Orion ED100 star test

Hi all, I've been thinking about why my ed100 star test looks the way it does, with well defined rings inside focus and fuzzy rings outside focus. I've seen a test indicating that the blue end of the spectrum is well corrected while the red end is less well corrected. So I've concluded that the outside rings are fuzzy because red is not focused to exactly the same point as green, while the inside rings are not fuzzy because blue is at the same focus point as green? I figure there are some refractor gurus here that can give me a tick or an E for effort?

For what it's worth...

http://geogdata.csun.edu/~voltaire/roland/startest1.html

Jon

Try running the same test through some colored filters to get a better idea what is going on. If you have a fairly narrow band green filter, that is the best choice. You may find that the rings are more similar inside and outside of focus when only one frequency at a time is examined. That that is the case, then what you are seeing is most likely spherochromatism--a sort of a cross between spherical aberration and chromatic aberration.

Spherochromatism is a shift in the level of spherical correction with wavelength. A scope that is well corrected for spherical aberration in the green portion of the spectrum, for example, may not be in the red portion or the blue portion.

- Jared