About Astronomik Nebula Filters


The following pictures may give you a glance of what the different filters do. Therefore a certain area of the sky was taken with and without filters. The performance of the different filters is clearly dependant on the object itself, because the best filter will always be the filter best suited for the emission lines of the object. However, darkening of the scattered background light as well as the spectral selectivity of the filter is clearly visible by these examples.

The following data are valid for all of the pictures:

Nikon D70 Digital Camera at 1600 ASA, objective 85mm, F/2.5. These are highly compressed parts of the original pictures. They were taken under a typical suburban sky, with medium background light. All pictures are raw, just the internal noise reduction of the camera was activated. Due to the spectral behavior of the filters the pictures are more or less colored - simply just a small part of the spectrum is able to pass the filter. If you use them for photographic purposes, you should take only the grey values or you have to be very careful with the colors produced. However, in this case we have left the color in just to give you an overview of the filter colors.



30s exposure without filter. The sky background is clearly visible, this is the maximum possible exposure time.


Picture taken with CLS-filter (CLS = City Light Suppression), also 30s exposure. Background is really darker, the contrast is remarkably improved. Because of the broad spectral pass of the filter the faintest star limit is not much worse than without filter, but one will get a greenish color. With this filter you can achieve an expansion of the exposure time by a factor of two to three.


Same picture taken with UHC-filter, also 30s exposure time. The filter has narrower spectral band pass, which leads to a more darkened background and now also to a loss in the limiting magnitude of the faintest stars. But if you compare the intensity of the Orion nebula, you will find nearly no difference to the pictures before.


Same filter, but 90s of exposure. Now the limit in faintest star magnitude is like the 30s-exposure without filter. Sky background becomes visible again. The Orion nebula is visible well structured - thjs is due to the two spectral bands of pass, one in the blue/greenish and the other one in the deep red.


Picture taken with the H-beta filter, 30s exposure time. The extremely small spectral pass takes most of the light intensity, background is really black now. The color is blue due to the wavelength passed around 486nm.


Again H-beta-filter, but 180s exposure time. H-alpha parts are clearly missing in the Orion nebula, which can pass the other filters.


30s exposure with H-alpha-filter. Only the far red can pass this filter, therefore the picture is colored red. Just the brightest stars are visible here, but look at the Orion nebula: it is still as nearly as intense as without filter.


Again H-alpha-filter, but 180s exposure time. Background is still dark, but the Orion nebula is now visible to the outer regions. Beside the left bright star at the top the flame nebula becomes visible at a very faint level.




More information on Astronomik filters you will find at the manufacturers site http://www.astronomik.com


and measurements on spectral performance of the filters at André Knöfel: http://www.astroamateur.de/filter