My understanding is that the filter allows the transmission of wavelengths of light typically associated with emission? nebula whilst blocking some other wavelengths. From an observing experience perspective it appears to enhance the contrast (for the loss of some brightness), which results in more detail being visible. As examples, I've found that the filter allows you to make out more detail in the carina, tarantula and lagoon nebs than would otherwise be visible, with my 10" dob anyway.
I'm a newby so would fully recommend doing some research to satisfy yourself before diving in!
Ben
Last edited by Bendy; 16-06-2017 at 01:14 AM.
Reason: Mistake