Astrozap Focusing Cap for Your 6-Inch SCT with Outside Diameters of 178 to 190 mm: 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. Simply turn the thumbscrew to the open position, and turn to close. You can forget the plastic plugs and extra tools!
Astrozap focusing caps are made from lightweight aluminum and powder-coated texture black. Astrozap Product Number: 423