Note: This does NOT work with the latest vesrion (3) of the autofocus software. The Arduino version is recommended.
This project began as a reproduction of a remote focus motor published on the internet by B. Sumpter(thank you!) (here). A 'gutted' servo controlled by a DPDT switch to reverse polarity (and hence direction) with a potentiometer to adjust the speed. That was quickly followed by modifications like a PWM supply and an H-bridge for better speed control in both directions. This worked great but lacked 2 important desires....
1) an interface that matches the movement of a focus knob
Both of these have been achieved (see Pictures and Videos page) in a very easy to build, inexpensive design, by utilizing a Pololu Maestro 6 channel servo control board coupled to a stepper motor via an H-bridge. Movement is accomplished by turning a rotary encoder. Autofocus by 'homebrew' C# code. The mount, made from aluminum stock purchased at a local hardware store, is attached via the focus lock screw.
Build the H-bridge/L293D(2)* circuit on generic pre-drilled circuit board(12). Add pins(13) for connection to the Maestro board and ribbon connector(6) for Stepper connection. Drill holes in the enclosure for power connector(11), encoder/thermistor connection (4 conductor phone cord) and mini-USB (on Maestsro board(1) to be mounted to bottom of enclosure(8). Create a shallow groove for ribbon cable(6) on top of enclosure(see "open" picture). The ribbon cable needs to be connected to stepper wires using solder and heat shrink tubing. Attach Meastro board(1) to the bottom of the enclosure using hot glue. The H-bridge circuit can be mounted with screws. Connect the 12 supply voltage to both the H-bridge circuit and the Maestro board (VIN, this provides power during manual mode without USB connection). Mount Rotary encoder(4) on top of another project enclosure(8) and connect to main circuit using phone cord via female/female coupler(14).
* (#) - reference to parts list number
Mount Stepper to focuser (details pending). Use a rubber grommet on stepper gears for better belt performance.
Set-up and Operation:
Download text file from download page. Download and install Meastro drivers from Pololu.com. Open Measto control program and load settings from the text file. The unit should now function in manual mode.
For Autofocus, download and unzip the scopefocus file. Run setup. Create a folder c:\test2 (this is necessary until user defined folder location capability is added to software) Ensure the device is connected by USB and start the autofocus software. Open Nebulosity and set Directory to c:\test2. Find available focus stars using frame and focus. Use manual control buttons to find rough focus. Ensure that you are "in" from the rough focus point. Select specific focus star using fine focus. Ideally medium saturation with 1 second exposure. Go back to the autofocus software. To find initial focus point set step size=8, repeat=1 and N=20. Click V-curve button. The motor will now move back from the initial location and generate a rough v-curve. If needed increase N to ensure it's actually gone past the focus point. The motor will then go to the focus point of minimum HFR. Then hit Fine-V, the motor will move in a predetermined distance then average 2 exposures with each smaller step to obtain a more accurate determination of the focus point. It will automatically go to this focus point when complete. The second video on Pictures and Videos page shows a screen capture of this process.
The "Std Dev" button will calculate the standard deviation of HFR over 10 exposures to give some idea of the error and to what degree the HRF value should be rounded off ("Round To").
"Temp Cal" is the start of a temperature correction feature. This will generate repeated Fine-V curves indefinitely to record changes in the focus position over a range of temperatures.
WO DDG focuser on C9.25 WO Megrez 88mm w/ thermistor