What does a 1/4 wave optic look like compared to 1/6 wave in focus? Tell me what I should expect to see.
I will first say that an improvement in wavefront is an improvement in wavefront, period. 1/6 is 33% less than 1/4 (0.16 / 0.25). Whether one would actually say "the SA is 33% lower" or not is up for debate - the SA p-v variance is 33% lower but you can't generalize it beyond that.
With only a 33% difference, I would not expect the in-focus image variation to be obvious. By obvious, I truly mean obvious. It will not jump out at you. I don't think even 1/4 wave to 1/8 wave would be an immediately obvious difference under most conditions.
What is SA? Spherical aberration prevents all wavelengths coming to a precise focus, by an amount measured in the number waves of light (typically below 1 wave length), due to the fast that spherical surfaces lead the center of the field to focus at a longer focal length than the edges. Because the image is not brought to a very precise focus, certain details can be essentially out of focus. Variation of SA by wavelength is called spherochromatism; prioritized correction of SA can leave certain wavelengths very precise, while others are less precise; red/deep red often suffer in doublet designs in favor of green, yellow, and blue, wavelengths to which the human eye is more sensitive. This definition of SA is important because it helps explain why I expect to see the things I do (as you'll see below).
We have to talk about seeing conditions when talking about observable differences in wavefront correction because aberrations add up. If seeing is poor, a 1/8 wave optic will display a more detailed image than a 1/4 wave optic, almost guaranteed. Of course, if seeing is poor, I don't think the owners of either scope would spend enough time outside to say "gee, if my optical figure was better, I bet I'd be seeing more detail".
Let us assume decent seeing, say, Pickering 7. This is probably the best seeing most people in the US get on average. Mine is maybe around Pickering 5 as I live under the jet stream. Pickering 7 is good enough to allow for some really nice patches of steadiness, but not so good that almost any optic better than 1/2 wave will look great. A lot of optics will look great if seeing is perfect. Seeing is almost never perfect for most people so it makes sense to use a more common (but good) approximation of seeing conditions for this question.
Let us also use Jupiter as an example of a very finely detailed object that stands to benefit from SA improvements.
With the 1/6 wave optic, I would expect to see slightly finer detail. For example, on Jupiter, I would expect to see slightly more definition in the belts and the swirls between them. I would expect to see sharper definition of textures especially in low-contrast areas like the tan/beige belts. Better edge definition (which is sort of analogous to contrast) is what I would expect to see. This makes fine details stick out better.
With the 1/4 wave optic, after having looked through the 1/6 wave optic, I would expect to see slightly less definition in the Great Red Spot, and in the swirls between belts. I would expect to see what could be described as a smudging effect in the belts, where I may have seen actual texture and hinting at deeper detail and intricate interwoven strings of clouds of slightly varying colors.
I have not spent enough time with Saturn under good seeing to say, but I would expect the same principles to apply. Sharper definition within the rings, clearer delineation of colors within the rings and on the planet itself, maybe even some "micro separation" between the rings (like Encke's division, which IIRC is not possible to see with only 6" of aperture) if seeing and magnification are good enough for that.
As I said, the differences will likely not be any synonym of obvious, but I am confident the differences would be visible if both scopes had enough time for thermal acclimation and the observer(s) used a single diagonal/eyepiece combination at the same magnification & exit pupil to swap between scopes. I would generally recommend at least 2 people do a comparison like this to challenge confirmation bias and to account for subjective differences due to visual acuity.
Dan, I think you know what you should expect to see, but I appreciate the challenge. As I said, an improvement in wavefront is an improvement, period. With a higher quality instrument, the observer can be more sure that if they are not seeing something, it is either due to the sky, or them, and not their instruments; because of this, along with cumulative aberrations, I would always recommend people seek instruments with SA p-v lower than 1/4 wave if at all possible, with 1/4 wave being the bare minimum (maximum?) that I personally would accept for a scope of any size. We have today the tools to make instruments to this specification in a cost-effective manner; there is no reason to accept anything worse.
Edited by jay.i, 10 February 2019 - 05:03 PM.