The Taiwanese company STC Optical has created a slide-in filter for Canon and Nikon full frame DSLRs that can reduce light pollution in astrophotography. It’s called the STC Astro-Multispectra Clip Filter.
The filter is installed in Canon and Nikon DSLRs by sliding the filter into the mirror box after locking the mirror up in your camera’s settings. After mounting your lens, the filter will sit firmly in place between your sensor and your lens.
Once installed, the filter helps to reduce the wavelengths associated with artificial city lighting while enhancing the wavelengths found in photos of stars (e.g. Hα, Hβ, OIII and SII).
Since the filter requires the mirror to be locked up, you’ll need to use Live View to frame your shots, as your optical viewfinder will be blacked out.
Here are some example photos taken without (left) and with (right) the STC Astro-Multispectra Clip Filter:
Last week we featured a similar product called the PureNight filter. Instead of sitting inside the mirror box of a DSLR, however, the PureNight filter is mounted to the front of your lens.
Source: PetaPixel (https://petapixel.com/2016/12/22/slide-filter-cuts-light-pollution-canon-nikon-dslrs/)