Telescope Reviews: Second Annual World Cup: Eyepieces

As I mention above, and as is clearly shown in the diagrams in Licha's article, Sparrow limit is a measure that at best would show two stars barely elongated, not even peanut shaped. It is a limit that probaly is more useful to describe a separation at which the "detection" of two stars is just possible. ( I referred to Licha's article when I wrote mine in 2003).

Observing stars at Sparrow limit would not allow you to see any distinct parts of the diffraction pattern. The star disks would be approx 40-60% overlapped. You would have difficulty to tell it was two stars. You would not see two distinct first diffraction rings, they would be blended into one. Not only would there NOT be any space between the two stars, but there would be no distinguishable depth to the nocth between the two stars. In fact, that is the definition of Sparrow limit, the point at which there is no detectable drop in the intensity of light between the two components.

Frankly, Sparrow limit is not what is normally referred to when discussing the resolution limit of a scope (see my citations above). It is not indicative of seeing doubles separated, and it certainly is not what we think of for discussion of extended object resolution. Sparrow Limit is about what I saw with my small scopes 2-3 years ago when Porrima was at closest, approx 0.5-0.6 arcseconds. Sparrow limit provides very little detail in the image for scrutiny, unless of course the goal is just to find the minimum detection limit of the scope by observing first noticable elongation.

FWIW, Rayleigh Limit is the definition of the size of the Airy disk. An understanding of the Rayleigh Limit (the Airy disk formula) allows you to calculate the size of the Airy disk and the central visible disk for the scope in use and the target you choose. Having that information, you can even predict accurately where the secondary will be positioned with respect to the spacing of the light or dark rings. Neither Sparrow nor Dawes provides that information.

Both Sparrow and Dawes are way beyond the scope of what are the intended targets for Jim's observing session. Even Rayleigh limit stars are beyond the scope of his intent, however, at least Rayleigh limit provides an understanding of the size of the features you see in the diffraction pattern when you do look at double stars. Even when you choose stars that are wider than the Rayleigh limit, although the separation between them varies with your choiceof stars, each of the individual diffraction patterns dimensions are still based on the Rayleigh limit (the Airy disk formula).

Understanding the dimensions of the Rayleigh limit is like having a ready made measuring micrometer in view in your scope. I trust you would understand how useful that might be for the purpose of quantifying differences in resolution.

edz

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