Lens types,
A perrenial topic around here is the relative performance of an APO, or virtually perfect scope, and a longish, well made Achromat. I don't mean to raise sand gratuitously here, but as a good-achro user, I can't help but wonder what I am missing.
I would like to examine and understand the common claim that an APO will "go to" higher power while still appearing "sharp", without "image breakdown", than a good Achro. My problem is partly that as an Achro guy who doesn't get around much, I have little opportunity to make the comparison myself. Moreover, my Achro appears to me to perform very close to theoretical expectations for its aperture, which suggests the question of how much better an APO could be. There's not much I can do about the first problem, so let me concentrate on the second.
I love double stars, look at lots, and through mindless perserverance have had some success with difficult ones. From firm optical theory, my 5-inch scope produces an Airy disk, or diameter across the first dark ring, of 2.0", but my observations, as well as Rayleigh's and the similar Dawes's limits (which hold for moderately obstructed reflectors, too), indicate that a typical visible "spurious" diffraction disk of a roughly 6th magnitude star is 1.0" across in my scope. That is sure enough how it looks to me.
If this 1.0" is magnified 60x, it looks 1.0' across, still too small for me to make out. 169x makes it 2.8', big enough to see as an extended circular spot, no longer hidden in the spiky glare of aberration produced by imperfect lenses, coatings, and eye, but round and clear. The highest power I commonly use is 217x, giving a spot 3.6' across, which looks comfortably big to my reasonably good eyes.
Vision treatises say that while a normal eye can resolve two narrow black on white lines as such when their separation is only 1.0', fully 2.5-3' separation, or extent, is needed for two points to be made out, or for an extended object to be seen as in fact extended.
So, my achro and eye seem to be performing, on double stars, according to theory and well-established empirical rules for a perfect telescope and normal vision. And, from my experience and reading, including your observing reports and comments presented here, it seems the APO indeed offers no significant advantage, other than aesthetic, on these most contrasty of all astronomical targets.
My planetary views are commensurate with both my double star experience and the optical and vision "rules" as stated above. Planetary limbs, for example, look sharp at around 150x (ie, 30x/inch), but at 217x(ie, 43x/inch) are perceptibly soft. I do use 217x on Mars, just because it is so darned small, and the features, although softened by the magnification, are still easier to see because they are made big enough.
A squishy part of my attempt to connect star and planetary images is the fact that dim stars' disks look smaller, and bright ones larger, than assumed by Rayleigh and Dawes. One might suspect that low surface brightness planets should be easier to resolve, ignoring the effect that dim things are hard to see. Saturn is currently 0.0 magnitude, and 23" diameter. Consideriing the area of the rings, this works out to a bit dimmer than 6th magnitude/square arcsecond, but still close to the Dawes limit standard star. Jupiter and Mars have considerably greater surface brightness, and might be expected to appear softer-limbed than Saturn by this sort of thinking (which I am not sure is valid, but consider as an objection to my argument that could be raised).
So now I come to the APO high-magnification claims. I can believe that due to better color control, low-contrast planetary features appear more easily visible in the APO, and also colors would appear true, instead of yellowed, as in my achro. But, how can it be that APOs look "sharp" up to the 70x or so per inch frequently claimed? At that magnification the disc of a roughly 6th mag star is a whopping 5.9' in diameter. If an observer can't distinguish such a bloated image from "sharp", something must be wrong with his eyes.
It is my guess that some APO fiends are guilty in such cases, not of lying, nor blindness, but of inarticulateness. They see something in the APO that beats the achro, and inaccurately call it "sharper". Am I correct, APO heads, or is there something that I just don't get?
Ron
A perrenial topic around here is the relative performance of an APO, or virtually perfect scope, and a longish, well made Achromat. I don't mean to raise sand gratuitously here, but as a good-achro user, I can't help but wonder what I am missing.
I would like to examine and understand the common claim that an APO will "go to" higher power while still appearing "sharp", without "image breakdown", than a good Achro. My problem is partly that as an Achro guy who doesn't get around much, I have little opportunity to make the comparison myself. Moreover, my Achro appears to me to perform very close to theoretical expectations for its aperture, which suggests the question of how much better an APO could be. There's not much I can do about the first problem, so let me concentrate on the second.
I love double stars, look at lots, and through mindless perserverance have had some success with difficult ones. From firm optical theory, my 5-inch scope produces an Airy disk, or diameter across the first dark ring, of 2.0", but my observations, as well as Rayleigh's and the similar Dawes's limits (which hold for moderately obstructed reflectors, too), indicate that a typical visible "spurious" diffraction disk of a roughly 6th magnitude star is 1.0" across in my scope. That is sure enough how it looks to me.
If this 1.0" is magnified 60x, it looks 1.0' across, still too small for me to make out. 169x makes it 2.8', big enough to see as an extended circular spot, no longer hidden in the spiky glare of aberration produced by imperfect lenses, coatings, and eye, but round and clear. The highest power I commonly use is 217x, giving a spot 3.6' across, which looks comfortably big to my reasonably good eyes.
Vision treatises say that while a normal eye can resolve two narrow black on white lines as such when their separation is only 1.0', fully 2.5-3' separation, or extent, is needed for two points to be made out, or for an extended object to be seen as in fact extended.
So, my achro and eye seem to be performing, on double stars, according to theory and well-established empirical rules for a perfect telescope and normal vision. And, from my experience and reading, including your observing reports and comments presented here, it seems the APO indeed offers no significant advantage, other than aesthetic, on these most contrasty of all astronomical targets.
My planetary views are commensurate with both my double star experience and the optical and vision "rules" as stated above. Planetary limbs, for example, look sharp at around 150x (ie, 30x/inch), but at 217x(ie, 43x/inch) are perceptibly soft. I do use 217x on Mars, just because it is so darned small, and the features, although softened by the magnification, are still easier to see because they are made big enough.
A squishy part of my attempt to connect star and planetary images is the fact that dim stars' disks look smaller, and bright ones larger, than assumed by Rayleigh and Dawes. One might suspect that low surface brightness planets should be easier to resolve, ignoring the effect that dim things are hard to see. Saturn is currently 0.0 magnitude, and 23" diameter. Consideriing the area of the rings, this works out to a bit dimmer than 6th magnitude/square arcsecond, but still close to the Dawes limit standard star. Jupiter and Mars have considerably greater surface brightness, and might be expected to appear softer-limbed than Saturn by this sort of thinking (which I am not sure is valid, but consider as an objection to my argument that could be raised).
So now I come to the APO high-magnification claims. I can believe that due to better color control, low-contrast planetary features appear more easily visible in the APO, and also colors would appear true, instead of yellowed, as in my achro. But, how can it be that APOs look "sharp" up to the 70x or so per inch frequently claimed? At that magnification the disc of a roughly 6th mag star is a whopping 5.9' in diameter. If an observer can't distinguish such a bloated image from "sharp", something must be wrong with his eyes.
It is my guess that some APO fiends are guilty in such cases, not of lying, nor blindness, but of inarticulateness. They see something in the APO that beats the achro, and inaccurately call it "sharper". Am I correct, APO heads, or is there something that I just don't get?
Ron
