Journal Entry #1: The Idea

Written By Brett Potter

I want to build a sky-tracking mount for my telescope, and I want it to be fully automatic.

Astronomy and astrophotography have always appealed to me because I love exploring the night sky, but I’ve often found myself deciding against setting up my telescope and equipment because it’s simply a tedious process. Every time I move locations, I’m frustrated by the effort required to precisely align everything and keep it aligned. My goal is something I can set down, turn on, and start using immediately.

To be more specific, I want to build a variation of something called an equatorial platform. An equatorial platform is basically a slowly-moving table that you can put your telescope on to help it follow the movement of stars across the sky. They’re great for a number of reasons, namely that they don’t require any electronics on the telescope itself, and they can support huge telescopes including large dobsonians.

Problems

There are three main problems I have with equatorial platforms, all of which stem from my desire for portability:

  • They require alignment to true north, which is pretty easy to roughly accomplish for visual observing, but requires more precise alignment for astrophotography. This is a non-issue if the goal is to set it up in a repeatable backyard location, but I don’t want to have to start from scratch every time I bring it somewhere new.

  • They require a level surface, which can’t always be guaranteed.

  • Equatorial platforms are designed to operate within a certain latitude range. They can typically be adjusted over a few degrees, but from what I can find there aren’t any equatorial mounts that have even a large enough range to cover the continental USA.

Fully automatic telescope mounts do exist, but they’re often expensive, bulky, still require extensive calibration, and can only handle telescopes that don’t weigh much. None of the commercially available solutions match the portability and ease-of-use that I’m aiming for.

With that in mind, here’s a list of self-imposed requirements for my project. Most are engineering requirements, but some are more subjective.

Requirements

  • Must operate in normal circumstances with no manual set-up other than powering on/off

  • Must be usable on tilted ground, but does not need to prevent the telescope from falling on extremely tilted ground

  • Must be usable anywhere in the world that a compass can get a reading

  • Must take less than 60 seconds from power-on to ready-to-track

  • Must fit within a carry-on suitcase

  • Must have a polar alignment error under 30 arcminutes to allow for astrophotography

  • Must have a battery life of at least 4 hours on a full charge

  • Must be able to handle a 20lb telescope

  • Must be able to stay on-target for at least 30 minutes uninterrupted

  • Must not require internet access nor connection to a phone/external processing source

  • Must be steady enough for long astrophotography exposures

  • Must not include any always-on lights

  • Must have easy-to-understand user interface

  • Must look good enough that I want to use it

So how do we make this happen? Well, I’m tentatively calling my initial idea a “Universal Polar Mount”.

There are two halves to the idea: the inputs and the outputs/mechanics.

Inputs

Theoretically, if we have an accurate idea of compass heading, GPS coordinates, and the tilt of the platform, we can figure out the earth’s rotational axis as it relates to the current position of the platform.

The biggest issue is compass heading. If we have an accurate compass sensor I think we can get a good idea of the direction of magnetic north, but that’s just the beginning.

From the GPS coordinates we can find two key pieces of information: the current latitude and the magnetic declination. Latitude is pretty straightforward, but I didn’t initially realize how big of an issue magnetic declination was. The basic idea is that compasses don’t actually point directly at the north pole- depending on where you are in the world, they might be off by as much as 30 degrees. To make matters worse, this difference, called declination, changes over time. I think I have an idea for how to account for this, but we’ll dive into that later in the project.

Lastly is the tilt of the platform. We’ll need a tilt sensor since we can’t rely on the platform being put down on perfectly level ground.

I’m anticipating having to do a ton of sensor data processing and vector math throughout this project to make all these inputs play nicely together, but my confidence is growing that these are all we’ll need.

Note: I acknowledge that the most accurate way to align with true north is visually aligning with polaris (the north star). I explored the option of using a built-in camera with image processing to do this, but that option still requires all of these additional inputs and adds a LOT more programming. If it turns out that the planned inputs can’t track accurately enough to meet my requirements, it may be necessary to add a camera for that last little bit of alignment.

Outputs/Mechanics

Once we know the spatial relationship between the platform surface and the earth’s axis of rotation, we can determine where the platform surface needs to be at every future point in time to remain aligned with the sky’s movement. We then need to give the platform the ability to move to that position.

In my opinion, the most elegant feature of a traditional equatorial mount is this tracking only requires one motor moving at a constant speed. It can do this because proper set-up and alignment eliminates the need to compensate for everything except the always-steady rotation of the earth. In our case, since we need to compensate for tilt, latitude, and compass heading in addition to the earth’s rotation, we’ll need three motors moving at variable speeds. Initially I’m imagining a yaw axis rotating the entire platform, and then two symmetric but perpendicular axes (pitch and roll) on the rest plane of the platform surface.

The motors will need to operate smoothly and accurately at extremely slow speeds, which will be a challenge both electrically and mechanically. Synchronizing three motors at variable slow speeds is difficult because even tiny inaccuracies can blur astrophotography images. The mechanical structure will also need to be designed so the telescope is very steady on the platform, with its weight directed down to the ground via a short and solid path.

Summary of Challenges

This is going to be a long ongoing project. Here isthe list of challenges as I see them currently, ranked by anticipated difficulty (easiest first):

  • Developing an intuitive user interface with clear error alerts

  • Finding smooth, low-power motors and motor controllers

  • Finding precise compass, tilt, and GPS sensors

  • Developing precise and reliable sensor data-processing algorithms

  • Designing a sturdy and compact mechanical enclosure

  • Designing a printed circuit board (PCB)

  • Developing calibration process for compass and tilt sensors

  • Compensating accurately for magnetic declination

  • Developing a robust mechanical yaw axis

  • Developing robust mechanical pitch and roll axes

  • Figuring out the math and programming to determine motor positions over time

  • Implementing accurate vector math for determining alignment

Thanks for reading! In my next journal entry I’ll dive deeper into selecting the sensors and doing some tests to see if my theoretical ideas hold up in practice. Stay tuned!

Brett Potter