Observing with Nebular filters

Nebulae contain gasses that emit light when a nearby star shines on it or through it. 
Examples of such nebulae can be found in Orion (“Great” M42), Lyra (“Ring” M57), Vulpecular 
( “Dumbbell” M27) and super nova remnants (“Veil” in Cygnus).  Comet tails also emit light 
by a similar process.  Hale-Bop had sodium tail.

Gasses emit light in only sharply defined regions of the spectrum unlike stars which have 
what is described as a white light or “a continuous spectrum”.  Filters that only pass the 
emission lines allow us to see the nebula in all its glory while all other light is removed.  
A filter that transmits only a few wavelengths is a “band pass” filter. Light pollution 
rejection filter block some areas of the spectrum while transmitting over a larger band width.  
These are broad band transmission filters.

Visible spectral lines from atoms in nebulae. (ref-1, 3)
Here is a list of the important nebulae lines in the visible region  700 nm(red) to 400(blue) nm 
(nm=nano meter  1nm = 10-9m ,  1A = 10nm)

Hydrogen Alpha		Red		656.3 nm
Hydrogen Beta		green		486.1 nm
Hydrogen Gamma	blue		434.1 nm

Oxygen III		green		495.9 /500.7	

Nitrogen II		red		654.8 / 658.4
						
Visible spectra lines in comets (Ref-2)
Sodium I		yellow doublet	589.0 / 589.6
C2 Swan Band (Ethane)	red		620 nm
	ditto		blue		430 nm

Deep Sky Filter, or Light pollution rejection.
This is a broad band filter that blocks mercury and sodium vapour.  It can be used for increasing 
contrast on galaxies and is suitable for CCD and photography.

Ultra High Contrast Filter
These have narrow band pass filters that lets through Oxygen III and Hydrogen Beta. Good general 
purpose nebular filter.

Oxygen III.
Provides the highest contrast for planetary and brighter emission nebula. Best with larger apertures

Hydrogen Beta
Provides the highest contrast for faint Hydrogen clouds like the Horse Head and California. 
Best with larger apertures



References
1)  Astronomical Spectroscopy, 	A.D. Thackeray (1961)
2)    Observing Guide to Comets,	 British Astronomical Association (1996)
3)    http://www.blackskies.com/tips.html