narrowband filters

I know this is sorta lame but what exactly is the difference between the narrowband filters when they say 3nm or 12nm? I figure they let in a certain amount of light per band. Should I start with a specific #nm?
I am about to start imaging with these filters and I am not sure which ones I should start with.
Thanks in advance
Jimmy

You pretty much got it. If you buy an Ha filter to get 656.3 nanometer wave length, you get from 650.3 to 662.3, (roughly) if you buy a 12 nm bandpass filter. But with a 6 mm bandpass, you get 653.3 to 659.3. The light allowed to get through is centered on the target wavelength, but some slips by on either side. And we are talking a bell curve here, not precise cutoffs.

The narrower the bandpass, the more centered the light is centered on the desired band.

Does it make a difference? Depends on your purpose, the instrument, and so forth.

You have to be careful with extremely fast focal lengths (F3) and narrowest of narrowbands. (THe light shifts a bit, and the filter loses efficiency. It does not pass as much of the desired light). Most of us will never have to worry about that difference.

If you are a serious, serious imager, or beset with major light pollution problems, and want to image under bright moons, etc. Go for the more expensive narrowbands from Astrodon, etc.

If you just want to get some good pics, the Orion set, with a wider bandpass, is satisfactory. The ATWB Image of the day from Tuesday (Jan 31) of the Horsie and Friends was taken from a moderately light polluted suburb with the Orion Set. If you get hooked, you have not lost too much on them when you sell them to trade up to Astrodons.

Alex