By Don Selle
It may seem obvious, but critical focus is one of the key attributes of a good astro-image. Critical focus is the technical term used to mean that the image is focused to the diffraction limit of the optical system (based on the physical properties of the system and how light interacts with it).
Most of us take critical focus for granted in the images we see and take daily. Our DSLR’s with modern lenses, our point and shoot cameras and our smartphone cameras all come equipped with exquisitely accurate autofocus systems. This technology has been available at the consumer level for over 40 years, first in film cameras, later in digital cameras and has been continuously improved ever since.
Not so for astrophotography. Telescopes don’t come equipped with the type of focus sensors that the early film cameras had. Astro-imagers had several different manual focus aids they could use, such as flip mirrors and Focault knife-edge focusers, but these were tedious to use and required considerable experience using them to achieve critical focus.