This is a "bench" comparison of the Hutech Idas LPS-P2 to the Astronomik CLS filter. Both are targeted at filtering out suburban light pollution by blocking those wavelengths that are emitted strongly by common street lights. Unfortunately the thrust for street lighting is to make the light bulbs not only highly energy efficient but also as broadband as possible. A typical example is the way the Low Pressure Sodium lamp (mostly an "ugly", very yellow, monochromatic light) has been overtaken by the broadband High Pressure Sodium Lamp. While the older LPSodium was easy to filter out, that is no longer true for the newer HPSodium. Immediately therefore we face the problem as to which type of light these filters should be evaluated against? I measured the spectra for 3 electric light bulbs that are commonly available; a 100W domestic tungsten light bulb (similar results may be expected from more modern LED street lights), a 160W mercury-vapour/tungsten blend bulb that is commonly used in industrial locations, and the latest-technology 23W Energy Saver bulb for domestic use. Ideally one should do the measurements against the actual, highly mixed, local skyfog, since in general we are surrounded by a plethora of light types, although there is often a preponderous collection of Mercury and Sodium lines emitted by street lights and supermarket parking lots. Nevertheless I think the conclusions are not affected greatly. We have to face the reality that our local skyfog is not defined by simple Mercury or Sodium (low pressure bulbs) spectral lines as we would like, or as the advertisements for these filters make it out to be. Lighting manufacturers are constantly on the look-out for ever more exotic phosphors to fill in the voids between the main Mercury lines. With time, public-space lighting becomes ever more "white" and broadband, and at a certain, high level of skyfog one has to accept that rather than filtering out the skyfog with notch filters, like the LPS-P2 and the CLS, it becomes more expedient to block ALL wavelengths except those of interest in the specific image one is attempting to capture - narrowband imaging. Read on, text beneath the slides.

To get a quick background on filters and their impact on imaging the popular Orion Nebula: http://www.samirkharusi.net/filters.html

To get a quick introduction to my rough-science spectroscopy: http://www.samirkharusi.net/spectrograph.html

For other topics peruse my primary website: http://samirkharusi.net/