Dew heater

A fisheye dome pointed at a clear sky radiates heat to space and drops below the air temperature. When it crosses the dewpoint, the dome fogs and the rest of the night is lost — the reference rig has logged dew arriving twice in a single night, between 02:30 and 05:00. A few watts of heat on the dome base fixes it, and Skylapse can run that heat itself.

How Skylapse controls it

• A BME280 on the Pi’s I2C bus (address 0x76 or 0x77, probed automatically) gives temperature, humidity and pressure. Skylapse computes the dewpoint from them.
• When the temperature closes in on the dewpoint the heater switches on; once there is comfortable margin again it switches off. There is a dead band between the two thresholds, so the heater does not chatter on a marginal night.
• The heater is driven from a GPIO pin through a MOSFET module (BCM 18 by default). It is forced off whenever the capture daemon exits, so a stopped daemon can never leave the heater on.
• Or skip the sensor entirely: the Heater tab has a manual on/off switch (v0.5.27+).

BME280, not BMP280

The BMP280 looks identical and costs a dollar less, but it has no humidity sensor — and humidity is the whole input to a dewpoint. Skylapse checks the chip id and rejects a BMP280 rather than compute nonsense.

Wiring it today

The reference build already has everything except the heating element:

Pi GPIO 18 ─► MOSFET module (signal)
12V rail   ─► MOSFET module (power in)
              MOSFET module (power out) ─► heater ─► GND

BME280 ─► Pi I2C (SDA, SCL, 3.3V, GND)

Any resistive element works: a ring of power resistors around the dome base (~3–5 W at 12V), a USB lens-warmer band, or nichrome. Keep it on the 12V rail, not the Pi’s 5V — see why the power is built this way.

A Skylapse heater ring is coming coming soon

A purpose-made 12V heater ring for the dome, switched by Skylapse exactly as described above, is in the works. Estimated ~$20. It will be announced on the Store page and in the GitHub release notes when it is real and in hand — not before. Until then, the resistor ring above works fine.