Why calibrate?

The Seletek uses a DS18B20 (or compatible) digital temperature sensor. These sensors are highly accurate from the factory — typically ±0.5 °C — but they can have a small systematic offset. For temperature compensation of the focuser, even a 1 °C offset can produce a noticeable focus drift, especially on fast optical systems.

Calibration takes only a few minutes and involves comparing the sensor reading to a known reference, then setting an offset in the software.

Calibration procedure

Seletek temperature sensor calibration screen — offset adjustment
Seletek configuration panel — temperature sensor reading and offset field for calibration.
  1. Let the sensor stabilise: Mount the sensor in its operating position on the telescope and let it sit for at least 5 minutes so the sensor temperature matches the local air temperature.
  2. Read a reference: Use a trusted thermometer (a calibrated digital thermometer or a weather station reading from a sensor near the telescope) to get the true temperature.
  3. Read the Seletek: Note the temperature shown in the Seletek software's temperature display.
  4. Calculate the offset: Offset = Reference temperature − Seletek reading. For example, if the reference reads 14.3 °C and the Seletek reads 13.9 °C, the offset is +0.4 °C.
  5. Enter the offset: In the Seletek configuration panel, find the temperature offset field and enter the calculated value. The displayed temperature will update to reflect the corrected reading.

Temperature compensation coefficients

Once the sensor is calibrated, set up temperature compensation in the focuser configuration:

  • Focus coefficient: The number of focuser steps per degree Celsius change. Determine this by noting the in-focus position at two different temperatures and dividing the step difference by the temperature difference.
  • Direction: Whether focus moves in or out as temperature drops — depends on your telescope design. Most refractors and some SCTs require a compensation direction that moves the focuser out as temperature falls.

A typical overnight temperature drop of 5–10 °C can require hundreds of steps of compensation — calibrating the sensor correctly ensures the compensation is accurate.

Ice water reference: For a very accurate calibration, use a mix of ice and water (0 °C at equilibrium). Immerse the sensor tip in the slurry for 30 seconds and adjust the offset until the Seletek reads 0.0 °C.