Confused: when to use just Ha narrowband, when to use duoband, when to combine with broadband

[BrendanC]

Hi all,

I started out with a modded DSLR three years ago, then got a 7nm Ha filter - all good, shoot galaxies, planetary and reflection nebulae and clusters as just broadband RGB without the Ha filter, use the narrowband filter for emission nebulae to add a ton of definition to the red channel.

Then I had an ASI1600M for a short while. Didn't take to mono at all, but understood that the broad distinctions still applied, except that I could also shoot S2 and O3 for emission nebulae.

So, I took a step back and now have an ASI 533MC Pro, which is great. I got a ZWO duoband filter too, and have enjoyed using it for a few months. But now, I'm starting to realise, I'm a bit confused as to exactly when is best to use each filter.

I expect I continue with broadband as before - galaxies, planetaries, reflection, clusters. I also get that if the Moon is above around 50% phase, I should use the Ha filter because the O3 of the duoband could be wiped out. So, that means I'm back to where I started, doesn't it? That is, broadband RGB , or broadband RGB with Ha. So, what's the advantage of using my duoband filter? At what point in between purely Ha when there's a Moon, and purely RGB, when there's less Moon, should I be using duoband? And what's the real benefit?

Thanks, Brendan

 

[Elp]

I think you're thinking too much about it. There's RGB and L imaging, and there's narrowband (SHO) imaging. That's it other than a few other specific filter imaging. A filter simply blocks unwanted wavelengths of light and passess the wanted.

Based on your local light conditions you then decide what you want to do. A OSC filter helps when you have certain types of surrounding light pollution, but a narrowband filter (SHO) with a mono camera will be more sensitive as it utilises all the pixels of the camera (even lrgb), but you can only shoot certain things in narrowband. Depending on what you are imaging you can supplement your osc RGB image with separate luminence, ha etc. An OSC duo narrowband filter will only work on emission targets rich in ha and o3, otherwise it's pointless using it as you're loosing the broadband signal you need for the rest of the RGB spectrum. 

[The Lazy Astronomer]

I don't really see any advantage to using an Ha filter with a OSC, may as well just use the duoband. Worst case is the Oiii would be completely washed out by a bright moon, but honestly I think that would be unlikely. You would most likely end up with at least some usable Oiii signal, and even if you don't end up with any useful Oiii, you're no worse off than if you'd used the Ha filter (assuming the same or similar bandpasses for both).

[BrendanC]

3 minutes ago, The Lazy Astronomer said:

I don't really see any advantage to using an Ha filter with a OSC, may as well just use the duoband. Worst case is the Oiii would be completely washed out by a bright moon, but honestly I think that would be unlikely. You would most likely end up with at least some usable Oiii signal, and even if you don't end up with any useful Oiii, you're no worse off than if you'd used the Ha filter (assuming the same or similar bandpasses for both).

This is kind of what I've been thinking too - just shoot duoband and then mix with RGB if I need to, for example for stars.

 

46 minutes ago, Elp said:

I think you're thinking too much about it. There's RGB and L imaging, and there's narrowband (SHO) imaging. That's it other than a few other specific filter imaging. A filter simply blocks unwanted wavelengths of light and passess the wanted.

Based on your local light conditions you then decide what you want to do. A OSC filter helps when you have certain types of surrounding light pollution, but a narrowband filter (SHO) with a mono camera will be more sensitive as it utilises all the pixels of the camera (even lrgb), but you can only shoot certain things in narrowband. Depending on what you are imaging you can supplement your osc RGB image with separate luminence, ha etc. An OSC duo narrowband filter will only work on emission targets rich in ha and o3, otherwise it's pointless using it as you're loosing the broadband signal you need for the rest of the RGB spectrum. 

I very probably am thinking too much about it! But it's been bugging me for a while and I wanted some opinions, so thanks for this. So, would you ditch the Ha filter, or the duoband filter? Or keep both?

[geeklee]

7 minutes ago, BrendanC said:

just shoot duoband [not Ha]

Agreed 👍

7 minutes ago, BrendanC said:

So, would you ditch the Ha filter, or the duoband filter? Or keep both?

Personally, I'd keep the duoband and ditch the Ha filter.  If you keep enjoying the duoband, there are versions with tighter band passes that you could put the Ha filter money towards.

[BrendanC]

Splendid, thanks all. 

I did a LOT of thinking before getting the ZWO Duoband. It is very wide for something called narrow! 15nm Ha and 35nm O3. Still, given I'm in a Bortle 4, and It was a lot cheaper second-hand than the really narrow Optolongs and Antlias and Altairs, I guessed it would be ok as an intro to dualband imaging with an OSC, and I've been pleased with the results so far.

It's just that I've increasingly been wondering at what point there's a crossover between when to use just Ha, and when to use dualband. Looks like there isn't one. Plus, at some point I will very probably upgrade, probably to the Altair because I like their quality assurance certification, especially after seeing Cuiv's videos about filters, so I wanted to get a grip on this before I did anything new.