Additional filters for astrophotography

Various filters have a long tradition in astrophotography. UHC or narrow bandpass filters in particular have been around for decades and have a permanent place in every filter box. These filters can usually be combined with converted cameras without any problems. It often does not matter whether the camera has been converted to full spectrum or whether a "real" astro filter is installed - only with wide-angle lenses is not every filter compatible or useful.

Many of these filters are designed to attenuate the light pollution of our industrial world. In order to have as little influence as possible on the image and the colors, certain spectral ranges of known light sources (e.g. mercury and sodium vapor lamps) are specifically attenuated. However, as our (street) lighting continues to switch to LEDs with a continuous spectrum, these filters will become less and less effective over time.

UHC and H-alpha filters

The omnipresent light pollution cannot be ignored and is not only a problem with a converted camera. Many filters have been brought onto the market to suppress disturbing spectral lines. Fascinating results can be achieved with the help of these filters - for example, a narrow-band H-alpha filter can produce usable results even on a full moon night. When you hold filters of this type in your hand, they do not look like glass but have a metallic character - a metallic reflective surface, sometimes with a color cast, is characteristic.

Even if these filters were designed and intended for unmodified cameras, they can usually be used for a full spectrum or astro camera without any problems. The only limitation is the angular dependence of the filters, which also causes problems with unmodified cameras (you can read more about angular dependence in the "Filter selection" section). Typically, these filters are used in telescopes or telephoto lenses, where the application is unproblematic. As already mentioned, the application can also make sense with a modified camera, depending on the shooting scenario. These filters are only incompatible with wide-angle lenses.

Filters against light pollution

There is another type of filter on the market that aims to reduce light pollution. So-called light pollution filters work on an absorption basis; when you hold them in your hand, the glass looks transparent and shows a blue or violet color cast. The filter effect is generally milder than with UHC filters, the light pollution is not completely blocked but "only" attenuated. Unlike UHC filters, for example, these filters are not angle-dependent and can therefore also be used on wide-angle lenses without any problems.

These filters are designed to suppress the visible spectral ranges of light pollution. However, practically all light sources also have a good proportion of infrared light, which they emit into the night sky. These IR components are not suppressed by the light pollution filters and can reach the sensor unhindered. This means that such a filter is not a solution for a full spectrum camera, which would still show greenish light pollution in the sky and magenta-colored foliage in the foreground with such a filter.

If the camera has been converted with a "proper" astro filter, then this effectively suppresses the infrared components. An additional light pollution filter can then noticeably help to reduce the light pollution and thus increase the contrast between the sky background and, for example, the Milky Way. Of course, all this has its limits and you can't expect miracles. There is no more important measure than finding the darkest possible place to take pictures, but these filters do help a little. Almost all landscape images that you take in the Astro Gallery were recorded with such an additional filter.

Clip-in filter

Clip-in filters are the generic term for small filter glasses that are located in a special mount and are installed directly between the sensor and lens, i.e. in the bayonet of the camera. They can be replaced by the user and are therefore very popular. However, a much greater advantage of these filters is that the light rays between the lens and sensor occur at a very limited angle and the angular dependence of the filter effect of many filters is not a problem. In short: you could use astro filters or UHC filters here without risking a color shift to the edges of the image.

Unfortunately, in practice, a clip-in filter consists of one piece of glass and therefore always changes the optical path. The filter thickness between the sensor and the rear lens of a lens is important and is included in the optical design of the lens. With normal and telephoto focal lengths, a small deviation is often not critical, but with wide-angle lenses this will inevitably lead to blurred edges and deformed stars. You can improve these image errors by stopping down, but this is of course only an option for daytime shots.

We therefore do not recommend clip-in filters, neither for modified nor for unmodified cameras, and instead recommend a dedicated astro conversion with a matching astro filter. This is the only way to guarantee the best image quality, especially with fast wide-angle lenses.