My refractor shows color fringes since it’s not an apochromatic design. Is there any filter or way to reduce these fringes?

Many refractors are two-element conventional achromatic designs, which leave what’s called residual color. This means the designs don’t fully remove all the color left over from dispersion of light from the two lens elements. The result is blue and red fringes around objects viewed or photographed through the scope. Daytime terrestrial views will often show blue fringes over shadows in the scene.

Filters can help eliminate fringes. A minus violet filter blocks the blue fringes (usually the most prominent) and lets the rest of the light through, improving the sharpness and contrast of a refractor’s view. However, it does make your view have a yellow-green cast.

Two general photo filters may also help. A skylight filter blocks blue in shadows for terrestrial photography and can similarly help for astronomy. A warming filter used in general photography will slightly shift the overall color of the view towards the red end of the spectrum, thus “warming” the scene while blocking bluer wavelengths.

Updated 12/27/13