Taking images of galaxies, nebula, and other deep-sky objects require precise tracking. To achieve this, astrophotographers use a computer to keep the telescope pointed at their target (a galaxy, for example). It is done by using a software to analyze the images captured by a camera and then send instructions to the telescope’s mount whenever the target drifts, so as to keep the telescope properly pointed. This automatic telescope guidance system is called autoguiding.

Autoguiding requires a means for a computer to send guiding signals to a telescope’s mount. For most mounts, especially the entry-level ones, a separate device can be used to enable the computer ‘talk’ to the mount—an example of which is a USB Guide Port Interface or GPUSB.


A USB Guide Port Interface (GPUSB) from Shoestring Astronomy. Note that the port on the left and the toggle switch on the right are simple modifications done to allow the GPUSB to be connected to my DIY Mount Controller, which will be discussed in future posts.

A GPUSB is a device that allows a computer to send guiding signals to a telescope mount. It has a cable that connects to a computer (through the USB port) and another cable to connect it to the mount (through what is called an ST-4 port). With a GPUSB, guiding software such as PHD2 or GuideMaster can now send commands to the mount, to move the telescope in any of the four directions: North (Dec+), South (Dec-), East (RA-), or West (RA+) during autoguiding operation.

I have been using a GPUSB in my autoguiding setup since 2013. Prior to this, I have used the Parallel port to send guiding signals to my mount. Parallel ports are easy to work with, however, I had to upgrade to a newer setup since such ports are no longer found in newer laptops. I suppose other enthusiasts have stumbled upon this problem as well. My solution was to simply use a GPUSB. Later, I learned that an Arduino can also be used.

In future posts, I will describe how I interfaced the GPUSB with my DIY Mount Controller and how to setup the guiding software that I use with the GPUSB.

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© Anthony Urbano (Manila, Philippines)