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Resolving power versus resolution?


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FLO lists these specs for my NexStar 4SE:

RESOLUTION: 1.36 arc seconds

RESOLVING POWER: 1.14 arc seconds

What does it mean exactly? I am trying to calculate the the Airy Disc produced by my scope, and my program gives me the first minimum of the Airy disc at 1.12 arc seconds which is nice and close, but what exactly am I comparing it to?

Is the resolving power the Airy disc radius (measured to first minimum)? If so, what wavelength of light are Celestron using (I'm using 510nm)?

And what is RESOLUTION - according to my calculations 1.36 arc seconds is approximately the middle of the first diffraction ring.

EDIT: (I originally posted with an eliptical central obstruction, I redid my maths :-) )

I plugged in an 8SE and I get Airy disc size of 0.49 arc seconds which doesn't match up to the Celestron figure of 0.57 very well...

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Ah... the 0.57 figure of Celestron is the Dawes limit for an unobstructed 8 inch instrument. Resolution is higher in an SCT as the central obstruction makes the Airy disk smaller, at the expense of contrast.

...so Celestron are only quoting rules of thumb, not taking into account the optical design.

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Ah... the 0.57 figure of Celestron is the Dawes limit for an unobstructed 8 inch instrument. Resolution is higher in an SCT as the central obstruction makes the Airy disk smaller, at the expense of contrast.

...so Celestron are only quoting rules of thumb, not taking into account the optical design.

Why would a central obstruction make the Airy disk smaller?

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My understanding is that the greater the aperture the smaller the Airy disc, hence the greater the aperture the greater the resolution.

In an unobstructed system most of the light is focused in the Airy disc, the diffraction rings are as a result of diffraction caused by the outer edge of the objective or objective cell.

In an obstructed system less light is focused into the Airy disc as a second cause of diffraction is added to the system by the edge of the secondary obstruction. The result is a greater amount of light is focused into the first diffraction ring, resulting in a slightly dimmer Airy disc. This also has an impact on contrast as the larger the central obstruction, the brighter the diffraction rings, in effect subtly smearing the image causing a loss of contrast.

The resolution of a telescope could be defined as its ability to separate two points. These points could be two stellar points or two points on a planetary surface. With two close binary stars -bright point sources against a black background- diffraction rings can overlap. It stands to reason then that the less light in the diffraction rings the easier it would be to separate the two points. For this reason a telescope with a small or no central obstruction is generally preferable. But aperture also plays an important part as the greater the aperture the smaller the Airy disc and the greater the resolution.

Telescopes however often resolve well beyond their parameters. For example, an unobstructed system of 4" (100mm), can resolve planetary detail that is technically impossible if we go along with Dawes limit. A 4" refractor will often resolve Enkes division in the outer A ring of Saturn's rings. So how does it do it?

It would be impossible for a 4" to resolve any single point on the Enke division, but because it is a linear feature, as many technically unresolvable planetary features are, a 4" can succeed in resolving the impossible. This is why unobstructed systems such as refractors tend to deliver planetary performance well beyond their aperture class.

Mike

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Why would a central obstruction make the Airy disk smaller?

The central obstruction robs light from the disc and puts it in the first ring. This increases Rayleigh resolution (defined as the radius of the first minimum of the diffraction pattern) while reducing real contrast (a lot more scattered light).

I was just looking for some hard data to compare my calculations to, to check my workings.

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