Getting some blue data for narrowband stars - H-beta or RGB blue - Ha OIII Hb as RGB - would that make sense?

[Daniel Karl]

Hey all ... it's cloudy and I'm waiting for a new flattener, so I got some time for stupid ideas

I'm imaging from Bortle 9 London, so anything but NB doesn't really get me great results. I'm surprised with the SHO I can get from here, and so far just stayed away from objects that
look better in RGB.

But sometimes I wonder if I could get slightly less ugly star colours if I could get some blue data ... H alpha,  O III and H beta as a kind of Narrowband RGB for stars.

Then again, since stars have a more or less continuous spectrum, narrowing the blue down to H beta would only give an advantage if my light pollution somehow were less in 
H beta than in the rest of the blue spectrum, otherwise I could just go for a blue filter? Then again, again, a H beta might be a better fit and also give some nice details on the odd object?

What do you think? Not worth or worth a shot? Am I missing something?

 

Cheers,

Daniel

 

 

[vlaiv]

Depends what you want to achieve.

If you just want to add Hb data and get some star colors - well, they will still look artificial / strange / maybe ugly.

You can extract true star color with Hb + the rest, but the thing is - you already have enough data to extract actual star color with only two components - Ha and OIII. That is enough data to calculate proper star color, but that involves quite a bit of math and understanding how things work.

Principle would be like this:

- derive temperature from Ha / OIII ratio for your camera + filters (this includes calibration from actual star as well as using Plank's law - approximating stars as black body radiators).

- Create starless version of the image in NB and compose that.

- Use stars from Ha exposure (nice, tight not overly stretched stars)

- Color them according to Ha/OIII derived temperature and temperature to sRGB ratio

- Blend

In fact, as a first step - I'd recommend just using white stars. That will remove dodgy star colors and make NB image more natural. You can even color them the same - slightly yellowish and you won't be far off from actual color.

[Daniel Karl]

@vlaiv I had not even thought about it that way (which shows that after 20years there is no trace of my physics degree left in my brain).
I actually did what you suggested on my last picture and just added the stars from my Ha stack to the starless SHO - I guess aesthetically
proper RGB stars wouldn't look great in this anyways.
 

(please ignore the star shapes - this is without a field flattener - the one I am waiting for

 
 

I was just wondering if it would help to have some blue data for some objects ... I tried to get M81 just to see how it looks, but only 800mm, London seeing and light pollution and a full moon was a bit too much I guess - 🤣

I should probably also take flats in future ... 

 

 

 

 

 

 

Edited January 19, 2022 by Daniel Karl

[ollypenrice]

I would only put RGB stars into a NB image that was replicating, fairly closely, an RGB image. I guess that would be HOO.

Then we need to think about why blue stars are blue. Are they blue because they produce more H beta emission? Not so far as I know, but I'm not a physicist. Wien's Displacement Law links the colour of blackbody radiation to temperature so all its radiation is displaced towards blue as its temperature increases. This makes me doubt that the proportion of H beta emission is causing the blueness of blue stars. I'll happily stand corrected.

Olly

[vlaiv]

41 minutes ago, ollypenrice said:

I would only put RGB stars into a NB image that was replicating, fairly closely, an RGB image. I guess that would be HOO.

Then we need to think about why blue stars are blue. Are they blue because they produce more H beta emission? Not so far as I know, but I'm not a physicist. Wien's Displacement Law links the colour of blackbody radiation to temperature so all its radiation is displaced towards blue as its temperature increases. This makes me doubt that the proportion of H beta emission is causing the blueness of blue stars. I'll happily stand corrected.

Olly

Not quite sure what you are saying here, but yes - star color is consequence of their emission that is very well approximated with black body curve.

In principle, you can derive transformation for Hb, Ha and OIII data that will produce proper star color - simply because you can produce temperature from three points on this curve (I think that you can with two as well).

Yes, it needs only two points:

Here we can rearrange expression so that T is function of B_lambda (measured value, all other are constants) and we can calibrate on a star of known temperature - and it would take only one channel to derive star temperature if all stars were the same brightness, but since they are not, we need two measurements to solve for two variables - T and brightness.

[ollypenrice]

2 hours ago, vlaiv said:

Not quite sure what you are saying here, but yes - star color is consequence of their emission that is very well approximated with black body curve.

In principle, you can derive transformation for Hb, Ha and OIII data that will produce proper star color - simply because you can produce temperature from three points on this curve (I think that you can with two as well).

Yes, it needs only two points:

Here we can rearrange expression so that T is function of B_lambda (measured value, all other are constants) and we can calibrate on a star of known temperature - and it would take only one channel to derive star temperature if all stars were the same brightness, but since they are not, we need two measurements to solve for two variables - T and brightness.

My point is really that any star-sized dot in the blue channel could be adjusted to give the right level of blue. Couldn't you do this just as easily with Ha as with H beta which traces the same gasses but with lower signal?

Olly

[vlaiv]

3 minutes ago, ollypenrice said:

My point is really that any star-sized dot in the blue channel could be adjusted to give the right level of blue. Couldn't you do this just as easily with Ha as with H beta which traces the same gasses but with lower signal?

Olly

Ah I see.

No. Because with stars all the light is produced by mostly hydrogen but it is different from emission of nebulae like Ha and Hb.

Emission lines in hydrogen come from energized electrons that jump energy levels in atomic hydrogen in nebula. With stars - process that generates light is much different - it is due to thermal motion of gas in outer layers and also due to fusion happening in the core (photons created that way are vastly more energetic than visible light but by the time they reach surface - they scatter so many times and give of so much energy to atoms in the sun that they blend in to thermal spectrum).

Funny thing about stars is that we don't have emission lines - but rather the opposite - we have absorption lines. If we observe spectrum of stars - we will see dips rather than peaks in Ha and Hb - simply because photons from thermal process that are on those frequencies are more likely to bump into hydrogen atoms and be absorbed. See this post as example

Process that forms color of the stars is the same one that happens here on the earth when you start warming up piece of iron.

Depending on temperature - it will glow with different color. Star of a given surface temperature has the same color as piece of iron on that temperature. We never really see blue color in iron because it needs to be much higher temperature (above 7000K).

With nebula it is fair to assume that large parts of it will be energized the same and ratio of Ha/Hb does not change much, but with stars - we don't actually record Ha/Hb light - but rather continuum around that wavelength. We exploit the fact that NB filter is not infinitely narrow and has some FWHM - like 3nm or 7nm - and record "around" Ha/Hb.

Ratio of spectrum continuum around Ha and Hb will depend on color temperature of the star and that is the information we can use to deduce actual color.

[gorann]

Thanks for that @vlaiv, it does explain why we have a sun that gives us here on earth sunlight with what we call a continuous spectrum (maybe with those dips you point out - or does it?).

[vlaiv]

1 hour ago, gorann said:

Thanks for that @vlaiv, it does explain why we have a sun that gives us here on earth sunlight with what we call a continuous spectrum (maybe with those dips you point out - or does it?).

Yes, sun also has spectral features.

(source: https://www.esa.int/ESA_Multimedia/Images/2017/12/Solar_spectrum)

 

[gorann]

rather good match with the human eye spectral sensitivity (http://www.yorku.ca/eye/photopik.htm), as expected since we evolved under this star. But is is striking how much of the red and IR part of the spectrum our eye misses.