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Does H-beta have the same distribution as H-alpha?


RikM

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As above really, does H-beta emission occur in the same place as H-alpha emission only with lower intensity?

For imaging, do I need an Hb filter or could I use my Ha, duplicate it and turn it blue (clipping layer mask) and drop the opacity to e.g.15% I want to get as close as possible to representative images using narrowband filters. I could imagine differences in spacial distribution of the emissions due to differential excitation by the embedded stars but I don't know if this actually happens or not?

If not, is the ratio of Ha to Hb constant?

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Generally the emission ratio is close to 40% - Hb to Ha.

Interstellar dust and "local" conditions in the nebulae can reduce the ratio slightly.

So in most cases, yes, you could use the Ha signal, reduced and re-coloured to represent Hb.

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For some/many H-alpha band is a sum of H-alpha and N-II. Planetary nebulae may even glow more in nitrogen than in hydrogen, not to mention that N-II is a doublet emission.

Only ultranarrow H-alpha filters can separate it from nitrogen.

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So would Ha minus Hb = NII?

I am not that bothered having Ha and NII come through together since they are both red and would map together to make a representative image anyway. I am just trying to work out why/if I need and Hb. I guess having the real Hb layer mapped to blue would mean I am not mistakenly making NII data blue by reusing the 7nm Ha?

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Sometime some use H-beta in imaging but it has some disadvantages - it can be less intense as H-alpha + [N-II], but can produce some colourization difference between those two filters. The biggest disadvantage may be the short wavelength. With some Moon light the background sky will be noticeably brighter in H-beta than in H-alpha as shorter wavelengths refract more.

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For comparison...

Here's Buil's spectrum of M42

http://www.astrosurf.com/buil/nebula/HII/m42/m42.html

Thanks for that, really useful. The relative intensities of Hb to Ha looks about 20%, which matches my estimate, and the 501 is much higher than 496 peak on the OIII doublet so that pushes it more towards green, which again matches my eyeball estimate. Looking at how low the SII is, makes me wonder why we commonly use that as the third filter?

What I am trying to do is find out the correlation between the wavelength and the Photoshop hue, saturation and brightness values for clipping layer masks to give the 'correct' colour. I would like to think that by then shooting equivalent data sets through each filter and applying equivalent stretch functions I could end up with a representative image.

At the moment I am using:

SII

Hue = 0

Saturation = 100%

Lightness = -63

Ha

Hue = 0

Saturation = 100%

Lightness = -53%

OIII

Hue = 163

Saturation = 100%

Lightness = -50%

Hb = (Ha @ 18% opacity)

Hue = 183

Saturation = 100%

Lightness = -50%

These are the closest matches to both my eyeball looking through the filters at a photo white card and using the colour sampler tool to pick out the average RGB values from a few spectra images with the wavelength scale or peak positions marked.

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