Tethered Focus Control for Photography Telephoto Lens


I am just starting with Deep Space Astrophotography. Have been doing wide field for several years. My new rig is a Celestron AVX equatorial mount. I am using a Canon 5DSR for my camera and I will be using my Sigma 800mm telephoto AF /f5.6 lens. I also have a Canon 300mm f/4 lens that has auto focus.

We see all kinds of focusers available for telescopes. These systems do an analysis of the star to access focus and adjusts the telescope’s focuser using a step motor or servo motor. My Sigma lens is an auto focus lens. Focusing is done in these camera lenses using Piezo stepping motors. I fully realize that it would be impossible to expect the camera to auto focus in a dark sky situation. However, I also know that it is possible to control the lens’ Piezo steppers via external control. For my daytime landscape photography I do focus stacked images all the time using a device called a Promote Control, that connects to the camera’s USB port and uses the lens’ auto focus Piezo motor to adjust the focus over a range of distance from near to far. There are also several tethered camera control software packages that are able to do this type of focus stacking function with a laptop connected to the camera’s USB port.

It occurs to me that it would be possible to have software in you tracking computer that does a similar function using the lens’ internal Piezo motors to focus on stars the same as a telescope auto focusing systems does via step/servo motors attached to the focuser of the telescope. It would seem that it should be fairly easy to do this.

Curious if anyone has heard of such a system?


If you look about, there are some examples of home-brew motor controllers and most importantly, interface software. Many AF motors used in astrophotography are based on a standard stepper motor interface. If it uses a standard winding pattern it may even be possible to use a commercial product and its ASCOM interface. You may just need to make up some wiring.