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Refractors are illogical any way you slice it. ............

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Just what fraction of owners build their own refractors??

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Yes mass production indeed.
But the figure of PTV5 7 you said I think is the top best of the serie with few units.
This doesnot explain the featureless views on venus with apo because this was not at a level of PTV4. I think they are lot of units like this.
The question of refractor doesnot resume to color aberration.
For capturing a detail of 1" size on Mars it can appear to be 0.3 contrast level in apo, .2 in the refractor achromat and almost nothing (say .15 or less) in my apo example. When approaching the optical aperture limits, my apo give nothing, the achromat still something accessible say .05-0.1 and the actual apo 0.06-0.12 only.
I think we get a lot in an apo will convince immediateley anybody by the contrast provided, but when pushing a little more this is not so evident. In both cases we get the same data but at a slight different contrast.
To-day, I think the achromats are fabricated not individually but by batch of one or few hundred of objectives depending on the aperure (until 6") on the same polishing machine.
PTV4 for refracting optics is good, for reflecting optics just on the minimum.
In europe we can get a zygo for 100 (for a general control) and still have the roddier to get similar data, but after the buy.
The come-back is here but confidential for achromats and visual observers.
Stanislas-Jean

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Stanislas-Jean

If both the APO and Achro are produced to the same standards and give a similar Strehl and PV error I think it is save to say that the APO will deliver the better visual image.
If you compare a badly made "APO" with loads of spherical aberration, with a premium hand corrected Achro, you might get to the conclusions in your posting quoted above.

Or do you want to tell us that any mass produced Achro will have a high Strehl?
Why should the production quality automatically be better on the Achro than it is on the APO?
I agree that a well made long focus Achro has its place but I am seriously having trouble following your logic.
We should make valid comparisons... it comes down to production quality not to the telescope system.

best regards
Chris

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See here for details:

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I have been in this hobby for a long time. A long time ago if you wanted a refractor it was either Astro Physics, Tak or TV. Orion also sold some rebadged Vixen stuff. Those were the choices then, Astro Tech didn't exist and SV was just starting with their 80mm scope that got some interesting reviews.

Then one day the 80mm Apo from Orion showed up and changed everything.

Today you can buy a nice refractor for a fraction of what they cost a decade ago. So I would say yes Achros are having a comeback and Apos are in a golden age!

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It is true. Achromats are making a comeback but apochromats are in their golden age... Never before so many, nor so many good ones.

I think the development of the affordable apo/ED scope has lead to improved achromats, particularly in terms of mechanical quality, the improvements in manufacturing trickle down from the apo/EDs to the achromats. A scope like the ES AR-102 did not exist 10 year ago.

I think there is a place for a decent quality achromat, the great virtue of the apo is that it can do about everything, high power, widefield all in one scope. A 4 inch F/6 achromat might do a pretty good job at low magnifications, a 4 inch F/15 achromat might do a pretty good job at high magnifications, but a 4 inch F/6.5 apochromat can do both jobs and do them very well.

But the place for the achromat... just like any scope, doing what it does best. If the role of the scope is narrow, say low power, widefield, then an achromat may serve the purpose...

Jon

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Achromats are making a comeback but apochromats are in their golden age... Never before so many, nor so many good ones.

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Then you'll have to tell that to the entire photographic industry.

That's where I got the original lead on the story.

Toodlepip,

Neil.

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Refractors are illogical any way you slice it. So picking an achromat over an apo for dso observing is no more crazy than picking a refractor over a dob for said purpose.
If you want a 6 inch refractor and don't want to sell one of your kids to buy it, the achromat is the only game in town. You use your scopes for their intended purpose. Play to the strengths of the design and you will be happy. I just picked a 6" achromat and I'm having fun exploring many classic dso's that my other scopes just can't show off, like this scope. So yes I think they are, and should make a come back.

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Technological advancement is absolute and does not grant exceptions. ... Just as the achromat rendered the Keplerian and other singlets largely irrelevant, so too the apochromat has rendered the achromat largely irrelevant (in a commercial sense), and indeed something new will eventually relegate the apochromat to oblivion as well. But that's tomorrow. Today, the relevance of refractors is all about apochromats. Far more energy is devoted to making better aporchromats than any other refractor design presently.

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