If you are an astrophotographer, or at least trying to be one, you probably know by now that for a good picture of the night sky you really need to get away from urban lights as far as possible in order to get nice and clear images. Light pollution is pretty much every astrophotographer’s nemesis.
As mentioned above you can get away from city lights or use photo filters. There are several filters that help with light pollution, yet at the price they are sold yo might want to re-think the getting away from the city lights idea. A cheaper version of the light pollution filter are the didymium glass filters, specifically the red enhancer filter. You would use red enhancer because the way the didymium glass works it will enhance the red colour of the spectrum but reduce the adjoining yellow and orange, which is good because almost all cameras filter out the red colour thus many details are lost form red-emitting deep sky objects. However do not expect miracles when it comes to enhancing the red colour from nebulae and such.
The images below are both shot at ISO-1600, f6.3, 30sec., 35mm focal length. Straight from the camera no additional editing. All images are shot from a fairly populated area, no much tall buildings, but still with all the light population that a town with 40.000 inhabitants can create.
This is another example, the only difference from above is the exposure is longer, 60sec.
As you can see the longer the exposure the effect of using the didymium glass as diminishing, but still the result looks pretty amazing than not using the filter.
HOWEVER, I noticed that the images made with the didymium glass filter had less stars, even though they appear to be more stark and sharp than the ones without (because those had all that red-to-pink haze all over). So, I took my try out a step further.
I applied the same edits on two images in Lightroom, cropped a slice to 100% and the result is below, as you can see the image shot with a didymium glass seems sharper and with greater contrast, but also less stars or fainter than the image without the red enhancer filter.
The conclusion would be that it’s a matter of choice. It would be best to get out as far as possible form city lights where you can get nice dark to pitch black skies. But if you can’t always do that, it might be advisable to use the didymium glass at least if you plan on capturing star trails or similar composite images.
For deep sky objects and Milky Way images, I don’t think it’s helpful much but, again, there’s the “red enhancer” part which is appealing since almost all DSLR cameras filter out the red colour of the light spectrum, hence you can’t capture the emitting red lights of nebulae. BTW there are at least two models that are astrophotography specific, or if you can find the person to do it, you can modify your camera not to filter out the red light, I am assuming that would probably ruin your camera for taking other pictures though.