Bahtinov Mask for Celestron 11" RASAThe Bahtinov will produce Cross-shaped stars, with one clear line that moves along the cross as you focus. When the line is centered on the cross, you should have achieved perfect focus.
Star in focus via a Bahtinov mask
How it Works
The slits in the mask are the focus assist, when the star is in focus it will appear on your screen display as an “X” with a line through the center (see above).
These mask will work equally as well on the end of a scope or on the end of a dew shield on top of that the mask itself does not need to be perfected centered on the scope this is due to the angle of the slits themselves.
Select a bright star, this will be your guiding object. Place the Bahtinov focusing mask on your telescope’s lens and secure it into place using the provided rubber tabs.
The slits in the mask design create a set of diffraction spikes on the star that will assist in the accuracy of our focus. It will appear on your DSLR display screen or computer as an “X” with a line through the center (when in focus).
The diffraction spike pattern from a bright star using a Bahtinov maskAs you adjust the focuser of your telescope, the central spike will move in and out of the X-pattern. If you are using capture software be sure to compensate for the lag time between exposures on the screen, typically 4-5 seconds.
The same process applies using Live view display on your DSLR camera. The difference is, the image will be much dimmer, as you are essentially looking at a live video feed rather than a short image exposure.