4
By actual test, an RS-232 link, using a USB to RS-232
adapter running at 460.8 Kbaud, with a 150 foot long 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 Kbaud, 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 in signal and
range.
The All Sky camera 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 – the
software can run continuously in the background while you use your
computer for other tasks. At these 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.
Mechanical Implementation:
The housing is lustrated in Figure One. 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. The
prototype shown in Figure one is black – the production version has a
white body so it doesn’t get as hot in the sun. The unit is 5.5x5.5x11
inches in size (14x14x28 cm). The cables are designed to exit out the
bottom of the unit so the connectors stay dry. If one runs the cables
into a building make sure the last section of cable before entering the
structure angles upward so water doesn’t drip down the cable into the
wall. The housing has holes in the side so it can be easily mounted to
a post with ¼-20 lag screws, or ¼-20 bolts.
