wpo - chromatic aberration forms stellar spectra!my astro homepage astro-optics page main spectro page
Chromatic aberration commonly reveals itself in the inability of a single element objective telescope to give a sharp colour-free image and has been the bane of telescope makers since the telescope's invention in the 1600s. Sir Isaac Newton, famed for his experiments with prisms and the solar spectrum, considered the problem insoluble and devised his all reflecting telescope [Newtonian] which is inherently free from this condition.
These experimental images record the effect of chromatic aberration caused by a single element lens eg 30cm fl f/10 meniscus, focused onto a Starlight Xpress MX5c colour CCD piggy-backed on my Meade LX200 SCT.
The images are bright stars focused near the blue region [centre]. The concentric rings of green, yellow, red and infra-red [colourless white outer circle] form in composite crude low resolution spectra of ~30nm resolution. Each star varies subtlety with colour temperature eg Arcturus has a stronger red component whilst Spica is noticeably stronger in blue. Typical RGB filtration is ~100nm bandwidth. It should be possible to improve the SN ratio on fainter stars by unfolding the circles into a traditional rectangular spectrum via software.
The lower monochrome version has the colour saturation set to zero eg not greyscaled which produces an erroneous effect for spectra.
images & text copyright - Maurice Gavin - Apr 2003