Perkin-Elmer 6" F50 LENS
This lens has a very long focus, compared to a regular refractor lens. As you can see from the photo below, it has a 25 foot focal length. It is parafocal for two wavelengths, "C" and "K". I didn't realize just how lucky I was for a while -- this is made for Hydrogen-Alpha! (H- and Violet Calcium.)
Not very confusing after you actually see what it means. Glass and other objects can bend light. This is called refraction. Hence the word refractor, as in refractor type telescope. Remember seeing a spoon or other object in a glass of water? It looked like the object was bent. This is refraction.
Sunlight is made up of many different colors. Colors are different lengths, so each will bend at different angles. The colors will be in focus in different places because of the different lengths. Water, as in rain or mist, also bends light and refracts it so we see a rainbow. This is why a rainbow has the colors separated - they are in focus, or refracted, in different places. A prism also has the same properties. Usually lenses are ground so that some wavelengths end up at the same place. So in this case, parafocal means two wavelengths or colors, "C" and "K" end up in focus at the same place. This is why some telescopes or binoculars have tinges of color around objects. Lens manufacturers make lenses with more than one piece of glass. Each piece refracts a different wavelength, so several wavelengths can end up at one focus.
This doublet lens produces a solar image about 2-3/4" in diameter.
label on the 6" F50 lens
Same lens, showing the push-pull capability
Lens shown mounted in the cell I made. I can move it remotely to change focus.