UPDATED (February 2016): PAGE UNDER CONSTRUCTION. This page is being updated to reflect the current changes and upgrades in my setup. I am now using other alternatives such as a GPUSB or an Arduino as a means to establish communication between the telescope and the mount since the parallel port (the port used in the original version of this DIY) is now becoming obsolete..

In this part, I will describe how to connect a computer to any mount by using a GPUSB connected to a built-in or improvised guide port.

First, we take a look at the telescope’s hand controller and try to understand its operation. All tracking telescopes have some sort of a controller. It does not have to be hand-held. Sometimes, the controllers are mounted directly on the mount’s housing. The purpose of the controller is to give the user a means to control the movement or the behavior of the mount by pressing some buttons/switches. For autoguiding purposes, we are only interested in two specific functions: a means to nudge (or move) the telescope to the east, and to the west. In a typical hand controller, such a function is achieved by pressing the arrow keys, particularly, the “left arrow” and the “right arrow“.

A typical hand controller. (1) left arrow button or RA minus, (2) right arrow button or RA plus.

For the telescope to move, one needs to press a key. The keys are in fact, some sort of a “push-button switch“. With that in mind, it becomes easy to visualize that the arrow keys of any hand controller are simply mechanical switches arranged in a manner similar to the one below:

Typical wiring diagram of a hand controller

In some telescope mounts, there is a port called an autoguider port. The most popular standard for this port is a connector called ST-4. It uses a cable connector called RJ-12. With this port, you can directly connect the mount to a GPUSB (using the RJ-12 cable connector), and then connect the GPUSB to the computer through the USB port (using a USB cable).

Take note that your mount’s autoguider port may follow a different standard, and it is best that you consult with the manufacturer regarding the details of the of the autoguider port, particularly, the pin layout. This discussion thread provides a wealth of information regarding the autoguider ports and pin layout of various types of mounts.

Regardless of the standard, all autoguider ports have a means to

  1. Move the telescope to the north
  2. Move the telescope to the south
  3. Move the telescope to the east
  4. Move the telescope to the west

(In this article, note that we are only interested in correcting the drift in the East-West movement, since, as described previously, North-South drift is best addressed with proper polar alignment.)

You may think of an autoguider port as a mere extension of the electrical connections of the hand controller’s switches/buttons. What is shown below is just an example. It can vary in different types of mounts.

Typical wiring diagram of an autoguider port (ST-4 standard)

Telescope mounts without an autoguider port may still be used for autoguiding, by manually tapping onto the appropriate electrical pathways. This would involve opening of the controller’s case and soldering a number of wires directly on to the circuit board. This will void warranty and must be performed with utmost care to avoid damage.

The simplest method is to tap onto the controller and solder wires directly on the switch’s contact points:

An example of a wiring diagram of an improvised port

In this example, if D is electrically connected to E, then the telescope moves to the west; and if G is electrically connected to F, then the telescope moves to the east.

It now becomes clear that in order to move the telescope, one only needs to “short” or electrically connect the appropriate pathways. In a GPUSB, its built-in opto-isolators perform this task.

(I will be posting here a diagram on how to connect a GPUSB to an improvised guide port. Make sure to revisit this page for updates.)

Related articles:
DIY Autoguider (Part 1: Introduction)
DIY Autoguider (Part 2: Setting-up the Guiding Software)
DIY Autoguider (Part 3: Wiring Diagrams)
DIY Autoguider (Part 4: Autoguiding and Polar Alignment)

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For DIY astronomy projects useful for astrophotography, click here.
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© Anthony Urbano (Manila, Philippines)

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