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Perceived size of stars.

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#1 Princess Leah

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Posted 23 May 2024 - 08:36 AM

Does the size of a star appear smaller with increase magnification due to a reduction of the size of the exit pupil?

Or is it a compound of smaller exit pupil AND increased magnification.

 

This assumes a magnification before the airy disc begins to be resolved.

 

Incidentally this may explain partly why I enjoy wide views with 4-5mm exit pupils. I like the variety of star sizes on show - even if is just an illusion!


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#2 TOMDEY

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Posted 23 May 2024 - 09:34 AM

For nearly all observers stars appear to be smaller when the pupil is smaller... even when viewing naked eye. This is why most of us can count more stars in the Pleiades at twilight than we can in an extremely dark sky. The explanation is that the smaller pupil removes the relatively aberrated outer region of our eyes' pupils. It also helps explain why we can see more detail in "little fuzzies" when we magnify to a smaller pupil. Some full-service ophthalmologists will measure and show you your eyes' impulse responses, along with a table of the corresponding Zernike aberration coefficients. Nearly all person's eyes are terribly aberrated when dark-adapted. Implants, Lasik, and PRK address this, most often improving day and night vision dramatically. The implants correct Zernike spherical and astigmatism. Lasik and PRK address the others. The result is (most often) improvement from deficient to ~hyper acute~ 20/15, 20/12.5, or even 20/10 in both bright and subdued light.    Tom


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#3 GlennLeDrew

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Posted 23 May 2024 - 12:51 PM

A number of factors play a role. Off the top of my noggin:

 

- A brighter star can appear larger due to 'bloom', or retinal saturation.

- At moderate to high magnification poor seeing can cause a point to become a turbulent blob of some size.

- At small exit pupils the Fresnel pattern of diffraction is resolved, with the Airy disk having perceived size.

- At large exit pupils the fuller involvement of one's imperfect lens/cornea can cause a point to become an aberrated splotch.


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#4 Princess Leah

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Posted 23 May 2024 - 02:20 PM

Marvelous replies as always.

 

One final thought.

 

If you have two scopes with same exit pupil, but different magnification - does the higher magnification help to focus the star to a finer point of light?

(Assuming no turbulence or airy disc been resolved).

(Although the bigger aperture might in some cases, cause blooming as Glenn mentions above, and make the star appear bigger.)


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#5 TOMDEY

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Posted 23 May 2024 - 04:28 PM

Marvelous replies as always.

 

One final thought.

 

If you have two scopes with same exit pupil, but different magnification - does the higher magnification help to focus the star to a finer point of light?

(Assuming no turbulence or airy disc been resolved).

(Although the bigger aperture might in some cases, cause blooming as Glenn mentions above, and make the star appear bigger.)

Somewhat surprisingly... the two (or more) telescopes with equal exit pupil normalize to the same angular size of the impulse response (Airy Pattern). The bigger scope may make the (same) star "look" bigger, just because it is brighter. Same reason that naked eye Sirius "looks bigger" than Polaris --- only because it's two dozen times brighter.    Tom


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#6 Princess Leah

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Posted 23 May 2024 - 05:22 PM

Yes I'd agree with that Tom. Very interesting. Thanks for your response.


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#7 MeridianStarGazer

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Posted 24 May 2024 - 08:42 AM

Probably already stated here, or I read in a different thread: brighter stars cast more energy into the further out diffraction rings. I saw some very bloated stars with 6 rings around them. Only when I look at Arcturus and similar.

#8 TOMDEY

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Posted 24 May 2024 - 01:02 PM

Probably already stated here, or I read in a different thread: brighter stars cast more energy into the further out diffraction rings. I saw some very bloated stars with 6 rings around them. Only when I look at Arcturus and similar.

Sort of --- but as a % it's of course invariant... the same for all stars.    Tom



#9 Cpk133

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Posted 27 May 2024 - 10:26 PM

Does the size of a star appear smaller with increase magnification due to a reduction of the size of the exit pupil?

Or is it a compound of smaller exit pupil AND increased magnification.

 

This assumes a magnification before the airy disc begins to be resolved.

 

Incidentally this may explain partly why I enjoy wide views with 4-5mm exit pupils. I like the variety of star sizes on show - even if is just an illusion!

Having a hard time understanding what the question is here:  Stars are point sources of light.  As you increase magnification you’re magnifying the diffraction pattern until it becomes visible assuming there’s enough energy for your eye to detect it.  The image produced by the optic is the same regardless of magnification.


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