Narrowband question

[Galaxyfaraway]

Please help me understand: these amazing colours you see on nebulae taken with narrowband filters, are these colours real or made up? I had this argument with my better half...She seems to think since people mix & match colours (and even synthesize them), one might as well paint your own nebula. Perhaps it's a non-sensical question in the first place since what's 'real' to human eye is also subjective...But if you stood in front of one, would you see all these amazing colours?

[ChrisLX200]

Well it's 'real' in as much as the different colours represent real differences in chemistry - the filters are specific to certain wavelengths of light emitted by ionised Hydrogen, Oxygen and Sulphur. However, the colours (or palette) used to display these different filtered channels as a composite image are purely arbitrary where the emphasis is placed on their ability to differentiate structure when viewed by the human eye. Natural colours they are not. The process is called 'colour mapping' and as the name suggests certain colours are assigned to each of the narrowband channels. So, the Hubble Palette (for e.g.,) is essentially Hydrogen=Green, Oxygen=Blue, and Sulphur=Red.

ChrisH

[pipnina]

Please help me understand: these amazing colours you see on nebulae taken with narrowband filters, are these colours real or made up? I had this argument with my better half...She seems to think since people mix & match colours (and even synthesize them), one might as well paint your own nebula. Perhaps it's a non-sensical question in the first place since what's 'real' to human eye is also subjective...But if you stood in front of one, would you see all these amazing colours?

you can use narrowband is different ways:

1: use it to boost the nebula's presence in LRGB images (e.g. OIII gets merged with blue and green, SII and Ha with red)

2: Use it to make up the hibble pallete using SII, OIII and Ha

3: Use it to make bi-colour images (e.g. SII for red and OIII for blue)

Only one of those will result in an image that's looks "natural" and that's the first one. If an image looks pale brown with green and blue it's probably HST and therefore false colour. Pink stars is usually a symptom for some reason.

[MarsG76]

You could also image Halpha in red and either OIII in Blue and Green channels ... OR  OIII in green and Hbeta in blue channels to simulate natural colors using just narrowband data.

In regard to the last statement, you wouldn't actually see any color if you were "right in front of one" since the nebula would be much more spread out and you most likely would see much less, if any nebulosity at all... remember that when you're looking at a nebula from a distance, you're seeing lightyears of nebula in a small area... much like if you defocus a star, the more out of focus (or spread) the light is, the fainter the star looks, same effect if you would be closer to the nebula and see less "light" concentrated into the same size area.

[Thalestris24]

Hi

H-alpha emission nebulae are naturally red, OIII green and SII far red. However they are all too faint to be seen by the naked eye. H-alpha and OIII can be picked up in their natural colours by a dslr (or an osc ccd) with long enough exposure times e.g. the Orion Nebula (M42) is quite bright, and the Ring Nebula (M57) emits a mix of colours including H-alpha (red), OIII (green) and H-beta (blue). An astro modified dslr is more sensitive in the red part of the spectrum. Much better discrimination can be achieved using a mono ccd and narrowband filters but in that case colours are assigned during post processing.

Louise

[Herra Kuulapaa]

There is always an option to do something really different by mixing RGB channels and narrow band data, although the results can be sometimes slightly weird

http://www.kuulapaa.com/Tahtidata2/M42_Outo_800.jpg

[glowingturnip]

In regard to the last statement, you wouldn't actually see any color if you were "right in front of one" since the nebula would be much more spread out and you most likely would see much less, if any nebulosity at all... remember that when you're looking at a nebula from a distance, you're seeing lightyears of nebula in a small area... much like if you defocus a star, the more out of focus (or spread) the light is, the fainter the star looks, same effect if you would be closer to the nebula and see less "light" concentrated into the same size area.

Indeed - your average nebula (apart from the globules) has a concentration of about 100-1000 molecules per square cm.  Compare that to 10^18 per cm3 for air, so a million billion times more rarefied.  So you probably wouldn't see anything even if you were right in the middle of one.

They're also very very cold - the Eagle nebula runs from about 10 Kelvin for the cold bits up to a toasty 40 Kelvin for the hot bits. (apart from the globules)

[Starfisher]

Hi

H-alpha emission nebulae are naturally red, OIII green and SII far red. However they are all too faint to be seen by the naked eye. H-alpha and OIII can be picked up in their natural colours by a dslr (or an osc ccd) with long enough exposure times e.g. the Orion Nebula (M42) is quite bright, and the Ring Nebula (M57) emits a mix of colours including H-alpha (red), OIII (green) and H-beta (blue). An astro modified dslr is more sensitive in the red part of the spectrum. Much better discrimination can be achieved using a mono ccd and narrowband filters but in that case colours are assigned during post processing.

Louise

so you can make some kinda rgb-narrowband combination actually? intresting