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Narrowband. A basic explanation, please.


Terrierist
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Not sure if there is simple explanation, but let's try one :D

Light is wave, electromagnetic wave. It can have different wavelengths, and indeed electromagnetic waves have very large range of wavelengths that define their characteristics (radio, microwaves, infra red, visible, ultra violet, x-rays, gamma rays).

Visible light has certain range of wavelengths and each wavelength we perceive as color of the rainbow (when you look at rainbow it is light separated into individual wavelengths).

Normal human vision sort of divides these wavelengths in "ranges" - red, green and blue and cells in our eye are sensitive to those "colors" - or wavelengths that fall into those ranges. Regular imaging tries to capture these as would be seen by human eye.

Light emitted comes from quantum mechanical processes - electrons changing orbits around nuclei of atoms. When electron jumps from high energy orbit to low energy orbit it gives off some energy and that energy is photon of light. Wavelength of photon depends on energy it carries. This is why there is different color to different light sources. Stars emit photons over whole visible spectrum, but some emit more "blue" photons than "red" ones and they appear bluish in color. Others emit more "red" photons than "blue" photons and appear redder in color.

Gas also emits light when energized - neon lights for example, or flames of different color depending on material being burned.

This is key for narrowband imaging - gas in interstellar space emits light via same process. Electron jumps from orbital to orbital. This kind of light is "monochromatic" in nature - always of the same wavelength / color. Narrowband imaging is used to only capture that sort of light - of particular wavelength and block all other light. This way we can observe gas structures in emission nebulae (which just means that gas in them is glowing with this sort of light). Depending on wavelength that we observe/record we can identify particular element - like hydrogen, or oxygen or sulphur (h alpha and beta, O III and SII for example).

So it is very much like regular imaging, but "concentrating" to capture only certain type of light, coming from known source and recognized by particular wavelength (this is how we can be sure that it is coming from that particular element).

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Just to add, narrowband imaging is done with special filters which have a narrow passband or bandwidth, and are usually used with mono cameras. The most widely used filter is the H-alpha one which passes a narrow bandwidth of light centred on the Hydrogen-alpha wavelength of 656.28nm. Others include Oxygen-Iii (O-Iii) and Sulphur-Ii (S-II). Naturally, narrowband filters are more expensive and need longer exposure times. The light from white light sources passes through narrowband filters since white light includes the narrowband wavelengths so most stars can also be imaged through them, as can the Moon.

Louise

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Simple explanation?

Narrowband images are made through filters which only pass light from gasses glowing with their own light. Each filter passes only the light given out by a particular gas. The imager photographs a target through, say, three different filters and so captures three gasses in that target. He or she then ascribes each of the three gasses to its own colour channel to produce a 'colour map.' This is analogous with, say, a geology map with limestone in red, granite in green and sandstone in blue. The colours are artificial but depict the distribution of rocks in the area.

Olly

Edit: This kind of imaging can be done very successfully in light polluted environments and an H alpha (Hydrogen) filter will eliminate a fair amount of moonlight. Monochrome cameras are by far the most efficient in this discipline.

Edited by ollypenrice
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2 hours ago, vlaiv said:

Not sure if there is simple explanation, but let's try one :D

Light is wave, electromagnetic wave. It can have different wavelengths, and indeed electromagnetic waves have very large range of wavelengths that define their characteristics (radio, microwaves, infra red, visible, ultra violet, x-rays, gamma rays).

Visible light has certain range of wavelengths and each wavelength we perceive as color of the rainbow (when you look at rainbow it is light separated into individual wavelengths).

 

THIS!!!

Great to read @vlaiv how do you explain to a newcomer about the RF of light and what is captured in camera?

But for me as a very, very simple beginner, what would be a simple narrowband system?

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Probably a black and white camera and a selection of filters , which you either swap manually or have fitted in a filter wheel .

So you would do a set of explosures in Ha , OIII and SII. and a set in plain black and white then process them all together.

(I've never done this so I'm guessing this is how its done from what I've read ) :)

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Hi

I wouldn't recommend narrowband imaging for a beginner since you generally need to think in terms of long exposures and autoguiding. So you need to have mastered 'normal' imaging and guiding first, really. Having said that, I'm not sure what exposure lengths you can get away with using the sensitive cmos cameras such as the asi1600mm-pro?

Louise

 

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Not wholly artificial.  Ha IS in the red part of the spectrum--and SII is even redder (that's why the HP assigns SII to red and Ha to green).  OIII is in the blue region.  So the colors of the HP (SHO) are tied into reality somewhat

Rodd

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