Why do pictures of stars have 4 cardinal points?

In summary, astronomers use diffraction spikes on pictures taken without a telescope to make the stars more recognizable.
  • #1
swampwiz
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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?
 
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  • #2
swampwiz said:
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
 
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  • #3
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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  • #4
Are there strong technical reasons why a telescope should have 4 rather than 6 diffraction rays?
 
  • #5
snorkack said:
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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  • #6
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:
https://en.m.wikipedia.org/wiki/Diffraction_spike
 
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  • #7
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.
 
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  • #8
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!
 
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  • #9
alantheastronomer said:
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
 
  • #10
davenn said:
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.
 
  • #11
sophiecentaur said:
Not total nonsense,
it is, specially this part ...

alantheastronomer said:
in fact it's a way to distinguish foreground stars from extended sources like planets or galaxies!
Stars are NOT foreground to planets :rolleyes:
and galaxies are not point sources so they are totally irrelevant
 
  • #12
davenn said:
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.
 
  • #13
sophiecentaur said:
. 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
 
  • #14
davenn said:
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.
 
  • #15
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.

 
  • #16
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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  • #17
davenn said:
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?
 
  • #18
pinball1970 said:
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.
 
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  • #19
glappkaeft said:
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. 😉
 
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  • #20
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.
 
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  • #21
glappkaeft said:
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 🤪
 
  • #22
sophiecentaur said:
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.
 
  • #23
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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  • #24
alantheastronomer said:
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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  • #25
sophiecentaur said:
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.
Yes, exactly!
 
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  • #26
alantheastronomer said:
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
alantheastronomer said:
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 onesDave
 
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  • #28
davenn said:
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
alantheastronomer said:
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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  • #30
alantheastronomer said:
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 ...

sophiecentaur said:
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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  • #31
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...
 
  • #32
pinball1970 said:
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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  • #33
James Demers said:
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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  • #34
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.
 

1. Why do pictures of stars have 4 cardinal points?

Pictures of stars have 4 cardinal points because they are a result of long-exposure photography, where the Earth's rotation causes the stars to appear as streaks of light in the sky. The 4 points are the North, South, East, and West cardinal directions, which are visible due to the rotation of the Earth.

2. What is the significance of the 4 cardinal points in pictures of stars?

The 4 cardinal points in pictures of stars are significant because they help to orient the viewer and give a sense of direction in the night sky. They also serve as a reminder of the Earth's rotation and its impact on our perception of the stars.

3. Why do the 4 cardinal points appear as straight lines in pictures of stars?

The 4 cardinal points appear as straight lines in pictures of stars because of the Earth's rotation. As the Earth rotates, the stars appear to move in arcs across the sky, but the camera's long exposure captures these movements as straight lines.

4. Do all pictures of stars have 4 cardinal points?

No, not all pictures of stars have 4 cardinal points. The number of cardinal points visible in a picture of stars depends on the length of the exposure, the location and angle of the camera, and the position of the stars in the sky.

5. Can you see the 4 cardinal points in real life when looking at the stars?

No, the 4 cardinal points are not visible to the naked eye when looking at the stars. They only appear in pictures due to the long exposure of the camera. When looking at the stars in real life, they appear as twinkling points of light without any discernible direction or pattern.

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