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:
- Ease of Setup and Setup Time
- Good Bearing Supports
- Tube Balance
- Hand Controls and Scope Guiding
- Timing Considerations, Servo vs. Stepper
- Electronic Hand Controls Really Need These Minimum Details
- Axis Clutches Save Telescope Lives Everyday
- The GOTO Conundrum
- Guiding Capability
- 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:
- My Apple developer account is expired for the second time. I already spent €99,- twice and would need to activate it again.
- 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:
- Adafruit Triple-axis Magnetometer (Compass) Board – HMC5883L. There are some choices here for the chip to use. The last post in this Google thread is insightful, as is a comparison of magnetometers on PitLab. The HMC5883L receives good reviews and a Python library is available for the HMC5883l.
- Adventures in Minecraft – parts kit. This kit contains everything to get started connecting a Raspberry to a board like the compass board.
- Raspberry Pi. Already owned this Pi 2.
Will this compass produce repeatable measurements? Will it provide a useful angular resolution? Before I can test that, it needs to be assembled.