FarPoint Astro Bahtinov Focus Masks make focusing for astrophotography easy. Works with all DSLR cameras or CCD imagers and is great for cameras with or without liveview. Simply place the Bahtinov Focus-Mask over the end of your telescope, find a reasonably bright star near your target, take a 3-5 second picture and view the diffraction spikes. Change your focus, take another picture and again, view your diffraction spikes. Is the center spike moving further from center? Then change the direction your focusing and try another picture. Watch the direction the spikes are moving and when the center spike passes the middle of the other two spikes, change direction and fine tune your tuning knobs until the center spike is in the middle. Your now in perfect focus. They are strong enough and light enough to fully support themselves whether you hang them from the end of a rigid dew shield or from a plastic dew shield attached to the end of an SCT. These are not paper thin masks. Bahtinov masks are made with light weight, thick, yet resilient plastic, will not break and will bend only when forced. You need not remove the dew shield to use these masks, just hang the mask from the end of the shield. This enables you to check your focus as frequently as you like throughout your imaging session. Our masks are designed to match the focal length of your scope. This focus mask was designed for 2.5" to 4.5" (65-114mm) dew shield diameters.