Parks ALP filter (broadband, 2") New $150 My Price $110
As described on Parks website:
" Anti Light Pollution
These ALP (Anti Light Pollution) filters utilize state-of-the-art multi-layer coating technology to selectively filter out the mercury vapor and sodium light pollution. These filters are of tremendous value in the city since they block man-made light, and in dark sky locations they block neutral oxygen transmission (natrual sky glow). ALP filters are not used for Lunar, planetary or terrestrial photography. Threaded to fit eyepieces and camera adapters. "