Carey mask

The mask is in the form of a thin card or sheet that is placed over the front aperture of the telescope.

There are four series of slits in the mask which form a diffraction pattern in the image plane.

Any slight error in focus will cause the 'X's to be offset, and this is very noticeable to the naked eye.

The third image is very close to perfect focus as shown by the equal spacing between the elongated spikes on the left and the right.

This indication of the direction of focus change needed removes much of the trial and error that can often be encountered when attempting astrophotography.