Binoculars for Stargazing

Why binoculars: Cost: From as low as £20-£30 you can get a decent pair of binoculars. Wide Field: Binoculars provide different views compared to telescopes: low power, wide fields, excellent for star fields. Can be breathtaking under dark skies. Ease of Use: Just hold them in front of your eyes. Also, an easy connection with what you see with your eyes. Portability: Can be with you almost always.

What you can see:

Basics: Aperture & Power “10×50” = 10x magnifying power, 50mm aperture. For astronomy: at least 35-42mm aperture (birdwatchers love these sizes) 50mm or more preferred (size & cost define our upper limit). Too much power is bad – too dim & too hard to hold steady. Many people feel up to 10x is OK to hold (I prefer 7-8x). Avoid variable magnification / zoom binoculars (12-20x80 etc). They are never good.

Basics: Exit pupil & Eye relief Exit Pupil is the diameter of the light cone coming out of the eyepiece. Exit Pupil = Aperture / Magnification. Large exit pupil = bright image. Young people up to 7mm exit pupil (e.g. 7x50) Older people perhaps up to 5mm (e.g. 10x50, 8x40) Eye Relief is the distance of your eye from the eyepiece. For eyeglass wearers, 12mm or more are required.

Basics: Field Of View Example: “315ft @ 1000yards”, “105m @ 1000m”, “6.0°” Apparent field of view = FoV° x magnification (e.g. 6.0°x10 = 60°) – similar to telescope eyepieces At least 60° is “wide-angle”. Under 50° is rather narrow. ?????????

Basics: Prisms & Coatings Porro prism better perf./price for astronomy. Roof prism preferred for birdwatching (more compact, waterproofing), requires expensive phase-coatings for good stargazing. BaK4 glass better than cheaper BK7 glass. Full Multi Coated (FMC) > Multi Coated (MC) > Fully Coated (FC) > Coated (C) > No coating > Orange/Ruby “coating”

Tips for Evaluating Binoculars Check for smooth operation of focuser, diopter and hinge. Collimation: Make sure the two images from your eyes merge into one. New binoculars should be collimated. Check for sharpness of focus, colours and chromatic aberration around bright objects.

Tips for Evaluating Binoculars 2 Look through the eyepiece from a distance to check for undersized prisms and BK7 glass (diamond shape). You can measure the effective aperture using a single LED torch / flash at least 15 cm from the eyepiece + tissue paper. Make sure you can easily view the entire FoV with or without eyeglasses (i.e. as you will be using them).

Buying Tips Avoid zoom binoculars, anything with a red/orange coating, unknown brands (e.g. based on Amazon reviews) – some well known brands with decent budget bins are: Nikon, Olympus, Celestron, Opticron, Helios, Bushnell, Bresser, Meade… Porro prism will give you better quality for the price for astronomical use. If you don’t have a limited budget, expensive roof prisms with phase coatings perform as well as good porro. Astronomer favorites are 7x50 or 10x50, as those have the most aperture you can easily carry around and hold steady. Astroboot (http://www.astroboot.co.uk/) often has some good quality models at heavy discounts. Lidl carries a couple of times a year usually a Bresser 10x50 for around £17. But test it first because many are uncollimated.

Examples up to 50mm £25-£35: Celestron G2 Porro 8×40 / 10×50, Bresser Hunter 8×40 / 7×50, Helios Solana 8x40 / 10x50. £40-£60: Opticron Adventurer 8×40 or 10×50, Celestron Cometron 7×50, Olympus 8×40 DPSi, Helios Fieldmaster 7x50 / 10x50. £60-£90 Nikon Aculon 8×42 / 10×50, Helios Weathermaster III WP 7x50 / 10x50 – Naturesport Plus 10x50 WA.

www.binocularsky.com