Posted on Feb 18 2012 at 03:19:54 PM in Astronomy
Sharp focus is critical to get good pictures.
I use an electric focuser because it adjust focus without introducing vibrations.
To fully determine the exact point where the image is focused I use a Hartmann mask with four triangles, because diffraction peaks generated greatly facilitate the determination of exact focus. You can make a template for this kind of mask here.
Twin diffraction peaks merge when they reach the focus.
This mask is a marvel when it comes to focus the camera, as it not only is more luminous than Hartmann mask and clearly shows when you reach exact focus, but also you can clearly see which side of focus you are at all time.
You can make a custom template for this type of mask Bahtinov Grabber (freeware) to help excact focus detection.