July 12, 2012

DSC – Digital Setting Circles with Sky Safari and Arduino

Category: Astronomy — Tags: , , , , , – vincechan @ 1:57 pm

OK, I’ve always been a visual observer, and have resisted jumping on the GOTO telescope bandwagon.  Part of the fun of astronomy was always using charts and star hopping to find a new deep sky object.   How else would one learn the night sky?

However, I must admit, having the power of a star atlas in something as small as your iPod is a real luxury.  And having that connected to your telescope makes things real easy, maybe too easy, but that will be a debate for another day…..

DSC on iPod using skywire


Here are the major components of the project:

  1. iPod or iPad with Sky Safari plus or pro
  2. Sky Safari skywire – goes between you iPod Touch to an RS232 port
  3. Quadrature Encoders – I used a pair of capacitive types CUI-AMT102
  4. Arduino – microcontroller
  5. Max232 – a microchip that converts a RS232 signal to TTL signal

Sky Safari plus or pro – this is an astronomy program that runs on your iPod or similar device.  It’s a pretty full fledged astronomy software package, considering what used to cost $100′s on a PC.  To use this as an interface, you need to use cable that hooks up to your iPod, sold by Sky Safari as the “sky wire”.  Basically, this sends out a RS232 serial signal to your telescope or device.   You need to set Sky Safari in the “Settings” menu, telescope setup to Scope Type:  Basic Encoder System, and under Mount Type: Alt-Azimuth Fork.  Also, in that menu, set the “Encoder Steps Per Revolution” to +4096 for both axis, and set “Get Automatically” to OFF.

Quadrature Encoders – the versions that I use are capacitive types, they are much cheaper than optical encoders.   The encoders are CUI-AMT102-V kit, order from Digikey: 102-1307-ND  Luckily, the pin outs on the encoders are 0.1″ pitch, so any 5-pin straight plug would fit.  On my Dobsonian telescope, the azimuth encoder was mounted to the center bolt of my rocker.

Azimuth encoder in Dob rocker box

Azimuth encoder close up of mounting

The altitude encoder is mounted at the center of rotation of the Dob’s rocker arms.  The required the fabrication of a tangent plate to hold the encoder stationary, relative to the rocker box.  I just happen to make mine out of “plexi” glass, as shown below:

Altitude encoder mounting

To interface between the encoders and the iPod-skywire cable, I used an Arduino micro-controller.  Initially, I tried to read the encoder pulses without interrupts, but that resulted in loosing steps if I moved the telescope too fast.  A relative  new encoder library for Arduino/Teensy was posted by PJRC, and can be downloaded here: http://www.pjrc.com/teensy/td_libs_Encoder.html  You’ll need to install the ‘encoder’ directory to your Arduino library on your computer.   If you are new to Arduino, they can be purchased from many suppliers: http://www.makercanada.ca/  You can download the IDE fro Arduino for free here: http://arduino.cc/

As the skywire cable requires a RS232 serial signal, you need to shift the signal using a MAX232 chip.  They can be purchased relatively cheaply either from shops listed in http://www.makercanada.ca/ or from Digikey #296-6940-5-ND.  I power the MAX323 with 5V pin from the Arduino.  The Arduino is powered with a 9V battery.   Here’s a picture of the prototype circuit:

DSC circuit in lozenges box

Eventually, I’ll find a better mounting box… ha ha.  Here’s the schematic of the circuit:

encoder schematic

Here is the source code for Arduino:

sky_safari_encoder_interrupt  – note: you’ll need to change the extension to .pde or .ino