If you want to capture the beauty and power of the sun, whether through photography or direct observation, you need a solar filter that can protect your eyes and your equipment from the sun's intense light and radiation. That's why we offer you our premium solar filter, which is specially designed for telescopes. Our solar filter is rated ND5, which means that it blocks 99.999% of the sunlight, making it safe for viewing the sun's surface and features, such as sunspots and eclipses. Our solar filter is made of Helios Solar Glass, which is a high-quality glass that has a reflective coating on both sides to enhance the contrast and clarity of the sun's image. The glass is mounted into an aluminum outer cell, which is durable and lightweight. Our solar filter is easy to use and fits most telescopes. It has a clamp-on design that allows you to attach it to the objective lens of your telescope securely. The filter is secured and centered to the telescope with nylon thumb screw(s) and felt tabs, which prevent any damage or scratches to your telescope. With our premium solar filter, you can enjoy the sun's beauty and power without any risk or hassle. SIZING: You must measure the outside diameter of your optical tube or lens before ordering the solar filter. To measure the diameter, you can use a ruler, tape measure, or caliper. You want to measure the widest distance across the end of your optical tube or lens. Measure from the outside of the casing on one side, across the center, to the outside of the casing on the other side. Order the solar filter size that is just larger than your measurement. You are comparing your measurement with the solar filter's inside diameter. For example: A measurement of 4" (102mm) across the end of the optical tube would require a solar filter with an inside diameter of 4.25" (114mm) because it is the size just larger than your measurement. With our Helios Solar Glass filter, you can enjoy the sun's beauty and power without any risk or hassle.