DayCam was developed to provide information on daytime cloud cover conditions at the observing site to help observers evaluate the probable night time conditions. Our experience has been that satellite and radar images provide some insight but that conditions in north-central Florida vary rapidly with both time and location and that there is no substitute for good real-time information gathered at the actual observing site.
Figure A illustrates the formation of a virtual image of a distant object in the spherical "gazing ball" by following two parallel rays. For paraxial rays, a ray directed at the center of the ball reflects back on itself and its extension lies on a radius of the ball. A ray directed at the focal point of the ball (which is a spherical mirror) reflects parallel to the optical axis. The virtual image is located at the place the extension of this ray crosses the center directed ray. Figure B shows the formation of a real image of this virtual image on the camera detector. Note that the virtual image is formed in the interior of the ball, approximately half way between the center and the surface of the ball. Thus the camera must be focused at a near distance (about 10 cm in the case of the AST@RHO DayCam).
In actual use, the camera was mounted in a 4 inch PVC pipe that served as a weatherproof housing. The remainder of the system was fabricated with 1 inch PVC pipe attached to a tall 4 inch PVC pier. The USB cable runs out of the pipe through a rubber grommet.
Here we see a camera (an Intel Pro Cam in this case) and a gazing ball (a silver Christmas ornament), set up at approximately the proper separation for use as an all sky camera. Currently we are using an IBM PC Cam for our camera but any inexpensive CMOS camera should work. CCD cameras are to be avoided because the solar image will cause major bleeding along columns.
Assistance in the construction and testing of DayCam came from Thomas Martinez, a high school student in the UF SSTP program who worked in the AST@RHO lab during the summer of 2001. Jacob Baron, also an SSTP student, provided helpful suggestions.