Now that we have already devised a simple contraption that allows a computer to convert guiding commands into light pulses, our next task is to devise a way for a telescope mount to “read” these pulses and translate it into actual east-west movement. This part of the DIY guide will describe the wiring diagrams that will enable any computer to talk to any type of telescope mount (i.e., with or without an autoguider port).
WARNING: I assume no responsibility for any damage caused to your equipment by following the information presented here. Please proceed with caution and follow instructions at you own risk.
First, we look at the telescope’s hand controller. 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 body/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. Note that there is no so-called “convention” when it comes to the type of connector and the pin layout used in autoguider ports. The most popular standard/connector however, is the ST-4 standard. Take note that your mount’s autoguider port may follow a different standard, and it is best that you consult with the manual/manufacturer regarding the details of the 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
For a port following an ST-4 standard, an action is initiated whenever the RA-, DEC-, DEC+, RA+ is electrically connected to the ground (GND), that is, whenever any of the pins 6, 5, 4, or 3, is electrically connected to pin 2, respectively. If pin 2 is electrically connected to pin 6, then the telescope moves to the east; and if pin 2 is electrically connected to pin 3, then the telescope moves to the west. In the same manner, if pin 2 is electrically connected to pin 5, then the telescope moves to the south; and if pin 2 is electrically connected to pin 4, then the telescope moves to the north. For autoguiding purposes, we are only interested in the pins that move the telescope eastward or westward. (Note that the movements I have just described are applicable only for observers in the northern hemisphere; all movements follow the opposite direction for observers in the southern hemisphere.).
You may think of an autoguider port as a mere extension of the electrical connections of the hand controller’s switches/buttons, as shown below:
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/housing and soldering a number of wires directly on to the circuit board. This will definitely void warranty and must be performed with utmost care to avoid damage. I have provided 2 possible ways to tap on to the electrical pathways:
If you were able to locate the wires that lead to the GND, RA-, and RA+, then you can just solder directly on to the electrical pathways:
Wiring diagram of an improvised port
We will arbitrarily call the connection points as A, B, and C. If B is electrically connected to A, then the telescope moves to the east; and if C is electrically connected to A, then the telescope moves to the west.
Or a simpler method would be to solder wires directly on the switch’s contact points:
Another wiring diagram of an improvised port
Again, we will arbitrarily call the connection points as D, E, F, and G. 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 pins/pathways. To control the mount using the light pulses generated by the computer, we will use an electrical component called a light-dependent resistor (LDR):
A light-dependent resistor (LDR)
An LDR is the simplest type of optical (light) sensor. It allows electricity to pass through (becoming a conductor) whenever light shines on it. In complete darkness, it becomes an insulator. Since we are dealing with 2 pairs of connection points (RA+ and GND, and RA- and GND), we will be needing 2 LDRs. One LDR will be paired with the red LED, and the other with the green LED. Such an arrangement is called an opto-isolator. Use superglue to join the two parts together and then cover it with black electrical tape so that no stray light would affect the LDR. (Note: LRDs are best replaced by dedicated opto-isolators. The purpose of using LDRs is to better illustrate the circuit’s operation. In my future posts, I will describe how the circuit could be further simplied.)
An opto-isolator (red) to control the eastward movement (RA-)
An opto-isolator (green) to control the westward movement (RA+)
Whenever the computer sends a guiding signal, the LED lights up. The LED will in turn, shine its light on to the LDR. The LDR in turn behaves like a conductor. Since the LDR is connected to the appropriate pins, it serves as an electrical pathway, causing the mount to move accordingly. The diagrams below describe in detail how to connect the electrical components.
For a mount with an autoguider port (ST-4 standard):
Click on the image to enlarge.
Option 1: For a telescope with no autoguider port (tapping directly onto the hand controller)
Click on the image to enlarge.
Option 2: For a telescope with no autoguider port (tapping directly onto the hand controller)
Click on the image to enlarge.
To test if the computer-to-mount interface works, perform the following procedure:
1. Connect the web camera.
2. Launch the GuideMaster (select the appropriate camera if needed).
3. Connect the computer to the mount using the diagram described above (always double check all electrical connections).
4. Click on the E button; the red LED should light up for 1 second, the mount in turn must move the telescope to the east.
5. Now click on the W button; the green LED should also light up for 1 second, the mount in turn must move the telescope to the west.
You are now ready to field-test the DIY autoguider! In my next post, I will discuss in detail the actual guiding operation. If you have questions, feel free to leave a comment. Clear skies!
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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© Anthony Urbano (Manila, Philippines)
