With all substrates fineoptically polished. These filters by far supersede the mediocre quality of common `flame polished` color filters. Fineoptical polish and 2 mm thickness allows these filters to be inserted anywhere into the optical path without introducing astigmatism or other defects.
A unique filter for many different astronomical tasks. Incomparable in accracy to combinations of generic colour filters made of raw polished glass – where surface inaccuracy adds up in the optical wave front to an unbearable degree!
You can now use this filter to focus perfectly with a bright image and since they are also parfocal, switch to a different narrow band filter without loosing the focus.