I've observed the section of Sh 2-91 near Phi Cygni (V = 4.7) several times -- a dark sky and an OIII filter are the key (as David's post suggests). Here's my last observation with my 18-inch Starmaster from the Sierra Buttes (7200' elevation) --
18" (8/2/05): Sh 2-91 was immediately picked up in the field of Phi Cygni (15' to the south) at 115x and OIII filter as a huge, narrow, faint filament stretching across 13'-14' of the field. It begins close to mag 8.2 HD 185735 and extends to the NE, passing just south of mag 9.6 HD 332384 (7.7' from the mag 8 star). It continues northeast, brightening somewhat and heads towards a 30" pair of mag 11 stars where it seems to spread out and dim. Overall, the filament is fairly straight, but gently arcs a bit near the center and at times appears split into two strands.
Another section I've seen is near 9 Cygni (V = 5.4). This observation is with my 24-inch from Lake San Antonio in central California --
24" (9/13/12): at 125x and OIII filter, I was surprised to immediately see a huge filament stretching at least 30' east-west across much of the field in the 21mm Ethos. The filament tapered at the east end and fanned out on the west end. The surface brightness was a bit uneven; brighter along the boundary, particularly noticed on the southwest edge and weaker within the broader fan portion on the west end.
More information, observations and labeled images is on Deep Sky Forum
Edited by sgottlieb, 20 May 2015 - 02:36 PM.