Skip to content



The Boltwood Cloud Sensor II is the must-have accessory for your remote or automated observatory. Protect your valuable equipment from weather damage. Aids in automatic scheduling of observations via popular observatory control applications.

The Boltwood Cloud Sensor II measures the amount of cloud cover by comparing the temperature of the sky to the ambient ground level temperature. The sky temperature is determined by measuring the amount of radiation in the 8 to 14 micron infrared band. A large difference indicates clear skies, whereas a small difference indicates dense, low-level clouds. This allows the sensor to continuously monitor the clarity of the skies, and to trigger appropriate alerts on your computer.

The device also includes a moisture sensor which detects rain and snow. To prevent false alarms due to frost or dew, a heater keeps the sensor slightly above ambient temperature. When rain or snow falls, the sensor is automatically heated to 70 degrees Celsius. This clears the sensor quickly when the precipitation ends, and ensures that the sky-measuring thermopile has a clear view.

The Boltwood Cloud Sensor II measures wind speed, using a specially-designed anemometer with no moving parts. The sensor will warn you when winds speeds are too high for safe operation of the observatory. Plus the sensor detects daylight and can be set to automatically close the roof to prevent any possibility of sunlight entering the telescope. The sensor also measures humidity, and provides a continuous readout of temperature, humidity, and dew point.

The included Clarity II Windows software monitors the sensor via a USB interface. It displays and logs all sensor information. Alerts can appear visually, or with sounds from either the PC speaker or sound card. The user can choose to be alerted for clouds, wind, and/or wetness; all with adjustable thresholds. The computer display can be set for a mini status window or a full information display.

The sensor unit itself will trigger an electrical contact closure when heavy clouds, high winds, or precipitation is detected. When connected to a dome or roof controller, this contact can be used to trigger closure even if the computer is not operating. This provides a ‘last line of defence’ against damage to your observatory equipment. The thresholds for roof closure are set through the Clarity software.

Cloud Sensor Graph II software is included. This provides a continuous graphical plot of the sky conditions, temperature, wind, humidity, etc.

ActiveX and file interfaces are provided for integrating the Clarity II software into your observatory control system.

The complete control software is available via the download tab (above)Both sensor firmware and Clarity II upgrades may be available.

The complete package, shown above, includes sensor with mounting bracket (shown attached to a pole), cable, adapter box, universal AC power supply with international plugs, USB cable, and PC software.

The Boltwood Cloud Sensor package includes the sensor unit, mounting bracket, a 50-, 100- or 150-foot (15m, 30m or 45m) data cable, 110/220 VAC 50/60 Hz power supply adapter, and software USB Stick. Compatible with Windows 2000, XP, Vista, 7, 8, 8.1, and 10. Open source Linux drivers are available.

Read an independent review of the Boltwood Cloud Sensor on

The Cloud Sensor mounted to a pole using the included mounting bracket. A 10 degree tilt helps rainwater run off the sensor face. The gold post is the wind speed sensor, and the smaller posts help prevent birds from roosting on the sensor.

The software window screen capture, above, indicates a clear sky. Winds are moderate, the temperature is below freezing, and the humidity is moderate; we are not near the dew point and the sensor heater is on low.

The screen capture, above, shows the clouds have just rolled in, and the wind is picking up. The roof closure contacts have been activated to protect the observatory. No sign of rain yet, but the moisture detector will see the very first drop.


Boltwood Cloud Sensor II Digital Thermopile Firmware

Newer Boltwood Cloud Sensor II units (serial numbers 4000 and above) use a digital thermopile requiring special firmware (version 67). This firmware is provided with your unit. We are also providing the last firmware version available to older units with analog thermopiles (version 65) for your convenience.

Warning: Do not update firmware except under instruction from Diffraction Limited technical support.

Incorrectly updating will brick your unit.

Download (v70, SN 4000+)

Older units S/N up to 3999:

Download (v65, SN 3999-)

Boltwood Cloud Sensor II Clarity Release 3.008

Control and logging software for your Boltwood Cloud Sensor II.


User’s Manual

Installation and operation instructions for your Boltwood Cloud Sensor II.


Linux Drivers

Open source Boltwood Cloud Sensor II drivers for Linux systems.


Boltwood Cloud Sensor I Clarity

Control and logging software for the original Boltwood Cloud Sensor I. This is NOT compatible with Cloud Sensor II.