B Why do pictures of stars have 4 cardinal points?

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I've noticed that a lot of these pictures are like this. Is it something to do with the error in the machining of the lenses?
 

russ_watters

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I've noticed that a lot of these pictures are like this. Is it something to do with the error in the machining of the lenses?
They are diffraction spikes created by the internal support structure of some telescopes. Some astronomers who don't have such telescopes even tape wires overtop of the scope for artistic flair: :cool:

Horsehead-HaRGB.jpg
 

Klystron

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Just to add that for some people with uncorrected astigmatism -- eyeballs that are not properly round -- bright stars resemble your picture even without a telescope. Generally, the brighter the star, the longer the points.

With a telescope or binoculars and without eyeglasses I can tune out most of the distortion by adjusting the eyepiece.
 
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Are there strong technical reasons why a telescope should have 4 rather than 6 diffraction rays?
 

davenn

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Are there strong technical reasons why a telescope should have 4 rather than 6 diffraction rays?
Depends on the number of supports for the secondary mirror and the brightness of the star

400px-Comparison_strut_diffraction_spikes.svg.png


here's one with 8 spikes

diffraction_spikes-star-2.jpg


generally refractors and reflectors like Schmitt Cassigrain's don't have spikes
because there are no or no need for supporting vanes
 
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If we talk about reflecting telescopes, for a particular design you could have even 6 spikes. But most common is design with 4 spider vanes to support the secondary mirror, like in case of Newtonian reflector. Check this link for more details:
 

sophiecentaur

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The diffraction spikes are in fact there for all the stars but they are very low level, compared with the centre of the star and for low brightness stars the spikes are invisible (don't even register on the sensor). It's only for bright stars that the spikes are visible within the contrast range of the (linear ) sensor.
 
Spikes only occur with point sources like stars - in fact it's a way to distinguish foreground stars from extended sources like planets or galaxies!
 

davenn

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Spikes only occur with point sources like stars - in fact it's a way to distinguish foreground stars from extended sources like planets or galaxies!

well, that doesn't even begin to make sense
 

sophiecentaur

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well, that doesn't even begin to make sense
Not total nonsense, I think. The relative brightness of the diffraction pattern vs star will relate to the areas of the spider and mirror. The level of the diffraction pattern will be many orders of magnitude up on the main image. You need a source that's smaller than the width of the diffraction spike and it needs to be bright enough for the (very diffuse) diffraction pattern to be visible.
 

davenn

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sophiecentaur

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it is, specially this part ......
Just bad wording, I think. Planets and galaxies are extended sources so they follow the same visibility argument. The word "foreground" could have been chosen better. "Bright enough stars" would perhaps have been a better term. Most astro images don't seem to have many spikes stars in them (you could show me an exception, I'm sure) because of the vast range of magnitudes. To get more spikes would involve severe burn-out of the brightest star images. That's the power of the logarithm for you.
Photoshop could (and probably does) present more (or fewer) spikes images.
 

davenn

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. Most astro images don't seem to have many spikes stars in them (you could show me an exception, I'm sure) because of the vast range of magnitudes
no argument there ... it's always seen on the brighter lower %'age of the stars in the image and of course as said earlier, by those telescopes with "spider" type mounting of the secondary mirror.
My CPC925 schmidt-cass scope doesn't have a spider support for the secondary, therefore no spikes
 

sophiecentaur

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generally refractors and reflectors like Schmitt Cassigrain's don't have spikes
because there are no or no need for supporting vanes
Has anyone come up with a design with a plastic membrane supporting the secondary mirror? I guess you would need two layers to stop the mirror tilting and that would involve four surfaces plus Newton's rings from any curvature. I'd bet someone, somewhere has tried it though. It could be cheaper than the optics of SC style optics.
Edit: A big Newtonian can get very heavy, so the initial cheapness could be outweigh by the need for a beefier mount. Things like this are usually down to total cost.
 

pinball1970

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I have seen this before and always wondered why it happens.
Top of pops was the usual one as there were always visible studio lights.

Link below @2.13 give a beautifully symmetric 8 pointer each segment looks be a perfect 45 degrees with some refraction on the 270degree line going clockwise.

 

pinball1970

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The two Ronnies outro gives some nice ones also with 60degrees this time. Why is that? Stars have 4 and these have 6 and 8?
 
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pinball1970

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Depends on the number of supports for the secondary mirror and the brightness of the star

View attachment 241287

here's one with 8 spikes

View attachment 241286

generally refractors and reflectors like Schmitt Cassigrain's don't have spikes
because there are no or no need for supporting vanes
Ok diffraction spikes, what is the mechanism in these spots? The TV camera?
 
Ok diffraction spikes, what is the mechanism in these spots? The TV camera?
For cameras the usual cause is the iris used to stop down the lens. In general any edge that protrudes into the optical path can be the culprit.
 

sophiecentaur

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For cameras the usual cause is the iris used to stop down the lens. In general any edge that protrudes into the optical path can be the culprit.
Not a fault, usually. You can buy ‘star filters’ for cameras. They have a lattice of lines all over so the diffraction pattern works for bigger and less bright sources ( chosen on aesthetic grounds).
At one time photographers did their best to avoid artefacts but you can see them everywhere these days. Effective in some cases, sloppy in others. 😉
 
To put it mildly I'm in the opposite camp of that particular never ending discussion. ;)

I tried the opposite and bought "filter step down rings" (used to fit a smaller filter to your lens) to be able to step down the aperture of some of my camera lenses (to improve the optical quality) without getting any diffraction spikes. The main reason was that camera lenses usually have many blades (LOTS of spikes) and that some of my premium lenses have slightly curved blades in the iris and thus produce not spikes but fan like shapes.
 

sophiecentaur

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To put it mildly I'm in the opposite camp of that particular never ending discussion. ;)

I tried the opposite and bought "filter step down rings" (used to fit a smaller filter to your lens) to be able to step down the aperture of some of my camera lenses (to improve the optical quality) without getting any diffraction spikes. The main reason was that camera lenses usually have many blades (LOTS of spikes) and that some of my premium lenses have slightly curved blades in the iris and thus produce not spikes but fan like shapes.
That’s an interesting take on things. I can sympathise but what can you do and what subjects do you photograph?
I have friends who are bonkers about Astro photography and, of course, they don’t use stopped lenses. AP has its own problems, of course but dealing with star trails is a piece of cake compared with getting just the right amount of filtering (v. narrow band) to bring out all the features of the chemistry.
I just take happy snaps of Saturn’s rings and the occasional nebula. I don’t post my efforts 🤪
 

pinball1970

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Not a fault, usually. You can buy ‘star filters’ for cameras. They have a lattice of lines all over so the diffraction pattern works for bigger and less bright sources ( chosen on aesthetic grounds).
At one time photographers did their best to avoid artefacts but you can see them everywhere these days. Effective in some cases, sloppy in others. 😉
I like them, they remind me of the 70s, glad it's come full circle and it's nice to have a technical explanation.
 
alantheastronomer said:
in fact it's a way to distinguish foreground stars from extended sources like planets and galaxies!
daven said:
Stars are NOT foreground to planets
you're right, I should've said "background stars" for planets, and reserved "foreground stars" for galaxies...
daven said:
and galaxies are not point sources so they are totally irrelevant.
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.
 

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sophiecentaur

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I'm referring to wide field images of clusters of galaxies like this one of NGC 7619 taken by
I assume the galaxy in question is one of the bright images without the star pattern; it is a distributed source and the star pattern is therefore distributed and not visible.
 

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