Using a Motorized Focuser

Do you sometimes get frustrated trying to fine tune your focus because the video image jiggles every time you touch the focuser?  I found adding an electric focus motor that attaches to the shaft of a focuser knob and connects to a hand held control provides me with vibration-free focusing without the need to actually touch the telescope at all.

I purchased my first JMI Motofocus in January 2012 for my Celestron C8 telescope for $180.  It can focus at 3 different speeds and has electronic braking.  When stopped, it holds the focus knob secure so you can’t accidently bump it out of focus.  Combining this with a Bahtinov Focus Mask enables you to easily nail the focus!

 I also purchased the JMI PC Focus Control (PCFC) so I could control the focus from my laptop. I have been using Motofocus and PCFC for 3 years and love being able to easily focus at the scope with the hand control or inside for Remote Video Astronomy using the PCFC (I plug the PCFC USB cable into a powered USB hub at the scope when I am remote inside).  The PCFC price is also $180, but you can purchase a similar PC control called PCUSB for $80 from Shoestring Astronomy that also works with the JMI Motofocus.   

JMI makes electric focus devices for many types and brands of telescopes in addition to Celestron, and this focusing technique works with any Video Astronomy camera.

Last year I bought a Celestron C5 for a portable scope setup, and recently also got a Motofocus for it too.  I can use the PCFC to control the focus of either scope now. You will need to make sure you get the right Motofocus that works with your particular telescope.  I purchased the one for the NexStar 6SE, which is compatible with the Celestron 5 SE, 6 SE and 8 SE.


So how hard is it to install?  Mine took less than 10 minutes.  I removed two of the three screws that hold the focus knob, slid the Motofocus over the focus knob and put the two screws back in place.

You connect the hand controller to the focus motor using the supplied coiled cable for jiggle free focusing.  The hand controller has two buttons for focusing in or out. A slide switch selects either the slowest speed or a slightly faster speed.  Holding either button down for over a couple seconds will begin to run the motor much faster for coarse focusing. 

I attached Velcro to the back of my hand controller to be able to stick it to the mount when not holding it.  Below shows the Motofocus connected to the PCFC controller in place of the hand control.

Here is the PCFC plugged into my laptop running the free software you download from their website. 

When you click on the icon for the focus software it reports it found the PCFC. 

Click OK to bring up the PCFC control panel.

You can click on buttons for a single Motor Forward Pulse, Motor Forward using Click/Hold or Motor Forward Full (or reverse).  After clicking Motor Forward Full it will run in that direction until you click on Motor Off, which is useful if you have just added a focal reducer and will need to change the focus a good bit.  You can fine tune the PWM Frequency, Motor Speed and Pulse Time for best operation with your setup.  When I plug the PCFC into a powered hub instead of directly to the computer I find it works best to have the Motor Speed set higher to supply ample voltage to the motor.

The Motor Time Accumulator gives you an approximate idea of how far you have moved the focus and you can reset the counter to zero.  This is just based on run time since this model of the Motofocus does not provide feedback of its position to the software.  (JMI makes more expensive models that include digital readouts and feedback of their position to the software).  You can download ASCOM drivers for the PCFC that enable it to be controlled by other software that is ASCOM compliant as well.


This setup has been very good for my needs so far. To me it has been well worth the price for its usefulness over these last few years.