This Astrozap focusing cap with built-in Bahtinov focusing mask will fit most 80mm aperture fast focal ratio ED refractors, such as the Astro-Tech AT80EDT ED triplet apo, or any scope with an optical tube or lens shade measuring between approximately 101mm (4”) and 110mm (4.3”) in outer diameter. Check your optical tube diameter for certain before ordering.
The inside of the focusing cap is lined with soft foam to protect the finish on your scope or lens shade. Three nylon thumbscrews tighten onto your scope’s front cell or lens shade to hold the focusing cap in place and allow the cap to fit the above range of scope diameters. When not used for focusing, the Bahtinov wheel can be closed as shown below allowing the focusing cap to be used as a dust cap for your scope.
The feature image below will show you a star’s appearance in a Takahashi Epsilon 180 reflector as seen through the Bahtinov Mask. The difference in the focuser position between the almost-in-focus images and the sharply focused image shown below is very small, only a few tenths of a millimeter, showing the sensitivity of the Bahtinov focusing method.
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