Quote:
Originally Posted by PKay
Hi All
Most of you will find this a very basic question, but I wouldn't ask if I didn't know...
When you use for example a Ha filter (656 nm).
Does the filter stop that frequency, or only allow that frequency?
What is the difference between a 5nm and 7nm filter (aside from 2nm)?
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That sort of filter is a bandpass filter (allows a certain frequency band to pass). In astro people often refer to it as a narrowband or line (spectral line) filter. In your example, the centre frequency for that filter has a wavelength of 656 nanometers (nm) and has a bandwidth (width of the range of wavelengths passed) of between 5 or 7 nm, and when specified fully, usually by the manufacturer, is based on a given signal intensity limit such as the 3dB, 6dB or other dB or possibly even %Signal Intensity bandwidth. The 3dB and 6dB bandwidths are commonly used in science, eng. , electronics etc... and for example:
* The 3dB bandwidth is the width of this band from the lower -3dB (~71% signal*) extreme to the upper -3dB (~71% signal*) extreme whereas,
* The 6dB bandwidth is the width of this band from the lower -6dB (~50% signal*) extreme to the upper -6dB (~50% signal*) extreme.
A pic is better and I'll post one if I can find it.
Best
JA
*Percentage quoted is for signal, NOT signal power