The SBIG AllSky-340 is our third-generation weatherproof all-sky camera, perfect for monitoring observatory weather conditions. Available in both monochrome and color versions, the KAI-0340 CCD sensor has 640 x 480 pixels at 7.4 microns square, and excellent sensitivity. Protected by an acrylic dome, the Fujinon FE185C046HA-1 lens has a 1.4 mm focal length at f/1.4, and provides excellent image quality all the way to the horizon. This sample image was a 60 second exposure from a light polluted back yard – with a first quarter moon behind the roof – and shows detail in the Milky Way.
One of the challenges of an All Sky camera is that the moon is in the field of view literally half of the time, averaged over a month. The reflections from within the dome and lens can interfere with detection of stars. The second test image below was taken earlier in the night, and has been DDP scaled to show the stars and the sky close to the moon. Some ghost reflections are visible, but the image quality is still excellent.
The AllSky340 has an RS-232 link to the PC for control and image download. While this interface is very slow in comparison to USB, it will tolerate very long 100 foot (30 meter) cable runs. Or, you can throw away the cable and use a bluetooth wireless link with an optional inexpensive adapter. The low power requirements mean that the camera can be powered by a solar array and battery for completely wireless operation. The camera can be located for a good view (e.g. rooftop) instead of for convenience for the wiring.
The RS-232 link reliably downloads the full image in 15 seconds at 460.8 kBaud over a 100 foot cable. Running at 115.2 kBaud over Bluetooth wireless, works up to 75 feet. With wireless links, one must minimize the number of walls you have to pass through. Each wall (2 layers of dry wall or wood) costs about a factor of two signal and range.
Although the RS-232 interface is slower, the AllSky340 can take an image while transmitting the previous image. Only a 1 second gap is required for image download, making this an excellent camera for meteor detection. The software can run continuously in the background while you use your computer for other tasks. At these slow download rates the computer workload is so slight your applications won’t even notice. The All Sky image is there when you want to view it.
The housing is illustrated in Image 3. The fisheye lens is mounted to a plate which can be translated, tipped, and focused relative to the CCD, so the full resolution of the lens can be achieved. This plate is also heated, to keep the lens free of condensation. The heat rises into the acrylic dome, warming it and keeping off the dew, and drying off raindrops. The inexpensive acrylic dome is easily replaced by removing a few screws, allowing for routine replacement in the field as the dome suffers the inevitable scratches and damage due to sunlight, windblown dust and disrespectful birds. Replacement domes are available from SBIG. The enclosure is 5.5×5.5×11 inches in size (14x14x28 cm).
This camera can also take exposures as short as 50 microseconds, so daylight operation is possible, allowing recording of cloud conditions 24 hours a day. A blooming streak will be noted vertically through the sun, but otherwise the image is excellent.
Note: the AllSky340C color version is no longer available, as the sensor is obsolete.