Note from Dave (Feb 2021): This project has been around now for about twenty years, and it’s less popular now than when it was first published for the simple reason that so many commercial telescope mounts now include “go-to” capabilities for a fairly economical price. To be truthful, I don’t even own a telescope anymore on which I could install these DIY digital setting circles. However, I still get a few queries about this project here and there, and over the years it’s been updated to use USB and bluetooth for connectivity. It’s still possible to use this system with modern software packages, too–the ASCOM driver will likely keep it relevant for quite some time, and even mobile packages like SkySafari will work with it.

Use the information on these web pages at your own risk. While I have tried to make the instructions as complete and error-free as possible, I will assume no risk for and will not be held liable for damages of any sort which result from your attempt to use this information. Your use of this information constitutes acceptance of this policy. There is always the risk when constructing electronic circuits that a mistake will result in damage to one or more components. Since this project also involves connecting the circuit to a PC, there is the added risk of damage to the PC if the circuit is built incorrectly. If you have questions regarding this project, please use the comment form below to contact me before you proceed.

What are Digital Setting Circles?

You probably already know that setting circles are markings on a telescope’s mount that help you point the telescope toward a particular object. Most telescope setting circles seem to be too small or poorly-marked to be useful, though. Digital setting circles also help you point the telescope, and they are much more accurate because rotary encoders attached to the telescope’s axes are used to measure the movement of the telescope, and a computer is used to monitor that movement and continuously report the direction that the telescope is pointing to the user. An electronic interface sits between the rotary encoders and the computer, monitoring the rotary encoders for movement and reporting that movement to the computer via the serial port.

Many commercial telescopes come equipped with digital setting circles, but commercial digital setting circles are expensive to add to telescopes that don’t already have them. I designed my own digital setting circles because I didn’t want to pay the steep price for a commercial system, and also because I thought it’d be a fun project.

My Digital Setting Circles System

My digital setting circles system requires four things:

  • A telescope on a mount. The type of mount (GEM, Dob, etc.) doesn’t matter, as long as you can find a way to connect a rotary encoder to each axis of the mount.
  • Two rotary encoders, one attached to each axis of the telescope mount in such a way that whenever the telescope moves (including by the clock drive, if any), the rotary encoders turn.
  • A decoder circuit that monitors the rotation of the encoders and communicates the encoder angles to a computer. This is the part of the system that I’ve designed and is described here.
  • A computer (PC or mobile device) that is running software that can get the encoder angles from the decoder circuit and compute the celestial coordinates at which the telescope is pointing.

You probably already have the telescope and mount, or you wouldn’t be interested in this project! The rotary encoders can be purchased commercially, but they aren’t cheap–expect to pay around $50 apiece for new ones. The computer can be an old (or new) PC notebook, and some mobile devices can be used via bluetooth, as well. There’s a fair amount of free or inexpensive software for use with digital setting circles.

That leaves the decoder circuit, and that’s the part of the system that I’ve designed and that you can build for yourself. The remainder of this web site is devoted to describing the decoder circuit and explaining how to build it. The links given above in the Navigation bar discuss the various aspects of this project, and I encourage you to read all of that information before you embark on building a system for yourself.

This is a project that is not terribly difficult to build. A PC board and a preprogrammed microcontroller chip are commercially available, and with those it’s simply a matter of putting the electronic components in the right places on the board and soldering them in place, and constructing a few cables. I figure a couple hundred of these decoder circuits have been built over the past several years. By far the biggest source of trouble for those who build it is in soldering the components on the board. If you’ve never soldered before, try to find someone who can give you a lesson, or at least try to do a few practice solder joints before you start building this circuit. Soldering isn’t difficult once you’ve seen it done and know what a good solder joint looks like.