This is an unusual object. NGC 6888 is a Wolf-Rayet nebula, a shell of gas expanding outward in response to pressure from strong stellar winds produced by the Wolf-Rayet star at its core. The high mass content of the shell suggests it is mostly interstellar medium material with the inner portion composed of material from the WR star. The Crescent Nebula is also quite beautiful.

The sketch at left captures the view in my 10-inch Starfinder Newtonian at 129X with an OIII filter attached. Its shape reminds me of the Greek letter, Omega. The nebula is very subtle, even with the OIII, extending some 18'.0x11'.5 in size. The brightest portion is a curving C-shaped emission. This is aligned roughly northeast-to-southwest, with the southwest end appearing brightest. The arc passes across a 6th magnitude GSC star at the northeast end and brightens noticeably in this region. The WR star powering this dramatic display is 7.5 magnitude HD 192163, centered in the drawing. Nebulosity is visible extending west from this star to the larger C-shaped portion and also to the southeast in a broad triangular pattern, which nearly connects the ends of the C.

Averted vision picks up a delicate haze southeast of NGC 6888. Digital Sky Survey images do not show this nebulosity but I'm not yet ready to discount it as averted imagination. Twenty-four field stars complete the view. You'll find the Crescent nebula about 11 degrees southwest of Sedr, the 2.2 magnitude star where the arms of the Northern Cross asterism intersect.