16 replies to this topic

[kingsbishop]

I have never properly understood why we can see smaller details on extended objects and not point sources I have only been told that it is because they are extended objects and not point sources but why?? How does diffraction know if it is an extended object and not a point sources becuase if we took the rest of the planet away except for one pinpoint it turns into a point source that sounds a bit like magic or some extraterrestrial being saying to the telescope “this is an extended object act like an extended object” if you know what I mean

This is really hard to explain what I mean so I am just hoping you get my drift and can give me an answer

[BQ Octantis]

Because Rayleigh is the point spread limit.

 

Pattern or line spread detail is limited by the Modulation Transfer Function, which is ~41% of Rayleigh.

 

And the limit for very high contrast standalone features is governed by the Edge Spread Function, which is more like ~10% of Rayleigh.

 

BQ

Edited by BQ Octantis, 12 January 2025 - 07:06 PM.

[kingsbishop]

Because Rayleigh is the point spread limit.

Pattern or line spread detail is limited by the Modulation Transfer Function, which is ~41% of Rayleigh.

And the limit for very high contrast standalone features is governed by the Edge Spread Function, which is more like ~10% of Rayleigh.

BQ

so in other words when you reach a point in magnification where your eyes can resolve 41% of the Rayleigh limit your at the point of empty magnification?

[BQ Octantis]

so in other words when you reach a point in magnification where your eyes can resolve 41% of the Rayleigh limit your at the point of empty magnification?

Pretty much. It's a little more complicated for visual than for imaging because magnification also darkens the image. The retina's contrast detection is nonlinear with respect to brightness, so that interaction also plays a role in what you can visually distinguish. For bright, low-contrast targets like planets, more magnification results in better contrast detection until the image darkens with no improved perceptual detail. This limit also varies with atmospheric "seeing" caused by turbulence in the light path.

[quilty]

Because Rayleigh and Dawes refer to stars only. Not to lines and other features. And it doesn't consider contrast based resolution
And because our perception refuses to stick to a simplifying single number (some do at least)

The Enke gap for instance. Many people calculate using numbers that it's impossible to discern it through a 7".
Others just spot it. And a third party (knowing all by half) find out that in the latter case it's not the Enke gap itself spotted but a widened, dimmed artificial representation of it.
Which is the very same case for any star except our sun. We don't spot stars but some optical artifacts produced by them and the optics and no one cares

Edited by quilty, 13 January 2025 - 07:31 AM.

[Sebastian_Sajaroff]

Cassini gap in Saturn is 0.7" wide
Theoretically invisible on a 3", however it’s pretty obvious on my refractor at 100x when the rings are a bit opened (like in 2022)

Edited by Sebastian_Sajaroff, 13 January 2025 - 07:58 AM.

[kingsbishop]

Because Rayleigh and Dawes refer to stars only. Not to lines and other features. And it doesn't consider contrast based resolution
And because our perception refuses to stick to a simplifying single number (some do at least)

The Enke gap for instance. Many people calculate using numbers that it's impossible to discern it through a 7".
Others just spot it. And a third party (knowing all by half) find out that in the latter case it's not the Enke gap itself spotted but a widened, dimmed artificial representation of it.
Which is the very same case for any star except our sun. We don't spot stars but some optical artifacts produced by them and the optics and no one cares

yes I know but how does a telescope know that if you know what I mean it sounds a bit magic without a reason

Say if we looked at Jupiter and then took away the whole planet except for one pinpoint how does nature know to act like a point source and not an extended object what is the reasoning behind why you can resolve smaller on extended objects than stars I know that you can but this thread is trying to find out the reason for this?

[BQ Octantis]

It is simply the wave nature of light coupled with a round entrance pupil. The constructive and destructive interference in the complex domain of all rays from the target through the system results in the patterns we see in the real image on the focal plane. The different stimulus types produce different system responses.

 

You learn this in signal processing—the point spread function is simply the impulse response of the system. The edge spread function is the step response. The modulation spread function is the frequency response. You also learn about the principle of superposition—where a real target is the sum of the various types of stimuli.

 

BQ

Edited by BQ Octantis, 13 January 2025 - 05:22 PM.

[ButterFly]

Have a look at this TomDey graphic showing the difference between lines and points.  The contrast transfer of the system (which always includes your eyeball) is the relevant consideration, and then it goes on to your brain for processing.  For the brain, the contrast to noise ratio becomes very relevant.

[ButterFly]

Say if we looked at Jupiter and then took away the whole planet except for one pinpoint how does nature know to act like a point source and not an extended object what is the reasoning behind why you can resolve smaller on extended objects than stars I know that you can but this thread is trying to find out the reason for this?

That's not nature.  That's you doing stuff.  Jupiter's light wouldn't know you smeared vaseline all over your lens either, but the nature of vaseline determines how that light gets spread out once it gets to the eventual focal plane (your retina).

 

In the same way, Jupiter's light doesn't know how big of an aperture you have to resolve fine details in the first place.  But, the nature of diffraction at the edge of the optics determines how that lights gets spread out at the eventual focal plane (again, your retina).  You can test that by making a diaphragm for your optics, and looking while tweaking.  Neither Jupiter nor its light cares what you're doing, and by the way, that light left Jupiter a while ago.

[quilty]

yes I know but how does a telescope know that if you know what I mean it sounds a bit magic without a reason

Say if we looked at Jupiter and then took away the whole planet except for one pinpoint how does nature know to act like a point source and not an extended object what is the reasoning behind why you can resolve smaller on extended objects than stars I know that you can but this thread is trying to find out the reason for this?

it's rather our perception which knows. We spot lines of same contrast more easily than dots. W're trained to spot lines anyway, three spots in a row make a line. (three same incidents in a row make a rule the ruler arranges all dots in a line). That's how we work, it's easier to consider a line or a rule instead of a bunch of unrelated dots or incidents
And how do we discern features better when they move? Same thing, we're trained (by inheritance) to specially spot moving features.
Coincidence makes causality (Well, too often it just fakes it but without further input causality is the simplest and most appealing assumption to explain coincidence) That's how we work

The reason is: Lines used to matter more than spots. Moving features used to matter more than dead ones.

"and by the way, that light left Jupiter a while ago."

and within those 20 minutes or so it has to find out what way you'll decide it to perceive it :-)

Edited by quilty, 14 January 2025 - 07:10 AM.

[BQ Octantis]

As it turns out, you can put the moon's edge across the martian disk to observe the result of two spread edges interacting…and then remove it.

 

 

The two edges are smeared deep into one another, and at the corners the energy is directed into a rounded fillet between the two. I've seen this many a time in the planetary forum for Jovian satellites merging into Jupiter's limb. Depending on the aperture and processing, sometimes it's a wart on the edge, and sometimes it's a point well within the limb.

 

For the martian egress (which I did not photograph), the martian disk rose over the shadowed edge of a lunar crater—so a black knife edge across it. The result was a crispy cord line that made Mars look like a little sun rising above the lunar landscape.

 

BQ

Edited by BQ Octantis, 14 January 2025 - 09:25 PM.

[Jay_Reynolds_Freeman]

Let's take a simple case, with a little hand-waving to simplify the physics. Suppose you are looking at a white field with a straight black line crossing it, like this:

 

        ________________________________________________________________________________

 

 

Suppose the width of the line is well below the nominal resolving power of your telescope. What happens then, because of how physical optics works, is that the narrow black line gets blurred out into a wider pale-gray line, whose width is more or less (factor of two or thereabouts) the resolving power of your telescope. It might look something like this:

   ______________

 

 

The CloudyNights editor doesn't have enough range of font size to show the effect clearly, but that should give you an idea. One way to look at it is that the total amount of "darkness" in the line is not changed, it is just spread out wider. The same kind of thing happens if you are looking at a narrow white line on a dark background.

 

Thus it is possible to observe the Cassini division with a telescope too small to resolve it cleanly -- instead of appearing as a narrow dark band in the rings, it will appear as a broader gray one.

 

If you had two very narrow black lines closely side by side, they would blur into one wider gray line -- you would not be able to see them as separate if they were too close together.

 

And incidentally, it is certainly possible to see objects smaller than the resolving power of  your telescope -- if it were not so, all stars except the sun would be invisible in amateur-sized telescopes.

Edited by Jay_Reynolds_Freeman, 27 January 2025 - 08:11 PM.

[Asbytec]

I have seen lunar craters, in near perfect seeing, with an angular diameter smaller than the Raleigh limit of resolution approaching the Dawes limit. One might reason a single Airy disc (of many) from the bright limb would obscure the crater itself. It doesn't. As mentioned, the Rayleigh and Dawes apply to point sources. Not extended objects which rely mostly on contrast (transfer to our eye) and seeing conditions. 

 

Ganymede's disc is approximately the same diameter as an Airy disc of a 6" aperture, yet we can see detail on Ganymede in near perfect seeing, as well. That is because Ganymede's disc is not an Airy disc where all resolution is lost to diffraction. It is a very small extended object where high contrast detail can be detected. There is a limit, surely, to detecting extended object detail, but I am not sure where that limit is. 

[quilty]

The point is you don't need the Airy disk size to spot things. As you can see at Io and Europe, tiny disks smaller than Airy (when you're right about Gany for the 6) but easily spottable.
Funny though to compare Europe's and Io's disks with them of Gany and Kallisto in the 6". The small ones are distinkt dots while the larger ones are always blurred no matter what seeing.
So Io's disk is mostly an optical artifact, similar to any star central disk (yet coloured differently and without regular diffraction rings) but Gany is a blown-up central disk or an optically blurred moon disk.

Edited by quilty, 03 February 2025 - 06:25 AM.

[Asbytec]

Yes, Ganymede is an extended object in most scopes. Anything less that 1/2 the Airy diameter (2.44 Lambda/D) presents a disc indistinguishable from an Airy disc. Io and Callisto are somewhere in between in modest apertures and become extended objects in larger apertures. 

[PKDfan]

Hi KingsBishop !

A specular point is difficult for the eye to detect while linear bounded edges are considerably easier to delineate.

Its as simple as that.

A scope requires we cleave apart miniscule variations of the spectrum and how defined those irregular textures edges are seen is a function of its optical quality.

Does a 1/4 wave scope differentiate the same view as if it was 1/20 wave ?

Of course not so each scopes unique figure decides the clarity of what is seen.


Sir Rayleigh Point Source metric is for splitting double stars so for extended forms of resolution especially those with Fine Scales of contrast or Linear bounded edges require the Edge Spread Function.

Cassini's division is the famous example of realizing great resolution of the light dark terminator boundary despite limited aperture.

Contrast line edges or specular points are two inviolate forms of information construction.


A point is an area of circumference with a brightness colour while linear forms of data are that data which intrudes upon the expanded spheres circumference.


The pie is either cut into pieces vrs the whole untouched pie are the two forms of information simplified.


Hope that helps a bit.


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