Astrozap Focusing Cap
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.
Astrozap's unique Focusing Cap designs fit most telescopes and performs a dual function. The cap itself contains three holes at a calculated points around the perimeter of the cap. When the center knob is rotated, the inside panel lines up with the holes to create the Bahtinov pattern that admits starlight, creating the unmistakable Bahtinov diffraction spikes that enable determination of the best focus point. Turn the center knob again, and the inside panel rotates, closes the holes, and converts the focusing cap into a dust cap.
All Astrozap Focusing Caps are made in the USA out of solid aluminum, and powder coated to resist corrosion and retain their handsome appearance for years to come. As one imager discovered, it is good to have an Astrozap Focusing Cap handy because one never knows when the electric focuser will quit.