I couldn’t find much info on the effects of a didymium filter / red intensifier for astrophotography on the internet. So after buying one I decided to run a few tests to see how they perform. In short, they’re a cheap and extremely effective way to cut out light pollution and enhance colours in the night sky.
Virtually all of the astro images you’ll see on this website have been captured using a Kenko Red Enhancer. The filter has been an essential bit of kit, helping me get some pleasing results despite the images being taken from a heavily light polluted area of England.
The comparison below shows the effect of the filter on two photos taken straight from the camera without any adjustment (white balance set to daylight). You can see how effective the filter is at cutting out the orange background glow. Using the filter does slightly reduce the light gathering power (some of the fainter stars are more difficult to pick out) but this can be compensated for by using a slightly longer exposure.
The next comparison shows the North America nebula region after some post processing in Photoshop. The filtered shot (left) clearly has more red colouration in the nebula, as well as keeping the colour in many of the orange/red stars. The result from the non filtered image is much more washed out, having removed most of the orange colour from the shot.
For a relatively small outlay, the filters are definitely worth using if you live in a light polluted area. I’m not sure if the filter would give much benefit in a dark sky location. I will have to get over to the North York Moors or the Dales to do a real test!