Telescope Reviews: Re: ED glass in binoculars

Milt makes a very good point above.

Color Blur or CB value is a ratio of Airy disk size. So a CB=1 is same size as Airy disk. CB=2 is twice the size of the Airy disk.

The forumla for color blur states that for a color free instrument you need to have CB=1, or explained another way, no residual color can fall outside the diameter of the Airy disk. Essentially all the light is contained within the Airy disk.

What's not mentioned in any of this is low power viewing. That's what is brought up by Milt in his post above.

When viewing at low powers, the human eye cannot perceive sizes as small as the Airy disk. Human eye resolution at 30x is not controlled by scope diameter, it is controlled by acuity and hence magnification. The spot size that can be resolved at 30X might be 4 to 5 arcseconds, not the scope Airy disk size of 1 arcsec. So the latitude of how wide the color blur can get before the eye sees false color, when magnification is not high enough to truly resolve the point, is dependant on how large a point the eye makes the object appear to be.

For example, for most people, regardless of scope size and hence it's potential resoltion, (which could be a 5 inch scope with res = 1 arcsec), the eye cannot resolve a 4 arcsec pair at 30x. It will be seen as one. As Milt points out, as long as the color blur is as small as the spot that can be resolved by the eye, (at 30x, perhaps 5 arcseconds), then false color is minimized. Oh, you may see some residual color around the edges, but not nearly as bad as you might think.

It doesn't take too much effort to think thru, as magnification increases, even though it may still be too low to reach the resolution limit of the scope, the spot size resolved by the eye gets smaller. So, for instance at 50x, the eye might be able to resolve 3" and at 75x, the eye might be able to resolve 2". The extent of color blur would need to be progressively smaller in each case to match the spot size, if the observer were to desire a color free image.

But as you can also see, that presents a very wide latitude in CB factor when using powers below that needed for the resolution limit. Low power binoculars are well down the chart. Even high powered BT style instruments are generally used in the wide latitude range, but may approach the resolution limit.

edz

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