How do I determine the size of filter to order? Spectrum Telescope filters slip over the sky end of the optical tube of the telescope. To get the proper fit, measure the outer diameter by using a caliper, ruler, or tape measure. Measure the widest distance of the telescope end starting from the exterior of the casing, across the center and to the exterior part of the casing of the other side. After you have your measurements, you should order a size larger than what you measured. This is because your measurement is being compared with the solar filter ID (inside diameter). For instance: If you measure 4″ across the optical tube end needs a 4.25” size solar filter. This will make it just a little bigger than the measurement you took. Your O.D. Measurement: 4 1/8” (105mm) to 4 3/8” (111mm)