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Electronic Focuser

 

Introduction

Having made the long exposure modification to the Philips ToUCam Pro II, I had to wait a very long time for any decent clear skies. Rather that just sit and wait I decided to see if I could make an electronic focuser to help with the astro-photography.

The electronic focuser project allows accurate focussing under computer control. This is most useful for eyepiece projection astro-photography where I found that touching the focuser was enough to knock the object from view, because of the higher magnification factors involved.

This project comprised:

bulletStepper motor, bracket and wiring connector
bulletUpdated DSC Interface board
bulletModified PIC software for DSC project

The stepper motor for the focuser is controlled by the DSC project board, using spare control lines on the ports. These controls allow the stepper motor to be enabled/disabled and moved in either direction at a rate determined by the software. These control lines are connected to a dedicated stepper motor interface IC to simplify the circuit.

 

Stepper Motor and Bracket

The stepper motor for the focuser was purchased from Farnell Components part no 7134423. The motor has a step count of 200 for 360 degrees of rotation which equates to 1.8 degrees of rotation per step. Check out the data sheet.

The motor is configured in unipolar mode, which means that four of the eight wires are connected to the +12V supply, and brings the number of wires between the focuser and the interface down to five. The bracket itself was made from a piece of aluminium I had lying around and it was simply cut it to size and shaped.

The original focuser was first stripped down and thoroughly cleaned as the grease used was of a very 'sticky' type. I then used normal grease instead which made the mechanism much smoother to use.

One of the focuser knobs was removed and a shaft coupling was fitted in its place.

I had to drill out one side of the coupling to the diameter of the focuser shaft first. This will connect to the stepper motor later.

Next the aluminium was made to size and holes drilled to allow it to be bolted on to the focuser. The stepper motor was then fitted and marks made on the aluminium for the motor screws.

After these holes were drilled, the position of the five pin DIN socket was marked out and cut. Finally the whole thing was assembled and the wires soldered from the stepper motor to the five pin DIN socket as in the picture below.

   

Later I used electrical tape to cover the exposed wires.

 

Interface Board

The interface for the focuser uses the same board that was used in the DSC project. I had three spare control lines available that I used as follows:

bulletPort B Bit 5 - Stepper Enable
bulletPort A Bit 0 - Step Direction
bulletPort A Bit 4 - Step Pulse Input

A new board had to be made for the stepper motor driver IC and some other discrete components. The circuit diagram is shown below.

Check out the data sheet for the UCN5804B.

A small piece of Vero-board was used to construct the driver board.

This board was place upside down on the original board and held in place using double-sided tape.

Five connections had to be made to the original DSC PCB, one for +5V, one for 0V and three for the stepper control port bits.

Another five wires were connected to a five pin DIN plug for the stepper motor connections (the wires in the picture above). To connect the motor to the interface box I found an old style keyboard extension cable made from coiled wire. This was ideal as when the telescope is moved the cable simply extends or contracts accordingly.

 

PIC Software

The PIC software was modified to add some new commands for the focuser.

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i    Focus in by 1 step

bullet

I    Focus in by 10 steps

bullet

g    Focus in by 100 steps

bullet

o    Focus out by 1 step

bullet

O    Focus out by ten steps

bullet

G    Focus out by 100 steps

The full range of the focus tube, from one end to the other, is in the order of 500 steps so focusing can be quickly achieved using the larger steps first followed by the smaller step and finally the smallest step.

Since the distance travelled by the focuser, from end to end, is around 60mm this means that each step of the motor is equivalent to 0.12mm. I could have wired the stepper driver to allow 400 steps per revolution which would have halved this figure to 0.06mm per step! If you wanted to do this simply connect pin 10 of the UCN5804B to +5V instead of GND.

 

Fitting To The Telescope

Here are a couple of pictures showing the electronic focuser fitted to the telescope.

       

In the first picture note the electrical tape covering the stepper motor wires. In the second photo the keyboard extension cable can be seen connected to the 5 pin DIN socket on the focuser bracket.

The telescope had to be re-balanced, to account for the weight of the stepper motor, but this was easily achieved.

As can be seen the modified TouCam Pro II is also fitted waiting for the evenings viewing :-)

 

Performance

In use the focuser seemed to work very well. Initial focusing was done using the larger step command (100 steps) until the object was near to focus. The smaller steps (10 and 1) were then used to sharpen the picture as much as possible.

As expected the weather was clear when I started to set up and then decided to cloud over a bit later. I persisted and later the skies cleared enough to get in some practice shots of Jupiter.

I found that when using a 10mm eyepiece for eyepiece projection, the vibration caused by the stepper motor was visible but the object was still kept well in the field of view on the screen and settled down very quickly once again.

In this photo of Jupiter I managed to focus on both Europa (on the left) and Io (on the right) as well as the planet itself. By moving the focuser only a couple of steps in either direction the moons disappeared from view showing the accuracy required.

 

 

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Copyright 2004 - 2007 Phil Davis

email: phil@philchris.co.uk

Last updated 2nd December 2009