... Baader has a filter that mimics a gold transmission called the "495 Long-Pass filter".
It might be worth a shot.
While at a dark site (Rocky Mountain Star Stare) last week I decided to try out this idea on the Lagoon Nebula (M8/NGC 6533) in Sagittarius. I was using 14-inch optics at about 75x with dark skies of ~22.0 MPSAS zenith (per my SQM-L device) around midnight. Unintentionally, I first "bleached" my eyeballs by viewing this object unfiltered.
Upon using a 610nm red long pass filter, I immediately noticed the red-colored dots of light, however there were much fewer stars (only about 1/2 dozen), no nebulosity, and the overall view was very dark. After several more minutes of observation, the number of stars seemed to double, with more faint stars becoming visible, as well as being able to see a dark lane dividing the east and west areas of stars. Upon still more viewing, some very faint nebulosity glow of the bright western area (NGC 6523), but not the eastern area (NGC 6526), was noticed. The vivid red dots reminded me of viewing carbon stars R Lep and T Lyr.
I then tried using a 495nm yellow long pass filter (since I had one readily available), but found it was not effective in reducing nebulosity.
The results of this experiment were very pretty/colorful, but not too encouraging as a means of removing just nebulosity from a star cluster, because the view was very, very dark and many stars of a cluster are also filtered out. [when using a red filter, and unsuccessful when using a yellow filter]
Edited by JimK, 03 July 2019 - 06:51 PM.