Guiding commands from the computer are sent through a port called ‘LPT1‘, or the parallel port (or sometimes called printer port). It is a kind of interface that allows a simple way for a computer to communicate with other devices.  We will try to convert these ‘commands‘ into a form that can be easily interpreted by your telescope mount. The simplest way to do that is to convert the commands into light pulses using Light-Emitting Diodes (or LEDs). These light pulses in turn will be used to drive what is called a ‘light activated switch‘ that we will connect directly to the autoguider port or hand controller. In this DIY guide, we will focus first on how a computer (with the use of the guiding software called GuideMaster) can generate light pulses, by connecting LEDs to the computer’s parallel 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.

The parallel port is mounted on a socket called DB25F(F stands for ‘female socket’) or DB25M(M stands for ‘male socket’). It has 25 pins (1 to 13 top row, 14 to 25 bottom row). For this project, we are only interested in pins 4, 5, and 25 (other pins will be utilized however in future upgrades). Shown below is a photo of my laptop’s parallel port.

A female parallel port (DB25F). Note the location of pins 4,5, and 25 (see arrows).

If your laptop/computer does not have a parallel port, you might want to search the net for a special type of converter called a ‘PCMCIA to Parallel Port Card Notebook Adapter‘.

For us to tap/connect onto these pins, we will need the male socket counterpart DB25M shown below.

A front (top image) and back view (bottom image) of the male parallel port DB25M. Again, take note of the location of pins 4,5, and 25.

It is important that we test if the parallel port is working perfectly. We will need to build a simple ‘tester‘ using a 330 ohm resistor and a red and a green LED. It will be soldered directly on the DB25M port. The tester will serve as a visual indicator of the current state of the pins 4 and 5. Connect the resistor to pin 25 (GND), then connect both the negative ends of the two LEDs to the resistor. The positive end of the red LED must be connected to pin 4, while the positive end of the green LED will be connected to pin 5, as shown below:

A simple Parallel Port Tester

Take note of the polarity of the LEDs. The negative terminal must be connected to the resistor.

The parallel port works in a straightforward manner: whenever the corresponding pin is energized, a +5V is sent on to the pin (with the pin 25 acting as the negative terminal). Thus in this case, both pins 4 and 5 serve as the positive terminal (corresponding LED lights up whenever the pin is energized). We now attach this tester to the DB25 female port. The LEDs may (or may not) light up (for many several reasons), this is not a problem/malfunction.

Parallel Port Tester attached on to the DB25F port.

We need to test the parallel port if it works with the guiding software and the guiding camera, thus we need to install the appropriate drivers. Install the camera’s driver. In this setup, I will be using a Logitech 4000 web camera. It comes with an installation CD which contains all the necessary drivers for the webcam. Follow the procedure provided in the camera’s manual.

Logitech 4000 installation disc

Download and install the guiding software (freeware) GuideMaster © 2005 – 2012 by Matthias Garzarolli .

Click on the image to go to the download page.

Connect the camera through the USB port and then launch the guiding software. Upon first use, you will need to ‘select‘ the correct camera. Go to Camera>Start Cam, then choose the appropriate webcam. You will also need to properly setup the camera’s settings. Go to Camera>Options and then set the camera to ‘Full Auto‘ and then select the highest resolution available (usually 640 by 480). The camera settings available will depend primarily on the camera model used.

We will now configure GuideMaster. Go to General>Setup. You will then be presented with four tabs. It is important that you set each tab with the following recommended settings (you may change them later, but for now, I do recommend that you follow them):

Telescope Tab

Guiding Scope Tab (In the example shown, a guidescope with focal length of 700 mm is used; enter the appropriate value for your setup.)

DSLR Settings Tab

Guiding Imager Tab (Note: For now, we will use an unmodified camera. I will post a separate DIY on how to modify a Logitech 4000 for SC1 modification, and thus take advantage of the other settings.)

Click ‘Save‘ when finished.  Go back to the main page. Notice the arrow buttons on the lower left panel: N, S, E, W. We are only interested in the E and W button (the E button nudges the telescope a little bit to the east, and W to the west).

Notice the E and the W button on the lower left panel, and the camera feed on the center right.

We are now ready to test the parallel port. With the tester attached on to the DB25F port, click on the E button; the red LED should light up for 1 second. Now click on the W button; the green LED should also light up for 1 second.  The tester demonstrates how light pulses can be generated through the parallel port. If you have made it this far, then you are now ready to connect your computer to your mount. In my next post, I will discuss how the telescope’s mount can interpret these light pulses and use them to nudge the telescope a little bit to the east, or to the west during 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)

For tutorials on how to get started with astrophotography, click here.

For DIY astronomy projects useful for astrophotography, click here.

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© Anthony Urbano (Manila, Philippines)

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