Testing a cheap light-pollution filter

My backyard sits in a solid red zone on the light-pollution map. The promise of a budget broadband filter is that it clips the worst of the sodium and LED glow and hands back some contrast. Does it? I shot a few matched frames to find out.

1. what it helps

On emission nebulae the difference is real. The same target, same exposure, came back with a visibly darker background and more obvious structure. Broadband filters work by rejecting the specific wavelengths streetlights pump out, and there is a lot of that here.

2. what it doesn't

On galaxies and star clusters it does almost nothing useful — those emit across the whole spectrum, so anything the filter blocks, it also blocks from the target. You just get a dimmer image that needs longer exposure.

nebula  (Ha-rich):  clear improvement
galaxy:             negligible, slightly dimmer
cluster:            don't bother
broadband target:   case by case

3. the honest verdict

For a cheap filter from a red zone: worth it if you mostly shoot nebulae, a waste if you chase galaxies. It is not a substitute for darker skies, but it is a lot cheaper than a tank of fuel to reach them.

misc.

• Filters add reflections. Watch for halos around bright stars and angle the filter if you can.
• Narrowband beats broadband under heavy light pollution, but it costs more and needs longer subs.