away from your eyes – in millimeters. For example, 8×42 and 10×42 monoculars have the same 42mm diameter objective lens. For stargazing, the bigger this number is, the better.
Yes, there’s a big trade-off with weight and portability, but there’s no point in stargazing with 25mm or even 32mm objectives. Wider, so able to let more starlight in, 42mm is considered the minimum size for astronomy, with 50mm ideal and 70mm a real treat. Again, there’s a moon get-out clause here; our natural satellite is so bright that the size of the objective lens is less unimportant.
Build quality
Ease of travel is one of the reasons that a compact monocular for stargazing is becoming more popular. These ‘mini telescopes’ are easy to carry, usually in a pouch or a bag, with many being supplied with a wrist strap.
When choosing, prioritize the magnification and objective lens size, but also think about whether they can survive being dropped or, just as likely, being soaked in dew. Waterproofing and fog-free lenses will help.
It’s also important to test the focus adjuster, which should be simple to turn smoothly clockwise and counter-clockwise but have just enough resistance to stay in place.
Analogue or digital?
You might think that a monocular is an entirely analog, handheld device, but in recent years, digital products have been produced, some of which even promise night-time viewing abilities.
However, these infrared-capable products – such as the Celestron NV-2 4.5×40 Night Vision Scope ($290/£269) — are designed for hunting, wildlife observation, sailing and surveillance, not astronomy.
Eye relief and comfort
Eye relief is often underestimated. It’s the distance between your eyes and the monocular lens, and it can greatly differ between models. It can vary from about 5mm to 20mm, with glasses wearers recommended to look for a monocular with an eye relief of at least 15mm.
Customization
There are a plethora of monoculars on the market that make all kinds of wild claims about magnification and “super zoom,” and very often come with a smartphone adaptor for taking images (digiscoping). These are mostly basic, small products that are tricky to use and not designed to be used at night.
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