Legend...

 

NGC 6781 (Aql)  = Name or Catalog Designation of the object (Constellation).
N 457/1850 mm = Telescope data (N = Newton, R = Refractor; Aperture/Focal length in mm).
205 × / [OIII] = Magnification (×) or Exit Pupil diameter (EP) in mm / Filters if used (UHC, [OIII], Hb).
fst* 6.6 (B) = Naked eye limiting magnitude (fst* = faintest star); Observing site code (see below). 
 

 

 

 

(D) = Darmstadt, West Germany, 240 m over sea level (poor sky conditions, fst* < 6)
(N) = Neutscher Höhe, West Germany, 440 m over sea level (dark sky, fst* 6 to 6.5)
(B) = Breitenbuch, West Germany, 600 m over sea level (very dark sky, fst* 6.5 to 6.8)
(S) = Sölden, Tirol / Austria, 2800 m over sea level (excellent sky conditions, fst* > 7)
(G) = Gran Sasso, Italy, 2200 m over sea level (excellent sky conditions, fst* > 7)