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Old 25-06-2021, 04:15 AM
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Don Pensack
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Don Pensack is offline
Join Date: Aug 2006
Location: Los Angeles
Posts: 471
Narrowing the bandwidth of a filter increases contrast by darkening the sky without darkening the nebula.

So the narrowest filter that passes the necessary spectral emission lines will be best for contrast.

Nebulae emit light at about 3 spectral wavelengths we can see well:

H- at 486nm (blue)

O-III at 496nm (blue green)

O-III at 501nm.(blue green)

So, if the nebula emits light at all 3 wavelengths, we want a filter that passes all 3 wavelengths, like a narrowband filter (Lumicon UHC, DGM NPB, Astronomik UHC, TeleVue Nebustar II, Orion Ultrablock).

That would include the large hydrogen gas clouds like M8, M20, M17, M16, M42/43, North America, etc.

If the nebula emits only the O-III light, then a narrower filter that passes only the O-III lines will improve contrast.

That would include planetary nebulae, Wolf-Rayet excitation nebulae (Crescent, Thor's Helmet, etc.), and supernova remnants (Veil, etc.)

If the large faint nebula emits only H- light, then a narrow filter passing only the H- light is best.

That would include the California Nebula, IC434 behind the Horsehead, and a lot of the Sharpless Nebulae.

Bear in mind, though, that a narrowband passes all those wavelengths, but it's bandwidth is a bit wider than optimum for just an O-III emitting object.

Hence, it's useful to have an O-III filter. The H- filter is only good for a few objects, so it's WAY down the list of filters to buy.

In the spectrum, O-III emission is at 501nm (strong), 496nm (1/3 as strong) and 493nm (1/100 as strong).

We need as much help as we can get visually, so we ignore the 493nm line and make filters for visual use that pass the 501nm and 496nm lines, like the O-III filters that pass both bands.

Photographically, we can narrow the bandwidth to just the 501nm line and make our exposure longer, so we can make the bandwidth narrower for higher contrast.

Visually, though, the photographic O-III filters (Baader is one) are dark and filter out 25% of the light from the nebula, making the nebula dimmer.

So for visual use, you want 2 O-III lines to come through, and for photography only 1.

Most visual O-III filters of 12-16nm bandwidth pass 2 O-III lines, so can be fantastic visual filters.
I would stay away from the 1-line O-III filters (3 to 10nm) for visual use
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