If you peruse posts on Cloudy Nights and other forums on the web, you’ll find claims that narrowband filters don’t work well with small scopes, that you need at least 8 inches (or 10 inches, or 12 inches…) of aperture to get any benefit from them. I’ve never agreed, having used the UltraBlock to advantage with my 4.25 inch (108mm) Newtonian to see through local light pollution.
Recently, I put this hypothesis to the test with my 4.25 inch (108mm) Newtonian on a night with particularly bad haze and light pollution. It was a night on which 3rd magnitude stars were hard to see. I found that planetary nebulas which were invisible in the 4.25 inch showed up relatively clearly using the UltraBlock. So as far as I can tell, you can use the UltraBlock filter to help you see through light pollution just as effectively with small apertures as with large. Here are the details….