A planetary filter in a metal frame of 1.25 inches. Color # 21, i.e. orange. It can be attached to any 1.25 "eyepiece with a filter thread and has a thread on both sides of the mount (the filters can be combined into color combinations).
Read our guide on filters and their applications HERE
(spectra of various bar filters, # 8, # 12, # 21, # 23A, # 25, # 38A, # 47, # 56, # 80A, # 82A, measurements on a Varian Cary spectrophotometer 100, July 2015)
The connection of practically all astronomical filters except for solar film filters (which are filters for the lens) is achieved by screwing the filter into the frame of the eyepiece from the opposite side than applying the eye (ie from the side that we put in the eyepiece extractor) ). The filters can be additionally connected with each other, because they have threads on both sides of the luminaire.