the proper name is NIR cutoff or blocking filter first optical LPF(LPF1) is anti-aliasing filter ( to reduce Moire effect) ( can be 2 glasses) and where dust removal piezo is attached to IR cut filter or ICF( sometimes called LPF2) is the filter to be replaced for IR conversion types of IR cut filters: transmissive (left) and reflective (right). The transmissive type is also commonly called blue glass. The top 2 photo is taken directly over the filters, and bottom photo taken slightly slanted common ICF are BG38 ( QB21) and BG39 (QB39) Schott BG18 spec S8612 BG40 and some others in order to do infrared photography, it is needed to replace this IR cut filter with a IR band pass filter: Eg GG420(JB420) 420nm to >2500nm or BCF700 400 to 710 nm note the difference of Ha increase nebulosity bright stars blur colour doped filter (old) vs sputtered interfering filter : The transition from blocking of light to transmission of light is lazy with color...