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SBIG is about to release our new All-Sky cameras, the AllSky-340 and AllSky-340C.
The model 340 is a monochrome camera and the model 340C uses a color CCD. This
will be SBIG's third version of a weatherproof AllSky camera for monitoring weather
conditions. The first had to be obsoleted when we stopped production of the
ST-237A, and the second went obsolete when the lens we were using became
unavailable, with no suitable replacement. With each version we gain some
knowledge and this is our best one yet. The sensor is the Kodak KAI-340 CCD, with
640x480 pixels, 7.4 microns square, and a high gain output stage for excellent
sensitivity. The camera electronics and CCD are from the soon to be released Smart
Guider camera, our new autonomous guide camera. The All Sky camera will precede
the guider since the software task is easier and is essentially done at this point.
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The All Sky camera incorporates the Smart Guider Camera inside an aluminum enclosure with an acrylic
dome to protect the fisheye lens. The fisheye lens is Fujinon's new FE185C046HA-1, with a 1.4 mm focal
length, F/1.4. We have tested this lens and the image quality is excellent wide open, all the way to
the horizon. Figure One below shows an All Sky camera image with the Fujinon lens and KAI-340 CCD.
The exposure was 60 seconds from a light polluted backyard - the first quarter moon had just set behind
the roof.
The new All Sky camera has some very interesting features that make it much more versatile than our
previous offerings. First of all, it 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 100 foot (30 meter) cable
runs. Or, you can throw away the cable and use a bluetooth wireless link with an optional inexpensive
adapter. And, the unit is low power and can be powered by a solar array (also optional) so you
don’t any wires AT ALL running from your PC to the camera! The beauty of this is now the unit
can be located where it has a good view of the sky, instead of good access to a power plug or PC. Your
roof is now the preferred location, above the trees and neighbor’s houses.
By actual test, an RS-232 link, using a USB to RS-232 adapter running at 460.8 Kbaud, with a 100 foot
cable, downloads a full image reliably in 15 seconds. At 115.2 Kbaud a full image takes 60 seconds.
Bluetooth wireless adapters will typically run at 115 K-baud, and the one we have tried here worked
reliably at a distance of 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.
These may seem rather long download times compared to the USB 2.0 interface of our last AllSky camera,
but the new AllSky-340 can take an image while transmitting, so with exposures longer than the download
time the camera is only insensitive for the length of the readout of the CCD, which takes place in less
than 1 second to an internal memory buffer in the camera. As a result, the camera is excellent for
meteor detection. Its field of view is wider than our previous meteor camera, so it should see many
more meteors near the horizon. One other plus â€" 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. It will not interfere with regular imaging using
the same PC. The All Sky image is there when you want to view it.
The housing is illustrated in Figure Three. 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. SBIG will maintain a supply of replacement domes. The prototype shown here is black â€" the
production version will have a white body so it doesn't get too hot in the sun. It is 5.5x5.5x11
inches in size (14x14x28 cm). |
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