Refractor CA Rankings

ScopeViews

Refractor Chromatic Aberration Rankings

I have attempted to create a standardised set of rankings for the chromatic aberration performance of refractors. I’ve 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 didn’t I choose an astro’ target? For one thing I would be limited to something near the celestial pole (so it’s visible all year) that includes bright stars. For another, it’s actually harder to get a sense of CA in astro’ photos, unless you zoom right in to spot violet bloating. Then again, given the climate here, if I’m unlucky it can be weeks between clear nights.

Obviously this isn’t quantatively scientific – the lighting won’t 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.

Don’t think this is a ranking of overall quality. It isn’t. The StarTravel102 comes at or near the ‘bottom’, but that doesn’t mean it’s a poor scope; actually I found it to be an especially good one … 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 – true apochromats

Thereafter things go downhill. Note too that the TV60, with a very similar lens to the TV76, nonetheless exhibits less false colour – that’s 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. It’s also impressively sharp.

As I test more scopes, I’ll add more images to this set …

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