Refractor Chromatic Aberration Rankings
I have attempted to create a standardised set of rankings for the chromatic aberration performance of refractors.† Iíve done this by taking a prime-focus shot of the same subject (a high-contrast daytime view of tree branches) in approximately the same lighting conditions.
Why didnít I choose an astroí target? For one thing I would be limited to something near the celestial pole (so itís visible all year) that includes bright stars. For another, itís actually harder to get a sense of CA in astroí photos, unless you zoom right in to spot violet bloating. Then again, given the climate here, if Iím unlucky it can be weeks between clear nights.
Obviously this isnít quantatively scientific Ė the lighting wonít always be quite the same and there are differences in focal ratio to contend with. However, I think it gives quite a good idea of how the telescopes perform in practice and with respect to each other.
Donít think this is a ranking of overall quality. It isnít. The StarTravel102 comes at or near the Ďbottomí, but that doesnít mean itís a poor scope; actually I found it to be an especially good one Ö within its own frame of reference, i.e. as a fast achromat. Chromatic aberration is not everything; overall optical quality can make more difference to the view, but is much harder to quantify.
Note that the first three are effectively free from false colour Ė true apochromats
Thereafter things go downhill. Note too that the TV60, with a very similar lens to the TV76, nonetheless exhibits less false colour Ė thatís the effect of aperture.
The FS78 is very impressive for a doublet, confirming much more scientific evaluations which rank both FS-78 and FS-102 in the triplet Super-APO category for false colour. Itís also impressively sharp.
As I test more scopes, Iíll add more images to this set Ö
TMB 175, F = 1400mm
Televue NP127, F=660mm
Takahashi FS-78, F=630mm
TeleVue TV-76, F=480mm
Sky-Watcher Equinox 80, F=500mm
Sky-Watcher Evostar 120 Achromat, F=1000mm
Sky-Watcher StarTravel102, F=500mm
Stellarvue Nighthawk 80, F=500mm