Post by Ron Walker on Jul 7, 2022 14:18:23 GMT -7
Why do we want to use lenses in a light and shadow pin hole projector. Basically to be able to project more levels of magnitude for a denser star field. In order to project brighter stars we need to enlarge the size of the holes. You might ask why not just make smaller holes for the dimmer stars which sounds good, but only good to a point. As the holes get smaller and smaller one runs into a diffraction problem where the projected image actually gets larger. I'm no physicist so I can't tell you why exactly but I know through experimentation that this does happen.
As light from a nearly point source flows through a pin hole it will expand until it hits the dome or screen and project a star dot. The larger the star ball the bigger a star hole one can have for a given projected star dot. Some pin hole projectors use star balls four feet in diameter. One does reach a point of diminishing returns and a compromise of eighteen inches appears to work very well. This allows for stars as faint as fifth magnitude to project without any problems. The problem with the bright stars is that they require holes as large as a quarter inch in diameter to allow the proper amount of light to pass through. As you might imagine that would project very large stars, looking more like the Sun (well it is a star but a lot closer), then a far away star. If we use a Plano/convex lens of the proper focal length, the normal widening of the light beam from the hole is adjusted into a collimated beam of light concentrating all of that light into a small bright star on the screen. The problem is solved. One needs to find a lens with focal length as close to the distance from the light source to the star hole as possible. If one is off by a few mm all is not lost as the projected star might still expand but not nearly as much as it would without a lens. Now all we need to do is find a source for these cheap lenses.
The days of cheap lenses is probably gone forever. Surplus Shed www.surplusshed.com/index.php is one place still in business but your looking at basic Plano/convex lenses in the $4 range unless they are having a sale. Remember also that the focal length of your lens should be close to the radius of your star ball. A 250mm focal length lens would work in an 18 to 20 inch diameter star ball. Since you are working with a pin hole you will only be using the very center of the lens so it can be most any size in diameter (your not projecting a full slide for example). Measure from the light source to the pin hole in the globe and get a lens with a focal length as close as you can get. The 150mm focal length lenses would work well in a 12 inch diameter globe.
As light from a nearly point source flows through a pin hole it will expand until it hits the dome or screen and project a star dot. The larger the star ball the bigger a star hole one can have for a given projected star dot. Some pin hole projectors use star balls four feet in diameter. One does reach a point of diminishing returns and a compromise of eighteen inches appears to work very well. This allows for stars as faint as fifth magnitude to project without any problems. The problem with the bright stars is that they require holes as large as a quarter inch in diameter to allow the proper amount of light to pass through. As you might imagine that would project very large stars, looking more like the Sun (well it is a star but a lot closer), then a far away star. If we use a Plano/convex lens of the proper focal length, the normal widening of the light beam from the hole is adjusted into a collimated beam of light concentrating all of that light into a small bright star on the screen. The problem is solved. One needs to find a lens with focal length as close to the distance from the light source to the star hole as possible. If one is off by a few mm all is not lost as the projected star might still expand but not nearly as much as it would without a lens. Now all we need to do is find a source for these cheap lenses.
The days of cheap lenses is probably gone forever. Surplus Shed www.surplusshed.com/index.php is one place still in business but your looking at basic Plano/convex lenses in the $4 range unless they are having a sale. Remember also that the focal length of your lens should be close to the radius of your star ball. A 250mm focal length lens would work in an 18 to 20 inch diameter star ball. Since you are working with a pin hole you will only be using the very center of the lens so it can be most any size in diameter (your not projecting a full slide for example). Measure from the light source to the pin hole in the globe and get a lens with a focal length as close as you can get. The 150mm focal length lenses would work well in a 12 inch diameter globe.
