Black Polymer Solar Filter for Telescope, fits ETX90, C-90, Questar 3.5, Wm. Optics ZS80II ED, Orion Obs. 70 & Astroview 90, Orion Explorer 90 108mm\4.25" Black Polymer Solar Filter for 90mm\3.50" Telescopes. This filter should fit the following telescope models: C-90, ETX 90, Questar 3.5, Wm. Optics ZS80 II ED or telescopes of similar tube diameter. Part #B4250. Please read carefully: 108mm\4.25" is the diameter of the filter. The filter fits like a cap by friction. To be absolutely sure that the filter will fit, please measure the outer diameter of your instrument and make sure that it is not larger. Black polymer is the most common filtering material for observing sunspots and granulation, through telescopes and binoculars. The sun will appear in a natural orange color when viewed through your telescope using this filter. This is a high quality product of Thousand Oaks Optical, Arizona. A manufacturer of safe solar filters for over 30 years. Words of the producer: "Stronger than Mylar with the filtering properties protected within the substrate. Guaranteed five years." WARNING: ALWAYS MAKE SURE THE FILTER IS WELL ATTACHED TO THE TELESCOPE/BINOCULARS/CAMERA. IN CASE NECESSARY, USE STICKY TAPE TO SECURE IT. NEVER OBSERVE SUN WITHOUT A PROPERLY ATTACHED FILTER!