There has been chatter around astrophotography message boards for some time now that a photographer can partially eliminate “sky glow” from the night images with a relatively cheap Hoya filter called a red intensifier. It is touted as the “poor man’s astrophotography filter” because it is a good deal cheaper than most purpose made astrophotography filters and it comes in the standard sizes to fit most camera lenses.
Briefly, sky glow is the ambient light given off by street lights, safety lights and other forms of illumination that obscure our view of the stars. The less sky glow, the easier it is to see star. This is why the view of the Milky Way is always better in the country, for example.
Without getting to pedantic, I’ll just say that the majority of sky glow falls into to distinct wave lengths of light that are both roughly in the red area of the visible spectrum. This is why many night shots have a rusty, muddy look. The color is due to the elements used to generate the light (mercury and sodium, I think… I’m no scientist) which each fluoresce at a different wavelength or color. The Hoya Intensifier (also called a didymium filter) is a filter that is designed to photograph autumn foliage and what it does is cut out the rusty orange found in some turning leaves and just let the red come through. This is great for astrophotography because that same wavelength of light also happens to be the one emitted by mercury lights (which make up many street lights). However, it doesn’t cut out the sodium lights so you still have some sky glow to contend with, just less of it. By playing with your image in your favorite post-processing software you can somewhat notch this now dominant red light out as well. It is an imperfect solution but it gets you a little better than half way there.
Last week I went up to my favorite spot for photographing lightning while a storm was rolling in around 2 AM to try to get some lighting shots. There was too much fog to get good photos of the lightning but I had brought along my Intensifier filters so I decided to give them a whirl and see how much sky glow they eliminated in what amounts to a tricky lighting situation. See, the thing about fog is that it refracts light around all over the place, which is why its hard to see when you are driving at night in the fog. The light from your headlights ends up creating a big bright cloud in front of you. Much the same thing happens with city lights on a foggy night.
Below are two exposures taken back to back (each at f/10 ISO 100 for 30 seconds). The first is without the filter the second is with it. You can see that the filter cleans up a lot of the haze in the image but it does make everything look pretty red. The top image in this post was further toyed with in Nik Color Efex Pro 4 and Nik Dfine 2 to try to bring out some contrast and detail. The image was also processed with Aperture to tone down the saturation of the red tones that the Intensifier left behind. I also bushed out some of the more obvious blue shades that show up when you start taking the red spectrum out of the image.
Anyway, I didn’t get any lightning photos but the clouds ended up looking pretty interested and foreboding and the whole scene was generally improved by the addition of the Hoya Intensifier (didymium) filter.
So, if any of you astrophotography folks out there were wondering what this filter did in pretty much the worst possible shooting conditions, you can now see for yourself.
And just for fun, here is a version I edited further in Nik’s Snapseed on my iPad.