Recently I’ve been investigating adding digital setting circles to my Dobsonian mounted telescope.

Digital setting circles is a blanket term that refers to any electronic mechanism that provides orientation feedback for a telescope as you move it around – the most basic version is a digital spirit level, like the Moore & Wright Mini Mag. Attach one of these to the tube of your telescope, and it will tell you the altitude it’s pointing at – from zero degrees (pointing at the horizon), to 90 degrees (pointing straight up, aka the “zenith”).

That’s great and all, but what you really want is to get this information into a program like Stellarium – an open source, multi platform planetarium app that simulates the night sky. If you can connect your telescope to Stellarium, you can have it display a “reticle” – a crosshair that corresponds to where your telescope is currently pointing, which is great for helping you find things.

The plan – Stellarduino

What I want to design is a cheap (and easy to build yourself) bit of hardware that acts as an interface between a Dobsonian mounted telescope and a computer running Stellarium, or some other planetarium software. I call this Stellarduino.

Bill of materials

1. A set of high-resolution rotary encoders. These attach to the Dobsonian mount and convert the rotational movement into a digital signal.  I’ll be using some expensive USDigital S1 and S2 encoders, purchased for ~$200 from Wildcard Solutions as part of an encoders and mount kit, though cheaper ones (like the kind you find in old optical mice) would suffice.
2. An Arduino, the brains of Stellarduino. Probably the larger DUE, though I’ll be using the ubiquitous $30 Uno until I run up against any limitations (floating point accuracy and no. of pins).
3. A 16×2 character LCD display, such as the $20 MIDAS MC21605C6W-FPTLR. This will be used to enter time and coordinates during alignment, and will display current Alt Az or RA Dec orientation once aligned.
4. Misc push buttons and wire.

How it will work

Using the Telescope plugin, Stellarium has been designed to consume the output from common GoTo telescope handboxes (like the Meade AutoStar) via a serial connector cable. So, in order to talk to Stellarium, Stellarduino will have to emulate one of these devices. To do this, the telescope must first be “aligned”, which means the altitude and azimuth orientation of the scope can be converted to the celestial coordinate system, Right Ascension and Declination. Alignment (from what I can tell) involves questioning the user for the telescopes’ geographic coordinates (latitude and longitude), and time (specifically UTC time). The user is then instructed to point the scope at an object of a known celestial location (typically a bright star), then at a second. By calculating the change in alt/az from the rotary encoders, it is then possible to convert between the two coordinate systems.

Project schedule

1. Write prototypes of the software using PHP. Build Stellarduino class library, figure out algorithms to convert RA/Dec to Alt/Az and back, figure out the alignment process and prototype it.
2. Port Stellarduino class library to Arduino.
3. Figure out best method of reading rotary encoders from Arduino.
4. Get LCD character display working with device so it reports telescope orientation using “dumb” alignment, that is, where the software assumes the telescope starts at zero altitude/azimuth. Dumb alignment milestone.
5. Port alignment process to Arduino. Stand alone alignment/push-to device milestone.
6. Figure out Meade AutoStar protocol, and emulate it. Final milestone.