Filters

There are several different types of filters I use when photographing astronomical objects.

Hutech IDAS LPS-P2 Light Suppression Filter

Hutech IDAS LPS-P2 filters

The IDAS LPS-P2 filter, manufactured by Hutech Corporation, is a thin-layered interference based light pollution suppression filter. This filter is designed to suppress the various common emissions lines generated by artificial lighting, yet allow important nebula emission lines to pass. This results in the enhanced contrast of astronomical objects, especially emission nebulae such as the California nebula. An important thing to note is that these filters do not make nebulae appear brighter. Filters attenuate light, so in practice, the objects being photographed are actually dimmer than if photographed with no filter at all. What they do however, is increase the contrast of certains types of nebulae by subduing the background skyglow through the attenuation of the major artificial lighting emission lines. These filters also do not block all types of light pollution. Broadband light sources, such as common halogen type automobile headlights, will not effectively be blocked by these filters.

The plot shown below shows the specification requirement for the spectral response of a typical LPS-P2 filter. Note that the response curves notches out to near 0% transmission for the various major emission lines of artificial lighting sources. This includes both Mercury (Hg) and Sodium (Na) artificial lighting sources.

Specification requirements of spectral response for a typical LPS-P2 filter

Also important to mention is that unlike other light pollution filters, the IDAS LPS-P2 filters are designed for balanced color transmission. This allows color photographs to be taken of broadband astronomical objects, such as stars, galaxies, and clusters, to be taken with minimal color cast.

Presently, I have two sizes of this filter. I have an LPS-P2-48 which is a standard 48mm (2") filter which mounts directly to my Canon T-adapter for use with a telescope (or eyepiece), and a Canon DSLR front filter version, LPS-P2-FF, which fits inside the camera between the camera lens bayonet and the viewing mirror. The front filter version can be quickly swapped in and out in a matter of seconds and can be used with practically all camera lenses and telescopes. It is basically a one-size fits all filter. However, because the LPS-P2 filter is an interference based filter, its spectral characteristics depend on the angles of incidence of the incoming light. Therefore, when the incidence angle exceeds 25 to 30 degrees, the LPS-P2 bandpass wavelengths are shifted enough to become ineffective in blocking light pollution emission lines. So in practice, this means they should not be used with wide-angle camera lenses. However, they work perfectly with almost all telescopes and telephoto lenses. I generally use them down to focal lengths of 50mm.

Baader H-Alpha 7nm FWHM Narrowband Filter

Baader H-Alpha 7nm narrowband filter	Step ring adapters for filters

The Baader H-Alpha 7nm narrowband filter, manufactured by Baader Planetarium of Germany, is used to isolate the Hydrogen alpha emission lines which are common in emission and planetary nebulae. This particular filter has a FWHM (full width at half maximum) bandwidth of 7nm which is centered at a wavelength of 656.3nm which is the wavelength of most red emission nebulae. I used this filter primarily to image emission nebula such as the North American nebula. The particular filter I have is a standard 48mm (2") filter which threads directly on my Canon T-adapter. For use with my Canon lenses that have filter thread sizes other than 48mm, I use various step-up and step-down rings which will allow the 48mm filter to be used with most any lens.

Because this filter is narrowband, it rejects most other wavelengths including light pollution. (Transmission of H-Alpha of this filter is specified at over 90% while completely rejecting light pollution.) This means I can photograph these objects in both an urban location with extreme light pollution as well as under the light of the full Moon!

Finally, there is often confusion to what parameter FWHM actually refers to. FWHM stands for "full width at half-maximum" and is an expression given by the difference between the two extreme values of the spectral passband response of the filter in which the transmission is at half its maximum value.