Modding an EQ3-2 Handset for Autoguiding

Introduction

Previously, even when polar aligned using a fine-tuned polar scope, my EQ3-2 mount could only track accurately for around 45 seconds at a focal length of 750mm. Obviously this limited my choice of targets, I could only image the brightest objects, and had even more problems when using narrowband filters.
The imperfect tracking can be caused by a number of things; not having the mount polar aligned perfectly, improper setup balance, wind, and periodic error - caused by sub millimeter inaccuracies in the surface on the motor grears. The solution is to 'autoguide' - have another telescope mounting on top of the imaging one, to follow a 'guidestar' and relay instructions to the mount so it follows the star accurately.
The only problem is that the Skywatcher EQ3-2/4/5 mounts do not have a connection to allow the mount to be controlled by a computer. There is a simple modification to allow it to do so however.

It involves using 4 pins of a computer's parallel port, through a transistor array, to control the 4 buttons on the handset controller.

The Modifications

Opening the handset controller reveals the inside workings.


On the underside of the PCB with the pushbuttons, I glued the ULN2003 chip (with legs cut off for space economy).


Each switch has a connection to ground and one to the grey ribbon cable.
The ground rail is connected to pins 5-8 on the chip, and the seperate switch connections to pins 13-16 individually.


The PCB is then screwed back down and a simple 5 way connector is connected to the chip, and built into the casing.


This is the final result after putting it all back together.
I'm pleased with the look of it, as the mod it very non-intrusive.


Here is the interface I made to connect everything to the parallel port of the laptop.
It allows the mount to be controlled, the SC1.5 modded webcam to be used for tracking a dimmer guidestar, whilst controlling the exposure time of the DSLR taking the actual images - all simultaneously.


This shows the connections to the back of the laptop.
(click for larger annotated version)