Up till now, I had been taking images of M-31 and M-33 with an achromatic Orion ST-80 refractor (400mm f.l. f/5), and the image here of M-33 shows that an achromat does to stars. It turns them into nice, purple donuts. This is caused by the way different wavelengths of light entering the edges of the lens come to focus at a different point than light entering the center of the lens. Lower cost refractors use these achromatic lenses which can focus the red and green at the same point across the whole aperture, but the blue focuses at a lightly different point, which causes the blue light to be slight out of focus when the red and green are completely in focus. This is what causes the "purple fringe" or chromatic aberration prevalent in achromatic lenses.

Follow up:

When using achromatic scopes visually, this is normally not noticed except on very bright objects such as second magnitude and brighter stars, as well as on the moon and planets. Visually, this can be dealt with using a minus violet filter, but these do not work well for color photography as they can cause a slight yellow hue to the photographs (again, this is not really noticed when using a minus violet filter visually as the yellow cast is very slight, but CCDs are much more sensitive than our eyes). There are some Photoshop actions available the can remove these fringes, but I, personally, think there's too much digital manipulation going on in the astrophotography community already (more on that later). Or you could shoot separate RGB channels in monochrome and stack them later, but that triples the amount of exposures needed. Much better to use an apochromatic lens.

Apchromatic lenses are those that correct for the curvature of the lens elements for all three color wavelengths. Offering more true-to-life color images but at the cost of being at least four times as expensive. My 80mm ST-80 can be purchased for about $120 new. An equal aperture apochromatic scope sells for about $500 on the low end. So, it's a matter of balancing cost vs. aperture when weighing the benefits of buying apochromatic glass. That's why I bought my 8" Ritchey-Chretien. I got the aperture at a lower cost because it is not a lens-based system. Mirrored telescopes generally don't suffer from chromatic aberration.

But, the AT8RC has a focal length of 1625mm. So, what to do about objects, like M-31, that are just too big for a scope of this length? Again, I had been using my ST-80, but those fringed stars just made me cringe. I could buy an apochromat, but I really didn't want yet another telescope that I would only use for a few objects.

So, I started looking at long focal length camera lenses. Lenses like the Canon L-series are apochromatic lenses, but man, they're pricey. For new, they are orders of magnitude more expensive than a similar-sized telescope, but they are faster and are easily used for terrestrial photography because they are auto-focus. New was out of the question, so I started checking eBay and KEH for something used. KEH's bargain level lenses tend to have cosmetic issues, but the glass is usually in fine shape. Who cares what it looks like?

Finally, I came across an EF 300mm f/4 L lens on KEH for about half of what a new one goes for. It was labeled as "etched", meaning the original owner had probably etched their name, address or whatever on the barrel. Turns out the lens was perfect except for two small scratched up rectangles on the barrel, which being aluminum, shouldn't cause a problem. The image of M-31 you see here was taken with that lens. Even though it's only 12 minutes worth of exposures, it shows a lot of promise. Most importantly, it's color free.