…continuedSeeking Summer's Dark Nebulae
The Trifid and the Cocoon
A dense dust cloud shapes the neck of the Swan Nebula, M17. This cloud looks distinctly darker than any other place in the field of view. Several narrow gray ribbons cross the Swan's body and lower neck diagonally. One superb night my 8-inch showed these features at 174x, before nebula filters were available. A 16-inch shows them routinely. John Casino's 36-inch Dobsonian with an O III filter revealed the linear absorption nebulae in M17 just as clearly as a photograph.
Under wilderness skies the 4-inch Astroscan at 16x with a UHC filter will find the triangular dark intrusion opposite the Star Queen in M16. But the Star Queen herself a figure of dark nebulae made famous by the green and brown Hubble Space Telescope image of her head and upraised arms required Bryce Heartwell's 10-inch reflector and a UHC filter on an excellent night. The Star Queen can be seen without a nebula filter in my 16-inch at 65x.
Alan M. MacRobert saw Barnard 361 in Cygnus through suburban light pollution. Using his 6-inch reflector at 45x, MacRobert called it "a well-defined and eerie dark nebula just south-southeast of [open cluster] IC 1369 . . . a big, dark hole in the Milky Way, its edges limned by rich starfields." From the round 30'-diameter main mass, my 8-inch reveals a narrow tail running immediately west of the pair of prominent stars ¼° west of the open cluster. B361 is beside the Funnel Cloud, 1° east-southeast of 63 Cygni.
Another quarry in Cygnus is the 2°-long Barnard 168, which meanders to and engulfs the Cocoon Nebula (IC 5146) at its eastern end. Through my 16-inch Dobsonian, B168 is more prominent than the faint 12' by 10' Cocoon. Dyer and Ling say that a hydrogen-beta filter may reveal the Cocoon with an 8- to 10-inch telescope.
Under black skies, the Astroscan at 16x shows very intricate structures around Gamma Cygni. Several dark bands can be followed between patches of emission nebulae within the Cygnus starcloud. Even 7x50 binoculars find dark windings around Gamma.
When the luminous starclouds of Sagittarius and Scutum cast shadows, when M7 and M8 stand out as bright naked-eye cloudlets, then you are enjoying the pristine skies that will make our mottled galaxy's more challenging dust clouds remarkably visible in silhouette.