The Secret Deepis the fourth title in my Deep-Sky Companions series – the other three books are Deep-Sky Companions: The Messier Objects, Deep-Sky Companions: The Caldwell Objects, and Deep-Sky Companions: Hidden Treasures. Like the third companion, The Secret Deep is an important work because it brings to light a new list of 109 deep-sky objects visible in small telescopes under a dark sky. None of the objects in the Secret Deep list appear in the Messier, Caldwell, or Hidden Treasures catalogues; I've included an additional 20 objects in Appendix B. Owners of this series, then, have at their fingertips more than 450 deep-sky objects to explore.

All the Secret Deep objects are visible from mid-northern latitudes, though five or fewer are best seen from more southerly locations in the Northern Hemisphere or further south. Still, the most southerly object in the Secret Deep list – globular cluster NGC 2298 in Puppis – lies at a declination of −36° exactly, so it is only 1¼° further south than open cluster M7 in Scorpius, the most southerly Messier object. From the latitude of New York City, NGC 2298 will be 9° above the southern horizon when highest.

I have taken great care to select objects visible through my new 5-inch Tele Vue f/5 refractor (see Chapter 1) under a clear, dark sky. As with some objects in the Hidden Treasures list, several of the Secret Deep objects are surprisingly bright – including some open star clusters visible in binoculars and to the unaided eyes, a few galaxies more apparent than the dimmest Messier ones, and a couple of planetary nebulae with central stars you can spy through large binoculars.