Do classic achromats outperform modern APOs?

A recent ad for a 4" Unitron claims it outperforms modern 4" APOs. I see this claim a lot, usually from someone selling an achromat to buy an APO... 8) Does anyone have any data to support this conclusion?

Doug Peterson said:

A recent ad for a 4" Unitron claims it outperforms modern 4" APOs. I see this claim a lot, usually from someone selling an achromat to buy an APO. Does anyone have any data to support this conclusion?

I've used 4" Unitron and a 4" APO. I'd definitely give the nod to the APO, and it provides low power, wide field views that are simply unavailable through a 4" f/15. It is also a lot more portable. IMHO, the 4" long focus achromat is pretty much a dinosaur. A well maintained Unitron might be a nice collectable, but it wouldn't be the best choice for use by an active observer.

Clear skies, Alan

First off, I don't think that particular scope is really a Unitron, since it doesn't have the original objective. Others might disagree if they feel that a Unitron is defined only by the mechanical parts, since they seem to have gotten their lenses from multiple sources.

The seller quotes impressive surface accuracy figures for the replacement lens. In that sense, it might be superior to a run-of-the-mill Chinese doublet "apo". Or maybe not.

I have observed with a purebred 4" Unitron, and was not very impressed.

I've seen a Unitron 4" beat an apo of similar (94mm) size. Having an f15 focal ratio makes it much easier to control damaging aberrations that (despite the fact we rarely hear about them) still afflict some so-called apos. Chromatic aberration is only one of several aberrations but some people think if the image is colour free, it must be good.
Too bad that isn't the whole story. A Unitron will not beat a top-class apo, but it can beat lesser ED scopes and "sort of" apos as well as overly fast apos.
-Rich

Hello:
I've use an achromat that beat any APO I've looked through. It just so happens that it was a 16" f/15 achromat and the largest APO I've looked through is 5"! So, if an achromat has 10 times the light gathering power of an APO, it can beat it. All else being equal, a N" APO should beat the pants off of a N" achromat, even if the achromat has a much longer focal length. But, not all APOs are equal. I'm sure that a well built and maintained 4" Unitron can beat a poorly built "APO" (I'll leave the debate of what's a true APO versus semi-APO to others).

Cheers
Mike Connelley