CONFRONTO CON IL FILTRO H-ALFA BAADER - Skypoint Srl

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Three of the most important parameters in narrowband filters are the FWHM, the maximum %T and the out-of-band
blocking. We have discussed the first two, and most people are somewhat familiar with what they mean. The out-ofband
blocking is very important. It refers to the degree to which light is rejected OUTSIDE of the passband. Poorer outof-band
blocking will let more light come through your filter, degrading its contrast.
Realize that these H-a filtes let light pass in about a 10-12 nm region, whereas the silicon-based CCD detectors used in
amatuer cameras are sensitive to light from the ultra-violet (UV) to the near-infrared (NIR) up to about 1100 nm. Thus,
more and more coating layers are needed to eliminate the potential light leakage of the filter outside its passband. If you
integrate the area under the curve from 300 to 1100 nm compared to just the area under the curve of the narrow
passband, a small amount of out-of-band leakage adds up quickly. This, in fact, is how you characterize the CONTRAST
of a filter relative to a chosen detector and light source.
You cannot see how much light is leaking through the filter outside the passband in %T plots, as shown above. The
signal would be