Any merit in stacking Ha & Oiii together for luminance?

[Adam1234]

I'm currently in the process of imaging the Western Veil Nebula in HOO, and I had a thought. Normally when I do HOO (or SHO) I normally use Ha as 'luminance' - is there any merit in stacking the Oiii and Ha subs together to increase the overall SNR and detail in the luminance? Or would the Oiii just end up diluting some of the details from the Ha?

[vlaiv]

Depends on target composition, but in general case (or in majority of the cases) - it will hurt more than help.

Rationale is as follows (very simplified): one of following 4 cases can happen and we need to observe signal to noise ratio - we are going to observe target signal to background noise (not quite how we define SNR - but simplifies analysis). We will also observe signal as on/off rather than continuous spectrum of values - again for simplicity. Method of stacking will be addition (again simplifies reasoning).

1. No Ha and no OIII - background noise - noise will be increased with stacking

2. No Ha and OIII - signal of OIII will remain the same as in OIII - no signal increase

3. No OIII and Ha - signal of Ha will remain the same as in Ha - no signal increase

4. Both Ha and OIII signal - signal will increase as we add Ha and OIII signal

First part will happen to background - so stacking OIII and Ha will certainly increase background noise. Signal will improve only in places there is both Ha and OIII signal - and in most cases signal is not equally distributed (otherwise we would have "monochromatic" targets with no color variation) - so this is probably the least likely scenario. In all other cases - signal remains the same as in individual stack - no signal improvement.

So there you have it - very simplistic analysis - signal will not improve in most cases while noise will raise in the background - ratio of the two will therefore go down.

 

[vlaiv]

Forgot to say - if you want to try "LRGB" approach with narrowband imaging - then get duo/tri band filters.

These "mix" signal in real time and background is added only once - while signal of Ha and OIII (and even SII) is recorded at the same time.

This provides the same benefits narrowband as L does to LRGB.

 

[Adam1234]

Thanks vlaiv