The ability of a telescope to resolve closely spaced objects is referred to as the instrument's resolving power, and is especially important when viewing tight binary stars. How close can two stars appear and still be resolvable as two? The single most important factor that influences the result is a telescope's aperture. In general, the larger the aperture, the finer the level of detail resolved. Of the many observational experiments that have been conducted to determine the resolution limits for telescopes, the two most often cited are the Rayleigh Criterion and the Dawes Limit.

Some amateurs can readily exceed these predictions, while others will never reach it. That's because a telescope's actual performance can be adversely affected by many factors, including turbulence in our atmosphere, a great disparity in the test stars's colors and/or magnitudes, misaligned or poor-quality optics, and the observer's visual acuity.

Dawes Limit calculates that 10x50 binoculars should be able to resolve binary stars separated by 2.3". Binoculars don't even come close. That's because binocular resolution depends more on magnification than aperture. Assuming 20/20 vision, the human eye can resolve stars 4 arc-minutes (240 arc-seconds) apart. To estimate the resolving power of binoculars, divide 240 by the binocular's magnification. For example, with 10x binoculars, stars need to be 24 arc-seconds apart to be resolved.

The list below from my book Cosmic Challenge shows the expected resolving ability for many different magnification values. Of course, your results may vary. But let’s put a few of these to the test.

The constellation of Draco the Dragon winds its way through our northern August sky, and while most of its stars are fainter than 3rd magnitude, it holds some fun resolution challenges for binoculars. Here are three of my favorites.

16 and 17 Draconis form a wide, easily resolved pair of stars in south-central Draco. 17 is the northernmost and slightly brighter of the two. 16 Dra, with a spectral class of F8V, has an apparent magnitude of 5.4, while 17 Draconis, a G0V star, has an apparent magnitude of 5.0. The two stars are separated by about 84 arcseconds. Despite their proximity in the sky, they are not a gravitationally bound pair, so they do not orbit each other. 16 Draconis is situated approximately 11.6 light-years away. Its companion, 17 Draconis, is located about 400 light-years from Earth.

Nu (n) Draconis is another wide double star that can be split through all binoculars. Nu is the faintest of the four stars in the Dragon's "head." This location led to its nickname, the Eyes of the Dragon. Nu is composed of two nearly identical 5th magnitude type-A white stellar jewels. In his classic book Celestial Objects for Common Telescopes, the renowned 19th-century deep-sky observer Reverend T.W. Webb called these stars "grand", an accurate portrait through modern binoculars as well. Studies indicate Nu is a true binary system, with both stars located about 100 light years away. At this distance, the stars' apparent separation of 62 arcseconds indicates a real separation of about 1,500 Astronomical Units. They have an orbital period of roughly 44,000 years. They lie about 99 light years from Earth.

How about a tougher test? Try your luck with Psi (ψ) Draconis, lying partway along the dragon's sinuous body. What looks like a single 4th-magnitude point to the eye is actually a pair of 5th- and 6th-magnitude stars separated by 30 arcseconds. That's right at the estimated limit for 8x binoculars, but again, they will need to be steadily supported for any hope of resolving this pair. Higher-power binoculars may reveal the pair’s color contrast. The system consists of a yellow-white main-sequence primary and an orange subgiant companion. Also known as 32 Draconis, Psi lies about 75 light years away.

For more binocular binary challenges, be sure to visit the CN topic Adventures with Binocular Double Stars. Since CN'er Fiske started the topic in October 2021, there have been over 580 replies to the topic.

Have a favorite challenge object of your own? I'd love to hear about it, as well as how you did with this month's test. Contact me through my website or post to this month's discussion forum.

Until next month, remember that half of the fun is the thrill of the chase. Game on!