A digital compass for a dobsonian mount

I need a goto system! My neck is hurting from looking in the both the finder scope as well as the main view, while I am failing to find anything interesting other than Saturnus, Mizar and Andromeda.

I am not alone. On the internet there is a whole topic about adding compass aids to Dobsonians:

  • 10 year running thread on cloudynights.com about adding laminated degree circles. It also covers adding a digital inclinometer to the scopes.
  • Very interesting project at simonbox.info where two potentiometers and a Raspberry Pi are used to measure Alt and Azimuth, convert to Ra and Dec using the sidereal python package, and interface with Stellarium.
  • The page Dobsonian GOTO Mount Design provides an excellent introduction into all aspects Dobsonian Goto systems, covering:
    1. Ease of Setup and Setup Time
    2. Good Bearing Supports
    3. Tube Balance
    4. Hand Controls and Scope Guiding
    5. Timing Considerations, Servo vs. Stepper
    6. Electronic Hand Controls Really Need These Minimum Details
    7. Axis Clutches Save Telescope Lives Everyday
    8. The GOTO Conundrum
    9. Guiding Capability
    10. Inertial Effects

Adding degree circles and an inclinometer is one step, but it still requires manual conversions between alt-azimuth and ra-dec. An electronic solution has as benefit that any desired transformations can be automated in software.

Yay! This telescope turns out to be a good hobby project that lends itself to improvement: improving bearings, adding some electronics, finally also a real use for all the high school geometry learned.

Simon’s project seemed like something I would like to copy. I already own a Raspberry and know Stellarium, and the demonstration video on Youtube where the telescope was moved and the crosshair in Stellarium moved was very appealing! I want this too! One caveat with this solution is that the through whole potentiometer is a discontinued product in the UK. In the NL store a similar (listed with 1W instead of 0.5W) product is available for backorder. At a list price of € 23,06 this was not something I would gamble on ordering when unsure when it would be produced.

Use a digital compass?

A friend of mine suggested ‘why not use a digital compass? You can buy a small board with a compass circuit that can be interfaced with i2c so it connects to a Pi easily’. This is intriguing, since attaching a compass to the telescope is less invasive and requires less cables than the two-potentiometer solution.

After a brief consideration of using the digital compass in the iPhone, I decided NOT to go the iPhone route:

  1. My Apple developer account is expired for the second time. I already spent €99,- twice and would need to activate it again.
  2. It is unclear to me how well the iPhone compass will work. The iPhone 5 got some bad compass reviews and when I point my iPhone with GoSkyWatch on a planet or moon, it is almost always wrong between 5 or 10 degrees from the target. Clearly with such a large margin of error I will not be able to use it as alternative for the finder scope.

So I decided to try construct and test a digital compass based on the following components:

Will this compass produce repeatable measurements? Will it provide a useful angular resolution? Before I can test that, it needs to be assembled.


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