General Information on Nebula Filters

All filters fit to the standard 1 ¼“ resp. 2” eyepieces filter threads.

General:

Interference filters by Astronomik and Baader block certain wavelength, e.g. potassium or mercury emission lines as emitted by streetlight lamps, and therefore enhance contrast for deep sky objects by darkening the background. They can be used for visual as well as for photographic purposes. Depending on the filter the bandwidth of the wavelength region which can pass the filter is varying (see below), and light sources with continuum emissions like stars or galaxies are damped more or less in their intensity. Light sources with emission lines, mostly galactic or planetary nebula, which emit nearly their full intensity in small lines of the spectrum, can pass the filter with their full intensity. Therefore these filters are called “nebula filters” in general. However, by blocking a certain part of the spectrum the filters do have a color. This is not a problem when observing by eye at night, as the human eye, when adapted to darkness, is not able to recognize different colors. When taking photographic images you have to look for this effect and to take care of the colors of your images, or just use a grey scale.

Remark on the wavelength which may pass the filter: for astronomical purposes the following emission lines are fundamental:

Line (nm)	Name	Description
486,1	H-beta	Mostly in Hydrogen nebula (galactic nebula) Transition n=4 -> n=2 in Hydrogen (Balmer series)
495,9	OIII	Mostly present in planetary nebula „forbidden“ transition in oxygen
500,7	OIII	Mostly present in planetary nebula „forbidden“ transition in oxygen
656,3	H-alpha	Mostly in Hydrogen nebula (galactic nebula) Transition n=3 -> n=2 in Hydrogen (Balmer series) This line is visible in deep red with the daylight adapted eye, however, at night this part of the spectrum is invisible (see graphics below)
671,7	SII	„forbidden“ transition in ionized sulfur
673,1	SII	„forbidden“ transition in ionized sulfur

The following graphics shows a comparison of the sensitivity of the human eye at night (grey) and during day (colored):

Visual filters:

The visual filters are made for emission and planetary nebula with dominant emission lines of OIII, H-beta and H-alpha. The pass bandwidth is varying, but all of them block the typical lines of urban illumination. Therefore contrast is enhanced especially in areas with high light pollution. The filters can be used photographically also, but you have to take care for the color information.

back