Sky Quality Meter Project

Introduction

Inspired by the Unihedron Sky Quality Meter and its straightforward design, I embarked on creating my own DIY handheld version. My goal was to build a device that not only displays sky quality in Mag/Arc sec² but also maps it to the relevant Bortle zone for quick reference. Thanks to Unihedron’s documentation on key components like the TSL237, a UV/IR Cut Filter, and a 30-degree lens, I found it easy to get started.

Parts List

• OLED Display: Aliexpress Link

• Project Enclosure: Jaycar Black Hand-Held Electronic Enclosure

• Arduino Nano: AliExpress Link

• TSL237: AliExpress Link

• 30-degree Lens: AliExpress Link

• UV/IR Cut Filter: Sourced from CCD Camera Module AliExpress Link

• 9V Battery and 9V Battery Terminal Lead

• LM7805 5V Battery Regulator: Jaycar Link

• Latching Switch

Code

https://github.com/joeldhenry/ArduinoSQM/blob/main/ArduinoSQM.ino

Assembly Parts

https://github.com/joeldhenry/ArduinoSQM/blob/main/cap-SQM.gcode

https://github.com/joeldhenry/ArduinoSQM/blob/main/front-v2.gcode

https://www.thingiverse.com/thing:6578554

Main Functionality

Unihedron provides the specifications for the TSL237 on their website, which details the coding required. The critical code segment is:

SQM_OFFSET - 2.5 * log10(frequency) * SQMGain

Here, the Offset and Gain are calibration variables. I set my gain to 1 and offset to 21, but these values can vary based on the TSL237 sensor and calibration accuracy. My initial calibration was done using the Sky Quality iPhone app.

Additionally, I created a table for Bortle Value lookups from Wikipedia:

String getBortle(double mag) {

  if (mag >= 21.99) {

    return "Bortle 1";

  } else if (mag >= 21.89) {

    return "Bortle 2";

  } else if (mag >= 21.69) {

    return "Bortle 3";

  } else if (mag >= 21.25) {

    return "Bortle 4";

  } else if (mag >= 20.49) {

    return "Bortle 4.5";

  } else if (mag >= 19.50) {

    return "Bortle 5";

  } else if (mag >= 18.95) {

    return "Bortle 6";

  } else if (mag >= 18.38) {

    return "Bortle 7";

  } else if (mag >= 17.80) {

    return "Bortle 8";

  } else {

    return "Bortle 9";

  }

}

Challenges

1. Compatibility Issues: The FreqMeasure Arduino library is not compatible with ESP32 or ESP8266, leading me to use an Arduino Nano. This is likely due to differences in clock cycles and frequency calculations.

2. Sensor Availability: The TSL237 is an older sensor and is harder to find and more expensive. While it’s available on AliExpress, newer sensors might offer better quality or results, though they would require significant code refactoring.

3. Light Leakage: I used putty and removed LEDs from the Arduino and OLED display to prevent light leakage into the sensor.

4. Calibration: Calibration requires understanding the code and output frequency, as well as a calibrated device, clear skies, and time. My initial calibration was done using the Sky Quality app, and further calibration was assisted by a fellow astronomy club member. Future code adjustments require connecting the Arduino to a computer, so I recommend making the device easily accessible.

Further Improvement

To further enhance the functionality and accuracy of this project, future efforts could focus on:

• ASCOM Driver Support: Adding ASCOM driver support to allow integration with various astronomy software, making the device more versatile and user-friendly.

• Improved Calibration Methods: Refining the calibration process to ensure more accurate readings.

• Newer Sensors: Exploring compatibility with newer sensors for potentially better performance.

• Bluetooth or Wi-Fi Connectivity: Revisiting the idea of Bluetooth or Wi-Fi compatibility if suitable hardware becomes available.

• User Interface Enhancements: Improving the display and interface for easier use in the field.