Unlike a simple Hartman or Bahtinov mask, the Astrozap focusing cap is a multi-purpose tool which allows for quick 'shutter-like' operation. Just turn the wheel to the open position for focusing, and then turn it again to the closed position to keep the dust out, or to image dark frames with your CCD or DSLR camera.

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. Simply turn the thumbscrew to the open position, and turn to close.