Narrowband versus color

[Ags]

Out of curiosity, what is the actual difference between narrowband and color imaging? For example, how many more photons (approximately) does a red filter let through from a nebula than a Ha filter would? Likewise, what proportion of the light let through by a green filter would be let through by an OIII filter?

Obviously the difference is huge for a broadband source like a star, but for a nebula is the difference much smaller? Or do so many emission lines contribute to a nebula that any single narrowband filter only capture a small part of the light?

I was just thinking about this, because I haven't seen anyone doing carbon or helium emission lines although there is enough of those elements for strong emissions - perhaps the lines are all in the far infrared...?

[Merlin66]

Think in terms of the ratio of the filter bandwidth...

If an Ha filter is say 5nm bandwidth and a "red" filter is 150nm, then you get 5/150 =1/30th the light.

Unfortunately as you've already guessed this is a gross simplification...for nebula all of the light is emitted in discrete wavelengths ie OIII, Hb etc and the narrow band filters are usually designed to have as narrow a bandwidth as possible centred on these wavelengths for maximum performance.

The reason you don't see much else being done is the large (or the expense!!!) of specialised filters for these wavelengths.

(Use a spectroscope and you can select the narrowband wavelength you like!!)

[Tim]

I toyed with the idea of getting some specialised filters, but they cost too much

With a Ha or SII filter in place, you could leave the camera run for hours and hours without it saturating the whole CCD. You couldn't do that with a plain red filter for too long, the difference is considerable.

[Ags]

With a Ha or SII filter in place, you could leave the camera run for hours and hours without it saturating the whole CCD. You couldn't do that with a plain red filter for too long, the difference is considerable.

On the other hand, people do use UHC, OIII and Hb filters for visual observations - so a fair bit of the light has to get through...