Allsky Camera

In an (ongoing) project to simulate the appearance of the sky, I decided I needed some actual observations to check my simulations against. Hence I built an "all-sky camera", an ordinary camera facing a curved mirror that allows you to take a picture of the entire sky in one shot. Because I couldn't find a large enough curved mirror, this version uses a 3 inch "blind spot" car mirror epoxied to the center of an 8 inch stainless steel oven attachment (!). This is of course quite cheesy; and I've been on the lookout for a better mirror.

The whole thing is shown on the right. The camera slides into the wire harness, and faces down toward the main mirrors. The entire assembly is all attached to a little wooden frame that fits on my tripod.

Because the camera isn't permanently attached to the mirror, I had to paint some calibration patterns around the mirror's surface. Hence the first stage in processing an allsky camera photo (1) is to locate, using a standard correlation matching program, the three little calibration symbols in the image and then shift the image so the symbols lie in the standard position (2). Using the known and now aligned geometry of the mirror, it's easy to project the image back onto the sky for both the inner (3) and outer (4) mirrors. In the images below, the sky is shown in a polar projection.

(1). Source image from allsky camera	(2). Aligned reflection map image
(3). Calibrated sky image from inner mirror	(4). Calibrated sky image from outer mirror

Because of the crappy mirror and the fact that my Epson PhotoPC 550 doesn't let you turn off color adjustment, the images weren't nearly as nice as I'd hoped. The processing was quite interesting, though, and I think I've learned enough to make the next version a good deal better.