Narrowband Filter Bandwidth

[BlueAstra]

I've noticed that narrowband filters are available in different bandwidths. The Baader Ha filter is available in 7, 8.5, 35nm bandwidths. The O3, S2, Hb filters are mainly 7, 8 or 8.5nm. Presumably the narrower bandwidth gives a 'sharper' image, more LP rejection, but less light and a longer exposure. They are also more expensive.

I'm not sure what the effect of the different bandwidths is, so when I start out on the narrowband imaging path with maybe a Ha filter with my C8 and SXH16 (in LP challenged area), should I jump straight in at 7nm or 8.5nm, or go for the cheaper 35nm?

[Merlin66]

The emission lines for OIII, Ha etc etc are of the order of <1nm so even the narrowest bandwidth will let background light through as well as the emissions....

The focal ratio can have an effect on very narrow band filters - a f4 type beam will cause the centre band wavelength to shift towards the blue..so a wider bandwidth on faster scopes would help.

The wide bandwidth filters just allow more background light to get through which can sometimes help in finding star images to focus on...

Hope this helps.

[moocher]

The narrower the band width the better. It does of course mean longer exposures but its worth it...

Alan

[Andrew*]

It does of course mean longer exposures

Alan

Does it, though?

As Ken says, the actual emission is extremely narrow in bandwidth, so as long as the peak has the same transmission % and is centred on the emission line, exposure for the nebulosity should be the same regardless of bandwidth.

Andrew

[narrowbandpaul]

spot on andrew!

[Whippy]

From my own experience, I used to own an Astronomik 13nm Ha filter that I couldn't use under a full moon while the Baader 7nm I now own can so my thinking would be that the more narrow the pass ont he filter, the less likely it's going to affected by other light sources such as the Moon and streetlights for example.

Tony..

[ollypenrice]

I agree entirely with Tony since I have the Astronomik 13 and Baader 7. The 7 is more contrasty and way better in moonlight. I must say I thought it a bit slower subjectively but have not done a real comparison. But in terms of image quality it gives far more bite. (Note the very different way I express things from Paul, who understands what is going on! I'm just a stumbling empiricist...) But get the Baader!!

Olly

[MartinB]

One thing that usually isn't considered when moving to tighter and tighter band withs is the exposure length required to get over the read noise of the camera. With my Baader 7nm Ha from a moderately light polluted sky and using a low read noise camera it takes a minimum of 30mins before the back ground sky glow outweighs the read noise. On a dark moonless night it is quite a bit longer.

The extra sky glow coming through a 13nm Ha might not be such a disadvantage and in practice, unlike Tony and Olly I've found the difference between the 2 barely perceptible. I'll see if I can dig out some comparable data

[Glider]

Does this all carry over to the wider 35nm filter?

[ollypenrice]

Is the wide one a visual filter?

Olly

[MartinB]

I think it is meant for imaging ?brought out because of the shift that occurs with very fast systems such as hyperstar. Not ideal for more regular scopes.

[dph1nm]

One thing that usually isn't considered when moving to tighter and tighter band withs is the exposure length required to get over the read noise of the camera.

Why would you need to do this? Adding extra sky to overcome read-noise only means you have an even noisier image.

NigelM

[MartinB]

Why would you need to do this? Adding extra sky to overcome read-noise only means you have an even noisier image.

Don't disagree with that at all. My thinking was that if a sub with a 13nm filter has a sky glow level that effetively washes out read noise after 20 mins there isn't much point going beyond this. However, the 7nm filter may reach that point after 40 minutes. If you aren't going to run 40 min exposures you will be hitting a noise obstacle and that will mean you aren't getting the quality of image potentially available. Ironically the problem gets worse the darker the site. Your images will still be better from the dark sky site but not achieving full potential.

At 15 mins a 7nm filter still has the potential to be better than a 13nm because of the reduced sky glow but not as much as you might think because the read noise is still a limiting factor.

[MartinB]

Since I still have an astronomic 13nm and a Baader 7nm Ha I will run a direct comparison. I can have them both in my filter wheel and now that I can automatically correct the focus between filters I will run them in an alternating sequence on my next project - HH. Goodness knows when this will be but the results should be interesting.