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Work in progress: The explorad combines a microcontroller, a display, and star charts in a head-up display for astronomical telescopes.

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This project was created on 05/04/2014 and last updated 3 months ago.

There are a number of useful things that make an amateur astronomer's life easier. The "Telrad" displays circles of 0.5, 2 and 4 degrees diameter in your field of view. Some astronomical charts also have these circles printed around objects, and together these make it easier to navigate through the night sky and find what you want to look at.

The explorad combines these features in a single device:
- A display and some more optical hardware is used to show telrad circles and some more information, projected into your view at the sky.
- Object information is stored on an SD card. Objects can be marked as a target for easier lookup, but the explorad can also display the position of other objects - there might be something interesting just around the corner!
- The telescope's viewing direction is determined with an electronic compass or with digital setting circles. With an explorad mounted on top, the explorad helps you point your telescope at an object and take a close look.

How the Telrad works:

Here's a little drawing that shows how the Telrad works. It just shows the main parts:

The Telrad contains a screen with three backlit circles. Light emitted from that screen is mirrored to a lens which brings the circles into infinity focus, and then (partially) reflected into the observer's view by a glass plate. The observer's eyes are focused at infinity, looking at a distant object. The resulting image contains the both the object and the red circles.

The size of the circles on the screen and the lens' focal length are chosen to have circles with 0.5°, 2° and 4° diameter. The screen can be moved to hit the right focus point. When you look through a Telrad, you'll see something like this:

(That's a selfmade Telrad clone I made)

How that relates to star charts:

Star charts contain circles with 0.5°, 2° and 4° around interesting objects and around points that are easy to find (bright stars). Here's a sample:

The star chart/Telrad combo can be used to easily navigate through the night sky to find an object for observation.

What the explorad does:

The explorad replaces the Telrad's screen with an OLED display. Why an OLED?

At night, the eyes adapt to the darkness. This adaption is easily disturbed by bright light, especially with wavelengths which the eyes are sensitive to. The eyes are least sensitive to red light, and that's why the Telrad's circles are red. An OLED display has no backlight which could bleed unwanted wavelengths, and we can choose to display only red pixels. That way the eyes are least disturbed.

If only circles are displayed, the explorad does the same thing as the Telrad.

What the explorad also does:

In addition to displaying the Telrad circles, the explorad can determine the telescope's viewing direction and use that information to do two main things:

  • display how the telescope must moved to bring a target into the field of view
  • display nearby objects on the screen which might be interesting to look at.

Determining the telescope's viewing direction is not too hard. A 3D accelerometer is used to measure the altitude (angle between the horizon and the telescope's viewing direction) and roll. I tried using a magnetometer for the azimuth, but that didn't work well - I got distorted readings caused by almost every object around the sensor. That's why I'm attaching an optical encoder to the telescope's base to measure the azimuth.

Demo Video

showing the current hardware, Telrad circles, Target tracking, and nearby objects display:

The display you see in the video will replace the Telrad's fixed 3-circle screen.


If the SdFat library is used in the project, the whole project probably has to be published as GPLv3 (I guess).

  • 1 × Teensy 3.1 Cortex-M4 prototyping board
  • 1 × Adafruit 128x128 OLED display
  • 1 × Pololu LSM303D breakout board Electronic Compass
  • 1 × Avago HEDS-5540 H06 optical encoder digital setting circle
  • 1 × input device Look left for the link to the proxy project
  • 1 × Navilock NL-507ETTL GPS breakout board with TTL output
  • 1 × Adafruit CC3000 breakout board For connectivity
  • 1 × A microSD card Mine is a SanDisk 4GB SDHC

Project logs
  • Semi-paused

    3 months ago • 2 comments

    In case anyone was wondering: I'm still working on this, but a 12" telescope is watiting to be finished as well. When it's done I'll post more about explorad and try to finish it.

  • Github repo, System Design Document and licenses

    9 months ago • 0 comments

    I've created a github repo, added a system design document (SDD) and added license information as required by the contest rules.

    The github repo is linked to in the project's link section. It currently only contains the SDD. Code will be added once it is easier to handle (bugs aside). License information was added in the project description, but that's preliminary.

  • I've got a video!

    9 months ago • 0 comments

    After a bit of fighting I can present this video. The annotation looked ok when I watched the video on youtube, but not here.

View all 24 project logs


Tiago wrote 10 months ago point
I hear you about the magnetometers, such a pain in the ass. Really sucks that there isn't any better alternatives.

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Christoph wrote 10 months ago point
I threw out the magnetometer, i.e. I'm using the electronic compass only to get altitude and roll values. When I figured out how to properly process the magnetometer values, I might use it again.

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pixelk wrote 11 months ago point
I'm working on a distantly similar project (an aid for polar alignment) and yours is very interesting to me. I try to work with Navspark's module ( but the truth is, the Arduino environment they ported isn't really ready yet. Too bad as the hardware is a nice addition to the teensy I use for the main board. I'm still trying to interface it with Sparkfun's LSM9DS0 breakout to have an all-in-one navigation reference.

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Christoph wrote 10 months ago 1 point
Which Teensy are you using? If it's an AVR-based one, try the Teensy 3.1 as an upgrade. It looks like it doesn't have as much power as the NavSpark, but I think Paul will soon have the Teensy 3++ (or whatever its name will be) ready.

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pixelk wrote 10 months ago point
Yes, I'm using a teensy 3.1. I pledged for the original Kickstarter for the 3.0 and since then I'm huge fan. Paul Stoffregen delivered as expected and the support is excellent.

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Mike Szczys wrote 11 months ago point
Nice start to this project! Thanks for submitting it to The Hackaday Prize. Have you thought about how this is a "connected" device (see the Basic Judging Criteria here: Part of it could be the input device, which you could package up with a protocol and make it extendable for other uses. But perhaps you want to make a way to save, share, and import information if many people had this rigged up to their telescopes? Just a thought.

I love the logo on the boot screen!

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Christoph wrote 11 months ago point
The input device will have a wired connection to the main device because the input device will (I've almost settled on that now) carry the battery for both. That way, the input device can be detached for charging and the main device can be left on the telescope without destroying its alignment (it must be aligned with the telescope's optical axis).

This device will be "connected" with a WiFi module for downloading the positions of solar system bodies, as they are more dynamic than objects in the milky way or deep-sky objects. I'm also thinking about a link between the explorad and stellarium.

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x3n0x wrote a year ago point
Looks really cool! Cant wait to see how it pans out!

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Michael O'Brien wrote a year ago point
When might you be putting up a parts list, even partial? I'm curious about performance and the like. Is that an OLED display? I commend the choice since you don't have to deal with backlight bleed :)

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Christoph wrote a year ago point
What you can see in the pictures is already included in the parts list, and the parts list will grow with the project.
The display is an OLED for exactly the reason you state - no backlight, only red. I can even dim it by adjusting the red value, because the display uses 16-bit rgb.
I will keep updating this project with details as I go along.

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Quinn wrote a year ago point
Cool project! Are you going to make the display appear as an overlay to the sky at infinity focus like the Telrad does?

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Christoph wrote a year ago point
Yes, it will have the same optics as a telrad and display everything at infinity focus. I'm thinking about adding a secondary display for tasks that don't require seeing the sky in the background.

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