A Digital Focuser for Camera Lenses
for use in Wide Field CCD Imaging

Using the Technical Innovations RoboFocus


 
Figure 1 - Losmandy DA bracket on Losmandy Dovetail plate, 1/4 aluminum connector plate, bent mounting bracket with RoboFocus for LX200 motor.
Figure 2 - Another view.

 
 
Figure 3 - Underside detail of connector bracket milled slots for adjusting belt tension around the camera focus barrel.
M 31 - Focused using the device pictured.

 
How To Build the Digital Focuser
Background

When shooting a fast speeds (f/2 -f/4) with camera lenses the "cone of focus" is very small and difficult to find.  I have found that I need to make adjustments of  .002" to achieve proper focus and these are very difficult to do by hand. When temperature variations cause change of focus the problem starts all over again.

I have adapted a RoboFocus, made by Technical Innovations,  for the task and it works remarkably well.  Since I also use Losmandy brackets as standard equipment in my observatory I designed it to work with those adapters and plates as well.  However, the basic design can be modified to your particular type of set up.

Making One

The parts list and directions are simple:

1. A RoboFocus for LX200. (It comes with a gear in place for the belt.)

2. A Losmandy Dovetail Adapter (In Figures 1-2 above its the one that says "Losmandy" on it.)

3. A scrap piece of 1/4" aluminum about 5-6" long and a couple of inches wide.  Its not too critical and depends on your set up.  I call this the "connector" plate. In Figure 3, you can see where I had it milled for a couple of slots.  These allow for putting a bit of tension on the belt so it will drive the focus barrel of the lens adequately.

4. The mounting bracket that comes with the RoboFocus.  It gets bent as shown and I hacksawed off the top of the slot where the motor shaft/gear come through.  It is then bolted to the connector plate through the adjustment slots. I used 1/4-20 hardware for everything.

5. An Endless Rubber Timing Belt to fit your system.  I get them from McMaster-Carr Supply (they are on the web) or call 562-692-5911.  The one in the image is a 23" outer circle and is their part #6484 K133. It is 1/5 pitch, 1/4" wide. (This size fits the supplied gear perfectly, you can use any length). They go up to 25",  I believe, and come in all sizes.  You can measure the size you need with a cloth measuring tape.  The belts are cheap and you can easily order a few different sizes to see how they work.

6.  Have fun and contact me if you have any questions.

 
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