External Temperature Sensors

Certain astronomical cameras (such as uncooled CMOS sensors, older USB webcams, or Raspberry Pi Camera Modules) lack internal temperature telemetry. Accurate temperature measurement is essential for:

1. Dark Frame Library Matching: Dark current scales exponentially with sensor temperature; master dark frames must be matched within $\pm 2^\circ\text{C}$ to $\pm 3^\circ\text{C}$ of ambient capture conditions.
2. Dew Heater Automation: Regulating heater duty cycle against ambient dew point calculations.

indi-allsky supports executing a custom script to inject ambient temperature telemetry directly into the camera image metadata.

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1. Configuration

In the web interface under Configuration → Camera:

• EXTERNAL_TEMP_SCRIPT: Specify the absolute path to your executable script (e.g. /etc/indi-allsky/scripts/get_ambient_temp.py).

Ensure the script has executable permissions and is readable by the indi-allsky service user:

sudo chmod 755 /etc/indi-allsky/scripts/get_ambient_temp.py
sudo chown indi-allsky:indi-allsky /etc/indi-allsky/scripts/get_ambient_temp.py

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2. Script Execution Protocol

When invoking the external script, indi-allsky supplies an environment variable named TEMP_JSON containing the temporary file path where output must be written.

The script must write valid JSON containing a key named temp with a floating-point temperature value in degrees Celsius:

{
  "temp": 14.85
}

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3. Implementation Examples

Python Sensor Script Example

#!/usr/bin/env python3
import os
import json

# Query your hardware sensor (e.g., I2C BME280, 1-Wire DS18B20, or weather station API)
ambient_temp_celsius = 18.25

output_path = os.environ.get("TEMP_JSON")
if output_path:
    with open(output_path, "w") as f:
        json.dump({"temp": round(ambient_temp_celsius, 2)}, f)

1-Wire DS18B20 Shell Script Example

#!/usr/bin/env bash
# Read from Linux 1-Wire subsystem
SENSOR_FILE="/sys/bus/w1/devices/28-00000xxxxxxx/w1_slave"

if [ -f "$SENSOR_FILE" ] && [ -n "$TEMP_JSON" ]; then
    RAW_TEMP=$(grep -o 't=[0-9\-]*' "$SENSOR_FILE" | cut -d= -f2)
    TEMP_C=$(awk "BEGIN {print $RAW_TEMP / 1000}")
    echo "{\"temp\": $TEMP_C}" > "$TEMP_JSON"
fi

Manual Verification

Test execution directly from the command line:

TEMP_JSON=/tmp/test_temp.json /etc/indi-allsky/scripts/get_ambient_temp.py
cat /tmp/test_temp.json