Refractor Optics Question

Hi, I have the following question regarding an apo refractor for the optical experts:


The inside and outside de-focused images of a star show perfect text book circular and centered rings, indicative of good collimation. However, 1/4 to 1/3 turn on either side of the focus shows different diffraction rings- the outside focus image appears to be bigger and brighter compared to the diffraction rings from the inside of focus. I recalled reading somewhere that they should be identical in good optics- or was I wrong?

What is the optical explanation for this, and is this indicative of some optical issue?

Thanks very much,
Ivan

Hi, I have the following question regarding an apo refractor for the optical experts (guess it does not matter whether this is APO or not):
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I am no expert but it is my understanding that to do a Startest with an Achromat you need to use a green filter so that the effects for chromatic aberration do not affect the result.

Sometimes I read of a fast achromat that has a "perfect star" test and I wonder....

jon

As already mentioned, repeat the test with a green filter.
Chromatic effects will cause the patterns inside and
outside to look different, unless it is some super-duper
APO.

If you still see the same differences inside / outside
focus with green filter, it could indicate spherical
aberration or turned-edge, depending on the details.

>>However, 1/4 to 1/3 turn on either side of the focus shows different diffraction rings- the outside focus image appears to be bigger and brighter compared to the diffraction rings from the inside of focus.>>

Yes, this is correct. They should be different. They will never be the same in a refractor unless it is extremely long focal length, greater then F15.

>I recalled reading somewhere that they should be identical in good optics- or was I wrong?>

Yes, you will read this all the time in optics texts, but it is based on old information about refractors (>F15 focal lengths) and is really only valid for pure reflectors. Most optics texts have never been revised to reflect the reality of the new fast doublet and triplet apos.

You can get similar inside/outside patterns if you use a green filter because that is the place where modern lenses are nulled. Slight differences in focal position of the other colors and sphero-chromatism at short and long wavelengths will make the ring pattern look different on either side of focus (the bullseye ring pattern is known as Fresnel rings). I realize that this looks odd for something very expensive like a fast apo, but it is quite normal and in reality has no effect on what you really want - sharp images at focus.

Roland Christen