The reason is simple — CTB-1 is extremely
dim. More than 28 hours of exposure time in Hα and 23 hours of exposure time in OIII narrow band
filters were necessary to capture it. Every single exposure with
G3-16200 camera on 30 cm corrected Newtonian telescope was 20 minutes
Narrow-band exposures reveal the gaseous bubble, but another
10 hours of exposures through regular red, green and blue filters were
necessary to show stars within the field of view in proper colors.
Resulting exposure time needed for this image exceeds 61 hours.