For the amateur astronomer, close or uneven doubles (with very different magnitudes) are a great test for telescope and observer, and of course the seeing conditions. Hartung’s classic book includes a myriad of doubles and multiples with statements like “10.5cm (aperture) will split them”, and to my embarrassment I find myself struggling with 20cm or even 35cm.

Hartung was of course a superb observer with acute eyesight, superb optics, and a dark sky site. Most ordinary mortals like us are lucky to claim one of these advantages let alone all three, so Hartung’s casual feats are usually wildly optimistic for the rest of us. However if you harbour a competitive spirit, then you can get a lot of fun out of picking some tough doubles and seeing whether you can separate them with your telescope. And splitting doubles is an excellent way of testing resolution (the capacity to distinguish detail).