Why do pictures of stars have 4 cardinal points?

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  • #26
davenn
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I think you're thinking of the very well defined, closeup, extended images of galaxies we are all familiar with. I'm referring to wide field images of clusters of galaxies like this one of NGC 7619 taken by astrophotographer Jim Burnell (jburnell.com) If you click on it you'll see two objects center right that are of similar brightness to some of the stars yet have no diffraction spikes - they are cluster members! Sorry for the confusion

even those ones are fuzzy blobs, extended enough not to be "pinpoint" light sources
I wouldn't expect any galaxy to have diffraction spikes .... in all the photos I have seen and the few
I have taken, none have ever shown spikes :smile:
It really does need point sources

Dave
 
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  • #27
davenn
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I'm referring to wide field images of clusters of galaxies like this one of NGC 7619

nice galaxy field ... will have to try and image that area ... it's gets above my horizon in mid August for a few months
A lot of guys in my area are imaging the Virgo cluster area at present ... M84, M86, M87 and a huge mass of others

Markarian's Chain of galaxies

Markarian's Chain of galaxies.jpg


By Dave Manning,
Telescope - Skywatcher Esprit 120mm Triplet Refractor; Field Flattener; Camera – Sony a7r Mk2, 18 x 5 min lights
He and I and a few others get out for astrophotography sessions from time to time
Left to right across the centre ... M84, M86, NGC4438 (upper), NGC4435 (lower), NGC4461 (upper) NGC4458 (lower),
NGC4473, NGC4477 ... and many, many others in there including lots of very faint ones


Dave
 
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even those ones are fuzzy blobs, extended enough not to be "pinpoint" light sources
I wouldn't expect any galaxy to have diffraction spikes .... in all the photos I have seen and the few
I have taken, none have ever shown spikes :smile:
It really does need point sources

Dave
Yes that's true, they are somewhat more extended. It's not that they might be confused for stars - If you look at the stars again, you'll see that their images are somewhat extended due to overexposure; they might be confused for cluster members but for the diffraction spikes! Also,there are clusters where unresolved cluster members mimic overexposed stellar images, but are found out due to a lack of diffraction spikes...
 
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  • #29
sophiecentaur
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their images are somewhat extended due to overexposure;
It is usually necessary to show some overexposed stars in order to see the fainter objects. The spreading of the images happens on the sensor but the diffraction spikes come before the image sensor. The spike amplitudes are the same, relative to the peak value for all point sources. A bright star will produce a disc of limiting peak white value on the output of the sensor but the weaker spikes are not extended because their luminance is way below limiting value. A sensor with greater dynamic range would show a diffraction limited image with a visible gradient around its peak and still show spikes for that image. Also, a web with thicker vanes could produce detectable spikes for fainter stars.
Otoh, every part of a bright, extended galaxy will produce its own spike and those spikes are spread over the angle subtended by the galaxy (with an image that is not into limiting) but the spikes are individually too faint to see.
 
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davenn
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If you look at the stars again, you'll see that their images are somewhat extended due to overexposure; they might be confused for cluster members but for the diffraction spikes!
No, for this reason ......

objects. The spreading of the images happens on the sensor but the diffraction spikes come before the image sensor
Yes, exactly
 
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Yes the spikes are produced before the image gets saturated, but the final result is an extended image which could be confused for a potential cluster member if it wasn't for the spikes...
 
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Top of pops was the usual one as there were always visible studio lights.
The rays in the TV camera images are caused by internal reflection/diffraction off of the lens's iris. Closely related to the telescope images, but of course there are no mirror supports in the TV camera.
 
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pinball1970
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The rays in the TV camera images are caused by internal reflection/diffraction off of the lens's iris. Closely related to the telescope images, but of course there are no mirror supports in the TV camera.
Thanks the guys gave a few examples, I always liked them (spikes) in older 1970s studio shows
 
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sophiecentaur
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Astro telescopes do not have adjustable apertures so the whole length of the optical assembly is circular cross section. That minimizes diffraction effects. Smaller aperture scopes use lenses sothere’s No need for a web to hold a secondary mirror.
 

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