Film Solar Filter ST425BP1 Your O.D. Measurement: 3 7/8” (98mm) to 4 1/8” (105mm) Telescopes that fit this Filter size: Meade ETX 90; Celestron: NexStar 60/80/80GT, FirstScope 90EQ; Orion: StarMax 90, Apex 90; StellarVue: NightHawk APLANAT, NIghtHawk Next Generation (with dew shield), SV809D (with dew shield); Questar 3.5; Williams Optics: ZS80 II ED, Megrez 80 Film Solar Filters These full aperture film are safe solar filters that permit the most possible light through the telescope which allows your telescope’s full aperture to be viewed. The daytime viewing is at its best with the full aperture when the atmosphere is least turbulent. Made from .002” thick black poly plastic, the optical thin film is durable and with proper care and normal use, can last for many years. Just as the off axis solar filter and full aperture glass solar filters, there is an aluminum cell securely attached to the telescope’s tube end by a nylon thumb screw. Made from high quality, the clear aperture is stock with optical thin film that has been thoroughly inspected. This is a durable, high quality, thin film solar filter that you’ll find anywhere on the market. It provides a natural yellowish-orange image when viewing and can also be used for viewing directly through the telescope and/or for photography. This filter comes packaged in a foam-lined box that is placed in plastic which absorbs any possible rough handling and shock during shipping. The box is also great for storing your filter when you aren’t using it. By following the instructions provided with the unit for care and use, you will have many years of use with this product. 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. Are the glass and thin film solar filters safe?Yes, all of our filters are safe, blocking up to 99.99% of the sun’s rays, perfect for unlimited visual use as well as photography. These filters are rated with a neutral density of 5. With proper care and handling, our filters will last for many years. All Spectrum Telescope filters meet ISO Certified 12312-2 while meeting or exceeding International Standards for safety. What can I expect to see using these filters? You can expect to see a solar eclipse, sunspots, and surface granulation.