Gamma Virginis with rainbows

Last friday I took this image of Gamma Virginis - a double star with 1.6" separation.
When I processed the image I noticed these rainbows.
I wonder why these came out.
Last time I did a similar image and didn't get any colors:
http://members.surfeu.fi/tapiol/astro/ds/SAO112921_091002.htm
Is this a collimation problem ? Focus should have been ok.
I thought Mak-Cassegrains were pretty immune of collimation problems.
StarMax 127, 10mm Baader, webcam

Thanks
Tapio

Attached Image:

What was the altitude of the star? If it was low toward the horizon, atmospheric dispersion may have done this. At low angles, the atmosphere does the same thing a prism does - it spreads out the spectrum.

Ron Wodaski
http://www.newastro.com