Light Pollution? What Light Pollution?
I am a relative newcomer to narrow band or emission line imaging as I have only been concentrating on it for the past 6 months or so. This page is designed as an introduction to what it is and what is involved. For more information and greater detail I would recommend visiting the following two sites
The biggest bane for the astrophotographer can be any of the following:-
Unfortunately it is impossible to do anything about the weather but the other problems can be overcome with the use of narrow band or emission line filters. All of the above apart from the weather are predominantly broad band light and although they can be tackled with broadband or light pollution filters, they are still a problem which is why everybody wishes for dark sites and clear moonless skies.
To give an idea as to how bad the light pollution is where I live, my house is just off a busy main road and I have no view of the eastern sky at all because my house is in the way, frequently my partner (bless her soul!) switches on the inside light which shines right out on the patio where I do my imaging, to the right we have neighbours who have a very bright security light in their back garden which comes on whenever anybody opens their back door, to the left our other neighbour has a movement sensitive security light which comes on if a cat walks across their garden and about 400 yards to the west is a busy tennis club that has bright outdoor lit courts. I have never found my emission line images to be affected by this.
There is however, a type of imaging that can be used even under moonlit skies, in the presence of artificial light pollution called narrow band or emission line imaging. This relies on the use of very restrictive filters which only allow very specific wavelengths of light through and in the process block out moonlight and artificial lighting. This effectively means that there are potentially more nights of the year (weather permitting) where imaging can be done.
Traditional RGB or LRGB imaging relies on the use of red, green and blue filters and sometimes the use of a luminance filter to create an LRGB image, this imaging creates an image with colours that are familiar to us as that is how the human eye sees i.e. in RGB. Emission line or narrow band imaging substitutes the RGB filters with Hydrogen Alpha (Ha), Doubly Ionised Oxygen (OIII) and Ionised Sulphur (SII). There are other emission line filters available such as Hydrogen Beta (Hb) and less common ones such as Nitrogen and Helium (these are usually special order). The first three mentioned here are the most common ones used so for the purpose of this article I will concentrate on those.
Emission line imaging is only generally suited to specific subjects however as follows:-
It doesn't work with reflection nebula or star clusters but that doesn't mean that you can't experiment!
Quite often spectacular images cane be made just using a Ha filter and creating a monochrome image. Below are two images of the same subject taken under similar conditions, i.e. a moonlit sky
The image on the left which was very kindly provided for this article by Justin Dighton was taken with 8x10 minute sub frames with no filter. The image on the right was taken by me in similar conditions using 12x10 minutes with a 13nm Ha filter. As can be seen the Ha filter has allowed more contrast between the dark nebula and emission nebula while at the same time combating the light pollution
Which band width?
You may have seen with narrow band filters the prefix 13nm or a different number, this is referred to as the band width and narrow band filters are available in different bandwidths such as 3nm, 6nm, 9nm and 13nm. The smaller the number the narrower the band width which will greatly increase the contrast between the nebula and the sky background, there is however a downside to this
Focal ratios faster than f4 can cause some problems and the likelihood is you will have to have the filters custom made by companies such as Astrodon or Custom Scientific, this will increase the cost as well. The reason for this is beyond my knowledge and further reading would be advised.
With traditional RGB imaging we are producing colour images that the eye is used to seeing so there is a specific colour convention when combining colour filters and deciding which filter is assigned to which channel, i.e.
With emission line imaging there is no specific convention for assigning which filter to each channel so this leaves quite a bit of latitude for experimentation. The two most common palettes are:-
There are other variations such as the reverse Hubble palette which swaps the SII and OIII around so OIII,Ha,SII = R,G,B
It is possible to use just two filters and create a synthetic channel, e.g. Ha = red, OIII = green and Ha and OIII combined = blue. I have manged this successfully with some images in my gallery, the veil nebula and the crescent nebula being two examples. Below are examples of the same subject with the same data, but just assigning the filters differently when combining
As can be seen from the above images there are some distinct colour variations and in some cases the different colour palettes help highlight different structures in the nebula.
Below is an example of the difference between traditional RGB and narrow band imaging for the same subject, the example in this case is the Soul Nebula - IC 1848, The narrow band image is with the HST palette
As can be seen, the narrow band image on the right shows more of the structure in the different gases of the nebula itself.
* A bit more care needs to be taken when getting the colour balance right and the best results are obtained by taking flats which is a bit more difficult with narrow band filters, although so far I have not used flats with any of my images on this site including the narrow band images.
I hope this introduction is useful and encourages people to give it a go, it is now my preferred method of imaging, not just because of the light pollution, but, also because I really like the end results.
My thanks to Justin Dighton for providing the monochrome B33 image