Colour fringing
Because we service telescopes, occasionally we get patients which are, sadly, unsalvageable. This one was one of those, an entry-level refractor with a terminally mangled focuser.
I took the opportunity to pull the front lens off and see how it worked. The telescope is (was?) a doublet-type refractor, meaning there are actually two lenses at the front, built especially to refract blue and red light to the same point.
Single lenses refract blue closest to the lens, green in the middle and red furthest away. This is the dreaded "chromatic aberration" that refractors produce. Visually, it's not much of a problem, but taking a photo with this type of lens looks ugly. Check out my photo of the Southern Cross. See the blueish purple fringes around the brighter stars? That's what happens when most of the light is in focus, but the blue and red frequencies aren't quite there.
Here's another one of my photos: this time a Lesser Black-backed Gull in Norway. Notice the colour fringing - yellow at the top and blue at the bottom - where dark areas meet light areas?
To handle this, modern refractors use two different lenses, each made of different types of glass. Like all lenses, these split ("disperse") light into rainbow, but because the two lenses have different properties, they do this by different amounts. I won't go into detail here, but clever lens designers have used these differences to design a pair of lenses that focus red and blue light to the same point.
So what does the lens group look like? Here it is, the two lenses separated. One is a convex "crown" glass, the other is a concave "flint" glass.
Finally, green light refracts differently again, so with a doublet, you will still have some chromatic aberration. If you want to get rid of more chromatic aberration, you need to put in a third lens (or more) - which can produce a better but expensive result, such as this Jewel Box photo.
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