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Abbe Dawes and Sparrow

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#26 fred1871

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Posted 29 November 2012 - 07:09 PM

You know what it is, Dawes has a specific look that's easier than Abbe. I think its easier to call a Dawes than an Abbe in the field. Yeah so in a real way with reservations a limit is what you make it. I guess that was my point condensed into a sentence.

Pete



Ummm... do we have a number for the Abbe limit, as transferred from microscopy to telescope use?

Or any idea what the double star image would look like at the Abbe Limit, if it is applicable?

#27 Asbytec

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Posted 29 November 2012 - 07:43 PM

"The formula for microscopes is 0.5l/NA which translates to 113/D = resolution "arc for telescopes (4.46/D inches), putting it between the Dawes and Sparrow limits."

http://www.cityastro...ag-contrast.htm

If the Sparrow limit is applicable on terms of being the point of zero contrast, the Abbe limit could apply to observing doubles. Its lower than Dawes 5% contrast between spurious discs, but higher than zero presumably between two 6th mag stars. In this case, I suppose the doubles would simply appear elongated to the eye.

#28 fred1871

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Posted 30 November 2012 - 12:32 AM

I suspect your description of it tells us why it hasn't been used in astronomy - it really doesn't tell us anything new, compared to the better known limits.

It may well be useful in microscopy, where things observed are perhaps more like planets than double stars.

#29 Asbytec

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Posted 30 November 2012 - 01:15 AM

Yea, I dunno, but it's application is probably better suited for Microscopy. Resolving fine details within cells at the diffraction limit, etc. They tend to measure distance rather than angular separation. The Abbe limit equation is better suited for giving distance in terms of wavelengths of light, though it can be converted to angular measurement.

As Pete says, there really is no hard floor limit, depending on how you define resolution. I never thought a 6" could show stars below Dawes, but it can reach to Sparrow (107/Dmm) and below. Seeing elongation at 0.5" arc was amazing (in sub arc second seeing, of course.)

#30 Asbytec

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Posted 08 December 2012 - 05:19 PM

Some thoughts on Raleigh and MTF on extended objects.

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#31 azure1961p

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Posted 08 December 2012 - 05:43 PM

I see where your going and thanks for the illustration. I for one want to experiment on Io and Ganymede with CO.

Thanks again I'm seeing how the darker value is less crowded by smaller central airy discs due to co .

This has some compelling possibilities.

Pete






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