The Star-ball

What to make it out of?

If I was to make one now, I would probably use something else, but back in the early 80's there was a more limited choice.

My first idea was to re-use something else, here in the UK we had pedestrian crossings that consisted of alternate wide stripes of black and white across the road, these were announced to car drivers by having two poles on either side of the road, atop of each was a large flashing orange globe. Once on the crossing the pedestrian has the right of way. These seem to have disappeared now, but went under the name of zebra-crossings, and the flashing lights were known as Blisha-beacons! So if I could only acquire one, it could be put to another use. Occasionally a car would hit one, and send the ball flying, but when I came to inspect one of these fallen Blisha-beacons I found it to be made of far to thick a material, and also a bit to small. So the idea was dropped. Finally I decided to form one out of glass-fibre, a 2ft (60cm) inflatable beach-ball was the ideal mould. So I set to work, outside of course as the fumes are very unhealthy. A word of warning to anyone planning such a venture, I started off in the shade of my parents house, but thought to put it in the sun to dry quicker, bad idea. The uneven heating caused the ball to expand, I quickly saw my folly, and returned it to the shade. But I have a slight bulged in my star ball now.

Me plus star-ball at a society meeting Larger version 34Kb

Putting in the stars

These are formed by varying sizes of holes, as I had chosen a limiting magnitude of 5, this required the smallest drill. All others being in proportion to it. But how small a drill could I get? I remembered seeing a 1/32 inch a school, so I visited my local hardware store. An Aladin's cave of goodies, the sort of place where you could get a single nut, as an assistant rummaged through a box of assorted odds and ends to find one that fitted your bolt. I said I wanted some small drills, how small came the reply, the smallest I answered. But was not prepared for the minutest drills I was shown. I eventually decided on the second smallest drill a number 79, as by 5th magnitude star. See what I mean (19Kb jpeg image)

The brighter stars required bigger holes, and as I was adding coloured filters to make the blue-white and orange stars look truer to life, this required even bigger holes. Sirius the brightest star is a 1/2 inch (13mm) hole, no longer a pin-hole! So all the stars of 1st magnitude and above have lenses to focus the star image down to that produced by the fainter stars. The lenses where the type added to the view-finders of SLR cameras. I did not try to explain to the camera shop salesman why I wanted so many +3 dioptre viewfinder lenses, but I got a discount on my order. The lenses (just the central strip) are held on each by two pieces of notched coke super-glued to the inside of the star-ball.

Lighting it up

The small filament bulb is held atop of a large kitchen plastic funnel, and is powered by a set of rechargeable batteries held under the funnel. As the star-ball is a fixed latitude type, for me I am 52°rees; north, left like this the stars that are yet to rise will project over the floor of the planetariurm. What I needed was a easy way to cut off such stars. An exterior horizon seemed clumsy, then I hit upon the answer. A self-leveling internal horizon, made up of a ball-bearing, some corrugated cardboard, string and a brass weight. All attached to the top of the funnel. The colour photo does not show it too well below, so here's a diagram (6Kb gif)

Inside the projector Larger version 22Kb

The Milky-way projector.

For this I drew out a large scale map of the milky-way, and applied various densities of Letraset® screen dots. See example area (13Kb jpeg)This negative brightness image was photographed onto a 5x4 inch black and white negative, which I then rapped around a small tube, with a light at it center. This assembly is attached on the base of the star-ball via a camera ball-and-socket joint, which allows the position of the milky-way to be adjusted to it's correct position (well almost). It appears dead-center in the above photograph showing the inside of the projector.