In last week’s BINTEL newsletter (you can read it here), I mentioned the Optolong L-Quad Enhance filter. This is a broad band light pollution filter that cuts out a very narrow range light from street lights and lamps while letting most of the visual light spectrum through. This make it a good choice for images of objects like star clusters, galaxies and emission nebula through a one-shot colour camera while at the same time keeping a very natural tone to the image.
This week we’ll touch on the Optolong L-eNhance and L-eXtreme filters, what they do and why you’d use them.
What’s the H-alpha, H-beta, OIII numbers and letters?
(A little bit of astrophysics about nebulae here. Don’t worry – it won’t be too scary!)
We can see several types of nebulae in the sky.
All nebulae are made up of thin, vast clouds of gas and dust. For an astronaut, they’d be as close to a vacuum as we could measure but they’re definitely “there”. Over time these super thin interstellar clouds are drawn together by their own gravity and collapse into a star. This is how our own Sun was formed some 4.6 billion years ago.
The light from stars close to these nebulae does a few things. It either bounces off the gasses and what we see is a “reflection nebula”. It can also be absorbed by the gasses. These gasses can’t keep absorbing the energy from the light and energy it carries forever. Something’s gotta give!*
The electrons in the gas atoms – and remember hydrogen is a very simple molecule with just one electron – will change energy in specific increments and eventually release the energy and fall back down to a prior level, at the same time emitting a photon. When it does that it emit light photo at a specific wavelength. There are several energy levels the electrons will jump to and fall back to. This means that no matter the wavelength of the light its energy it absorbs, the wavelength or colour it emits is very specific.
The terms H-alpha, H-beta etc refer to the wavelength of light produced by these the changes in energy level. O-III emission** is produced by a different process, but it still only emits light at a single wavelength.
H-alpha emissions are wavelength in the deep red part of the visible spectrum are are responsible for the red and pinkish colours in gas clouds like this image taken by Rob Watson and posted to the BINTEL Society Facebook page.
Planetary nebulae are the remnants of an exploded star that are spreading out into space. They’re illuminated by central collapsed star which are usually small and hot. The chemical process in these nebulae are more complex and another wavelength of light – O-III – is emitted. This is seen as a greenish colour and this in a region where are our eyes are quite sensitive to.
Light pollution arrive in wide variety of wavelengths caused by a multiple sources. The Optolong L-Quad Enhance filter stops light from major sources of light pollution and lets remaining wavelengths through. The Optolong L-eNhance filter takes another approach – it blocks a large chunk of light apart from the H-alpha, H-beta and O-III processes through.
This means that a colour camera*** will mainly see light produced by the processes mentioned that occur in emission nebulae and planetary nebula.
As an example, here’s the Optolong L-Quad Enhance filter:
You can see how is transmits most of the light. It’s only blocking light around common light pollution sources.
See how this compares to the Optolong L-eNhance filter.
The Optolong L-eNhance will only let light around the H-alpha, H-beta and O-III wavelengths. It’s not simply a darker filter!
Going one step further, the Optolong L-eXtreme filter only lets through light around the wavelengths produced by H-alpha and O-III, not H-beta. This further trims down the light let through and handy for imaging certain astro objects.
Again, not just a darker filter, narrower frequencies of light being let through.
Q: Why can’t I used a filter like the Optolong L-eNhance or L-eXtreme filter for safely viewing the Sun in its H-alpha light?
This is because these filters let through the light the around these wavelengths. Light at specific frequencies and a little either side of them. There’s so much light coming in from the Sun, even this small amount of extra light can be very dangerous! The only way to safely view the Sun is either through a white-light filter which fits over the front of the telescope and vastly reduces the amount light of *all* visible wavelengths, or through a dedicated Solar telescope.
To wrap, up there’s no cure for light pollution, but there are a number of options to help reduce its effects. Have a chat with BINTEL about what you’d like to observe and image and what can be used to help.
* Yes, I know. This is a major oversimplification! There is a vast amount of information about these processes on the web or contact me and I can explain it further.
**O-III emission are produced by collisions of electrons, rather than electrons falling back to lower levels as occurs in H-alpha and H-beta. Oxygen is a only a tiny fraction of the gasses in nebula, but because these collisions are far more energetic than energy level changes, more light is emitted
***This filter can be used with a mono astro camera as well