CN FORUMS ARCHIVE Telescope Reviews: Seeing Encke!

you'll get no argument from me!

Your eyesight could well be correctable past the 20/20 average..for example, baseball star Mark McGuire can see about 20/10 with his vision corrected but only 20/400 naturally. The reason such great correction like this is possible (and the reason it is more often *not* possible) is due to a very specific physical advantage, as I will explain a bit later.

FWIW, I'll delve into this subject a bit further...

The illustration I gave previously is rough but instructive overall as an illustration of the variability of human eyesight. In assessing innate human eye acuity, magnification isn't in play. I was refering to the relative ability of the eye to discern a shape of a specific degree of arc as measured by standard testing at a specific distance, the 20/xy ratio. (spatial resolving power) No magnification enters into this equation. It is the same for natural or corrected vision.

For example, someone with 20/20 vision, corrected to that or innate, can discern an object which subtends a 5 minute angle at 20 feet distance. Someone with 20/10 vision, corrected to that or innate, can discern an object which subtends angle 1/2 of the size than that of a person with 20/20 vision. This ratio remains the same to infinity, at any given distance. The person with the better vision sees everything at the same size as anyone else--at any given magnification including zero: They just are able to resolve more detail per specific distance up to and including infinity, the reason for which I'll get to later. At any rate, my point was only that the eye's optical acuity varies and thus presents an important factor as to what different people comparatively are able to see through the same telescope at the same time.

Now, magnification through a telescope works the same basic way as walking closer to the eyetest chart. The more magnification, the closer to the 'eye chart' you come, and the greater the object angles become and the more you can discern. Think of the eyechart as being the focal plane and decreasing distace to it as equivaent to increasing magnification in a telescopic system. Only by "increasing magnification", by walking up to ten feet of the chart, can a person with 20/20 vision resolve the same detail the person with 20/10 can resolve-- at twice that distance. Now we can see why, if a person with 20/20 is using the maximum usable magnification on a specific telescope, then that person will likely be able to discern considerably less detail than a person with 20/10 vision; and see *less* detail, generally, in that particular telescope. In fact, the person with 20/20 will require a *bigger* telescope, capable of two times higher useable magnification, just to be able to discern what the person with 20/10 can see at half that magnification.

With the foregoing in mind it's easy to understand why a person with less visual acuity than another will see less through the same instrument at the same time with the same magnification; and also why the person with lesser visual acuity would need *more* magnification to duplicate the visual details seen at a lower magnification by the person with more acute vision--and perhaps a larger telescope to do it with.

This observation possibly goes a long ways towards helping explain the occassional discontinuity between different observers (current and historically) and what can be seen in a specifric aperture/system. For example, concerning an extended detail like Encke's represents, let's say common wisdom requires about 10" and 400X to even have a chance to see it. But increase visual acuity 25% or more over average acuity (represented as 20/20 corrected) and then those occasional sightings in 8" or even 6" instruments all of a sudden have another quite rational explanation beyond simply wishful or mistaken thinking.

Vision, corrected or innate, can reach 20/10 though not much past that if any at all. Focusing--or glasses or contacts-- cannot correct eyesight to a resolution level exceeding that which it is biologically capable of. (Lasik researchers dispute this precept however, see references below). Younger eyes often can see or be corrected to better than 20/20; older eyes often can see or be corrrected to no better than 20/25.

Now as to why the eye is thus limited---What determines what resolution power an eye is capable of? Why can some eyes only be corrected to say, 20/25 while others can be corrected to as much as 20/10? The answer lies in the density of the sensing cells (cones and rods) within the retina. Generally, the greater the density of these cells, the better level your eyesight is correctable to, both for day and for night vision. The key area to the optician's measurement of visual acuity, however, is restricted to the fovea centralis. The greater the density of cones in this discreet area of the retina, the better your 20/ score will be.

Most of us already know that we actually have two sets of 'eyes' within each eye and each 'eye' functions within discreet areas of the retinal surface; one 'eye' is designed for low illumination and another 'eye' is designed for higher illumination. The greatest cone density (and the source of one's best visual acuity) is located in the fovea centralis and the acuity of that area is what an optometrist measures when he/she measures visual acuity and fits corrective lenses on a person. The retinal area we normally use for focused vision (reading etc) is the macula, the cone rich area surrounding the fovea.

Then there is the other 'eye', which is peripheral to the macula and is populated primarily by rods. This retinal area becomes the most sensitive and provides the functional acuity in low light situations, where cones simply cease to function well. However, low light (rod) acuity is merely a fraction of what high illumination (cone) acuity is. Increased light (image brightness) helps us see better because we can then use our more sensitive cones within the fovea. "Visual acuity during scotopic (pure rod) vision is very very poor: 20-200 vs. 20-20 for cone vision: 10 times better!" ref.

This factors into why the brighter image given in a large telescope is also helpful & important in discerning more detail. The more illuminated the image, the more we benefit from the increased resolving power inherent in our 'cone vision'. Put another way, the less we depend upon scotopic vision the better we can see detail. This also illustrates the underlying reason why we want to conserve as much of the light entering a telescope's system as possible..the more light, the more illuminated the image and the better we are able to maximize the visability of details using a given aperture, all else being equal (no increase in light scatter etc).

All this leads naturally to a simple question: is the visual acuity as measured at the fovea centralis (cone acuity) translatable in any direct proportion to that of the rod areas of the eye? In other words, does extra sharp cone vision (better than 20/20) necessarily mean one also has extra sharp rod (scotopic) vision as well>? Or could the acuity vary, even be opposite? I can't find definative information on this although there would seem to be a relative correlation; maybe someone else can find more data relevant to this question.

Understanding the role of the eye within the telescopic system can lead to some interesting observations. For example, if you want to know what part of your eye is active in providing whatever resolution you are seeing you can do a simple experiment: see if averted vision (rod vision) improves the resolution of the object's details--if it does than you know your vision is limited by the resolution of your rod vision and therefore acuity is vastly decreased from what it could be--and you know that more illumination (object brightness) would improve things. This is handy when adjusting the brightness of an object with filters, such as the moon or bright planets. You wouldn't want to decrease the brightness to such a level that your averted (rod) vision starts to dominate, for example.

Another example is the impact of chromatic aberration--the rods are totally color blind and sense only the 505 nm wavelength of light. This is why any improvement of CA within an optical system ceases to factor visually whenever you are observing an object below the illumination threshold at which cone (color) vision begins to function--which is why CA's presence doesn't negatively effect the view of faint extra solar objects (nebulae, galaxies) as it does the bright planets & stars, moon etc.

Understanding how the eye works also exposes the reason higher reflectance coatings on such components as diagonals and other reflective optical surfaces provide such widely acclaimed benefit. This would be truly win-win but for the increased light scatter these coatings (enhanced alum, die electric, etc) also generate which arguably can make a wash of the whole effort. "Arguably" indeed...as every eye is different, so are the effects each one observes in a given system configuration. Does the increased light scatter really cancel or exceed the gain in illumination levels enhanced aluminum, for example, provides? No wonder there is so much dispute among observers concerning the nature and relevance of the discreet effects small optical variations engender! It's entirely possible that the relevant answer to that question could lie more in the optical differences between individual eyes than anywhere else.

Anyway, when comparing optics, we shouldn't forget that the figure of our eye is just as relevant to a telesope's performance as the figure of it's own optics. The system doesn't end at the eyepiece by a long shot.

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Mardi

4" achromat, ETX-70, 8"cat.
Whitepeak Lunar Observatory Website

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