I have now been taking astrophotography pictures for 2 years and I have been perfecting my skills, even though there still is a lot to learn. However one always longs for better quality and more definition, or more stunning colors. I had heard that some filters can enhance astrophotographs and it is not rocket science, it is just a matter of physics!
Modern cameras are able to pick up more signal than the human eye can, because of their higher light sensitivity, and possess a slightly wider spectrum of 'visible light', even though they are not able to capture UV or microwaves either. Nonetheless the atmosphere of our planet is a real nightmare for astronomers and astrophotographers, as the different gases it contains block or distort some electromagnetic radiations (EMR) such as visible light. Plus the water vapor and haze blurs up a lot of shots, let alone light pollution.
Astronomers found several techniques to counter these problems. Build telescopes in altitude or in space, or use giant laser beams to correct the atmospheric deviation. In camera-based astrophotography, there isn't such a solution, unless you are Bill Gates. In order to increase the light signal and get rid of as much light and atmospheric pollution as possible, one just simply (it still expensive, don't get me wrong!) need to filter it, as it has its own wave-length emissions.
I recently bought two CLS filters from Astronomik, one for my Canon 70D (EOS-Clip) and one other for my sony (that I will try to mount myself). CLS filters block the wavelengths of the light emission from mercury and sodium vapor street lamps while letting the major part of the spectrum of the visible light pass, and enhance H-alpha emissions (red color found in nebulae). In short, it is supposed to block most of the undesired light and enhance the contrast of deep-sky objects.
I had my first trial in Tenerife, where I tried a first shot with the EOS clip filter and my 70D. Here is the result:
I took 5 shots of the milky way (f/2.8, 16mm, 45'', ISO 3200) without and with CLS filter. I stacked them in Nebulosity 4.0 and post-processed in LR (same settings, except for the temperature that has been adjusted to make up for the color shift caused by CLS filtering).
Conclusion: On the one hand, CLS is very efficient to bring out a lot more detail in the milky way and deep-sky objects (even the faintest nebulae are visible in red). The contrast is way more obvious in the picture with CLS. On the other hand there is way more lens abberation with the filter, and a significant color shift. Plus I probably had to increase my ISO to 6400 because of the light blocking. I briefly tried the filter with my sony a7r2 and 28mm at f/2.0, 2.2, 2.5: not a good idea as the lens distortion occupies 50% of the frame! So the filter is really made for f-stops higher than 2.8! It is also super hard to focus manually with the filter and I wouldn't recommend using it with manual focus lenses. I spent 15 minutes trying to focus (the focus point is shifted) on my Rokinon 10mm f/2.8...
I can't wait to practice more with the filters and try them on deep-sky objects at higher focal lengths! In the mean time here is a last shot, without filter that I forgot in my library and that I took the same night, right afterwards for a time-lapse.