Using a Bahtinov Mask
1: Attach your camera to the telescope, center a fairly bright star in your telescope and achieve rough focus.
2: Place the Bahtinov mask over the front aperture of your telescope by “hanging” it on the three plastic lugs from the front dew shield.
3: Take a short exposure. The image of the star will show diffraction spikes (see image below).
4: The “center” set will most likely be shifted to one side or the other. This is showing the defocus.
5: Adjust the focus of your telescope and re-expose the star, once again showing the diffraction spike pattern.
6: Repeat this process until you get the central diffraction spike centered against the star and the other spikes.
7: When you achieve a symmetrical pattern, you have achieved the best possible focus. Remove the mask (which easily slips off), center your target object, and take your images!