Why are stars spiky? - Deep Sky Videos
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- Опубликовано: 24 сен 2012
- Why do photos of stars often have spiky, pointy bits of them? These are called diffraction spikes and some people even add them artificially!
Here are explained by Professor Mike Merrifield (University of Nottingham) and Nik Szmanek (astrophotographer). Mike tweets at / profmike_m
Some image credits:
Philip Perkins: www.astrocruise.com/
Nik Szymanek: ccdland.net
Fred Espenak: astropixels.com
Deep Sky Videos website: www.deepskyvideos.com/
Twitter: #!/DeepSkyVideos
Facebook: / deepskyvideos
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Videos by Brady Haran
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periodicvideos.blogspot.co.uk/... 
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Super useful video, thank you! I'm a NASA scientist and engineer developing a potential new satellite and I'd forgotten all these kinds of details from my undergrad astronomy classes. This video felt like I was sitting in on office hours! Good times :)
What an excellent editing job done on the existing footage! You're good Brady... you really are.
Aside from looking through a reflecting telescope to view diffraction spikes you can also seem them with the naked eye by looking at a high-contrast situation though a screen door. For example, looking out my back door at night, which has a sliding screen door, at insecurity lights off in the distance - I see diffraction spikes on the lights with the screen obstructing them, no spikes with no screen obstruction.
I just realized, this series has somehow become part of my life. Really, it would be a great loss to stop making these marvelous stories.. Thanks for contributing to human knowledge!
I really liked his white and gold jacket
+Ângelo Vargas Looks more blue and black to me.
I have been greatly enjoying all these videos Brady, and I was wondering if you have or were ever planning on making an episode of DeepSkyVideos to introduce beginners into astronomy. It would be great to have a high quality resource to be able to reference when just starting out with buying and setting up your own telescope. Do's & dont's, how best to take pictures etc. I just love the universe and would love to have a better view of it especially since I live far in the northern hemisphere.
This was a great video Brady
I have 2 ideas to solve this:
1) Make the supports turn around(spreading them out during the taking of the photograph), so that the effect is the same in all directions, but this might cause the spikes to be spread out on all axes.
2) Glue the secondary mirror on a piece of perfect glass (perfectly flat, maybe even perfect on molecular level?), so then there is nothing blocking the light.
First idea would probably work okay, but then you are just doing less damage to more area, easier to just make sure the spikes aren't over what you want to look at.
Second idea is what I was thinking, and Nik actually said that some telescopes do embed the secondary in a sheet of glass. But making a perfect enough sheet of glass for one of the big telescopes would be really expensive - and impossible for telescopes like the Extremely Large Telescope.
Like anything, it just comes down to cost. Cheaper to rotate the diffraction spikes away from what you are looking at than to design them out.
1. By making lots of supports you actually distort and blur the image (sometimes addons of lots of parallel stripes are used for calibrating optics, as there is a very particular kind of distortion).
2. Putting glass for holding the secondary mirror is done for Makusov type of telescopes. More to it - the glass is not flat, but shaped as a lens to counter "coma" distortion attributable for all reflector telescopes. The problem is that such solution is not really an option for telescopes larger than 50 cm in diameter.
Brady, THIS is why I really dig your channels! Learned something brand new today. I figured it was because of the telescopes, but I thought it was more due to the curvature of the lens :D
This channel answers a lot of my childhoods why's and how's
Great video Brady! Thanks.
That was very informative. Thank you Brady for making these videos :]
There's a really fantastic set of java applets produced by Paul Falstad. If you google "falstad" the first hit should be the ripple tank simulation. You can play around with waves and diffraction patterns.
Well I just learnt something today I wasn't expecting. Thanks Brady.
At first I thought this was going to be a boring atmosphere video. Then I looked at the tittle a bit harder and thought that maybe I should watch it. Glad I did.
Really interesting. I have a 8in SC w/no spikes and an 17.5 Newtonian w/spikes. Now I have a deeper understanding about them.
Thank you.
who could dislike such an informative video.
Can you have the secondary mirror constantly spinning? This way the struts that are holding it aren't in a fixed position so the diffraction pattern gets even out?
0:48 Does that mean as the light passes through the aperture, the light/information through the other end is scrambled (distorted)? A similar idea would be how an image starts to pixelate as it's increased far beyond it's original size. You can see in the diffraction image that the waves seem to expand/stretch.
Very insightful video thanks for uploading :)
How abou you make the secondary mirror roatatable along with the supports for it?
In a long exposure, if you rotate it along equally, you'd end up with averaged-out, faint, pretty much invisible disks instead of spikes, right? Or are the spikes so bright that even smearing them out like that wouldn't be good enough?
Marvelous vid and explanations!
As a photographer, I'm more concerned about the blurry diffraction spikes vs the crisp clean ones.
Can someone explain what could be causing the fuzziness of the diffraction spikes as opposed to a crisp one?
Can a lens designer correct the fuzziness to produce a crisp starburst?
What does he need to do?
I think if I cannot do anything about the diffraction spikes, I'd rather have a crisp one than a low contrast blurry one.
Wow, I never knew that....that's why your videos are so awesome! :-)
great video !
It also creates discernible focal points in a large cluster,
kind of like putting an x thru a point to have it stand out from the background.
very interesting.
This discussion is particularly interesting considering we now have these beautiful images coming down from the JWST, and how different they are from the awe-inspiring Hubble images that have defined my entire life! The children of today are so lucky to have images of the cosmos that are sharper, have even more background galaxies to gawk at, and have webb's gorgeous hexagonal diffraction spikes 🥰
Cool - that's the idea!
Why dont they make the supports transparent? Like a non reflective plastic or so
I was wondering why a glass support wasn't used, I understand that a lens on the front might cause some abberation and whatnot, but can't these be made accurately enough and large enough to be of use on the larger telescopes?
Excellent explanation.
could we use transparent glass for holding the mirror?
Finally, my question about how the supports hurt the images is answered.
What I would also like to know, is what I think was mentioned at the end, why not use transparent supports for the secondary mirror. Also, why use mirrors in the first place and not just lenses? Is it because it's easier to make large mirrors than large lenses to get more light?
Please Brady make a video about this, thanks.
Very interesting, never thought about that actually.
It all depends on the purity/doping of the glass. You can dope glass to eliminate lots of spectral bands if desired.
Could you hold the secondary mirror by magnets? I feel like it would affect certain wavelengths, but would it affect all of them, and enough so to merit that idea really stupid?
What was that sound at 2:37?
Mirrors are also used more than lenses now because of the extreme difficulty of grinding very large lenses and the fact that lenses get very heavy. The weight of a lens can warp the glass or just cause it to otherwise break. Mirrors are also able to be adjusted and focused relatively easily, so it's basically a win-win all around.
This blew my mind!
this is soo interesting thanks bradyy
I know what you mean, Brady has a way of making things so entertaining, suddenly it's 3AM.
how would you damp vibrations?
Brady, can you do a video on quasars?
Can you elaborate on this different set of artifacts. I realize the mirrors are more difficult to grind, but it's possible and doesn't inherently cause artifacts. (As a matter of fact a parabolic reflector still would be parabolic, but from a different part of the curve, i.e. not symmetrical about the axis.)
I would imagine the difficulty would be both stabilisation and precision. The requirement of the struts is to hold the secondary mirror perfectly, something I'd imagine magnetic fields have a problem with over large distances.
would it be possible to hold the mirror in place using magnetic forces? Big energy consumption would be involved, but in a big enough telescope that would also provide a super-precise way to keep the mirror in a fixed position.
What am i getting wrong?
The last part about refraction telescopes not having these diffraction artifacts makes me wonder about something. I've read on phys.org that there are new, ultra-flat, distortion free new kind of lenses that have been developed recently. People mostly talk about applications in consumer electronics and "tiny" stuff such as fiber-optics. Any chance it could be used for building telescopes as well?
The three major problems with lenses as opposed to mirrors is that lenses deform under their own wight, they need better maintenance (cleaning from both sides), and they block some parts of the EM spectrum. There is also chromatic aberration, it's due to the fact that different parts of the spectrum refract at different angles, so you'd end up with images where the colors are not aligned, but this can be fixed.
if you wanted to minimize these spikes, couldn't you position the secondary mirror on a piece of glass or something transparent?
Why doesn't the secondary mirror cause any effect on the image when it's massive compared to the struts?
couldn't your take two pictures of the same star with the strusts rotated, and put the two images together so the thing that is different in the pictures (the diffraction spikes) gets removed?
Well,that was educating!
Is it possible to magnetically levitate the central mirror perhaps?
Well, the distortion can't be that large for a perfectly flat pane ... although I imagine it doesn't stay perfectly flat when you attach a weight (such as a secondary mirror) to it. But I actually didn't think of chromatic abberation. OTOH you should be able to correct for it since mostly narrow band-pass filters are used anyway.
As I was watching this the question that kept coming to mind was: "Is it possible/feasible to have the secondary mirror supported by electromagnets surrounding it, locking it in place?"
Obviously, the floating mirror part would need a few magnets in it, but they could just be behind the secondary mirror and facing the electromagnets.
Any thoughts?
It should be mentioned though that any glass does have an absorption spectrum, and there will be SOME frequencies which it will happily block and deny you images in. But if you're designing a visible-light telescope (for example) a glass disc wouldn't be a problem.
I believe right near the end the gentleman showing his telescope actually names a specific type of telescope that does this. Large glass lens with a reflecting mirror in the center.
Not necessarily a bad idea, just something you need to take into consideration when designing or buying a telescope. What mkirefu described is basically a Schmidt-Cassegrain telescope (also mentioned in 6:44).
By correcting I meant in software. CAs are only really a problem when you measure several wavelengths at once. When you have a narrow bandpass filter, your whole image will be displaced by a certain amount which can be easily corrected for in software.
I can't imagine that the cost of a glass plane is really an issue when looking at the huge telescopes that are being built and the incentive to find new exoplanets. I can imagine what the secondary mirror of such a telescope will do to glass.
There are designs that do that. Google Stevick-Paul folded scope. You see smaller amateur scopes that use unusual designs like that to get the secondary out of the way. The problem is that both grinding the mirrors to the correct shape and alignment of the optics gets a lot trickier.
Of course, now that big scopes are all ground and aligned with computers, I suppose the reasons to not use those designs are largely gone but like the video said, diffraction spikes aren't usually a big issue.
Why don't they give the secondary mirror an offset from the optical axis, so that it's out of the light path?
Can't the secondary mirror be held in place by a glass pane or something else transparent?
Back to the proper physics :) Very good video.
1:24 - Is this the same reason I see this pattern when looking at lamps at night? They always carry this kind of halo, and it kind of goes through the colours of the rainbow as it fades out radially. Hmmm....
I would assume it is because off-axis telescopes usually create coma and astigmatism effects. So you end up with blurred images or stretched images. This can be corrected with lens, but lenses are very expensive for even small telescopes, let alone large 40"+ telescopes.
A part of that is because the light is deffracted when it passes between your eyelashes (which acts like appertures). If you squint at a lightsource when it's rather dark you'll see the phenomen becomes stronger.
An even cooler phenomen you can observe if you're nearsighted is the following: Press the edges of you thumbs and index fingers together to form a tiny hole. Remove your glasses and put the whole right in front of your eyes. Suddenly you can see at a distance due to diffraction.
Thanks for the video, very much appreciated ;)
I see. I guess this is also true for pmma (acrylic glass)?
why isnt the secondairy mirror placed on a piece of thin glass, would also prevent dust
Ok, wow, would never have thought that it had any thing to do with the support structure of the telescope, it makes sense though when you explain it, But it brings up another question, what if you can still see these spikes with your naked eye? is that normal?
Good stuff..always is!
Why don't they use a supertransparent "lid" made of glass or plastic to cover the aperture of the telescope and fixate the secondary mirror on it's center? Such mirror would be like floating, with no visible structure holding it in place. I don't know what optical effect the transparent lid would cause, but I think it could work...
Why not just a put a flat piece of glass on the front and attach the mirror to the inside of the glass plate? Would that work?
Why do the diffraction spikes appear on bright/near stars, but not on distant dimmer stars ad galaxies?
That's exactly what they mention at the end of the video: telescopes with the Schmidt-Cassegrain design have that, but putting a piece of glass in the optical path creates additional losses and aberrations. With very large telescopes it's probably not practical to put in a huge pane of glass that is strong enough to keep the secondary mirror in place.
Aside from stars, what other heavenly bodies or simple objects around us that have diffraction of light?
Interesting bit: which strut causes which spike? Basically, a vertical strut would make the light spread out horizontally, causing a horizontal spike.
The reason for it not showing up with difuse source like a nebula is that a star is really bright - literally as bright as the sun (per square arc-second - think about that...), just not as big, and overexposes on just about any sensor. The spike is a smeared image of the star. The nebula is not so bright, so it's smeared image is very faint.
Such telescopes do exist, but glass lowers the speed of the device and, more importantly, it filters out some invisible wavelengths that professionals might be interested in. It would also likely add its own absorption pattern.
go to wiki, reflecting telescope. it's called schiefspiegler which is of course german for askew mirror (personalised masculine)
this channel should have far more views. whats up with people these days? being all the time at parties and being useless?...
The glass would decrease the sensitivity of the telescope (make it darker) and unless it's absolutely perfect, it will introduce glitches like chromatic aberration, diffraction, etc. And it wold be another layer of glass to keep completely clean of dust.
For really big telescopes such a structure would be nearly impossible to build Nik mentioned that some refracting telescopes are built that way.
I was thinking about this the other day...Funny thing is, I assumed that the similar light flares that you get from looking at lamps and such with the naked eye, were the same. I guess not? Could be the same effect but different cause.
The fact that I learned the fact that there's a fact that those star spikes aren't there and just look like there there made me starstruck.
So the shape of every star we ever drew or pictured, was that way because of the telescope. You just blew my mind!
you would have to have mirrors with weird eliptical shapes and to get that on a mirror is super herd, making a mirror for a reflecting telescope is hard work, polishing it and giving the final perfect shape takes hours of different grits of abrasives ...
what about quantum locking to hold the secondary in place?
I dont Get any defraction spikes when I take a picture, I have and 11inch schmidt cassigrain telescope with a corrector plate instead of struts it this why I dont get andy spikes
Pardon my ignorance but why doesn't a nebula, for example, have refraction spikes and stars do? And how come every star has refraction spikes and not only 1 star? Does that have anything to do with the filters that are used in telescopes? I would greatly appreciate a serious and informed answer. :)
I would like to hear more about smith cassegrain telescopes, where the secondary mirror is mounted in a lens. What are the problems with this approach?
Schmidt correctors being a single glass element create a bit of chromatic aberration that pure reflecting system wouldn't have. It's minor but some of the brighter stars show a bit of a blue halo around them. It really isn't that significant especially compared to an achromatic refractor.
In another comment I explained how it can perfectly be part of a paraboloid. Just think a paraboloid of which you take a piece, away from its axis. The light will still have the same focus, but it won't be in the light path. The mirror won't be symmetrical, but it will still be a paraboloid.
Maybe your iris is supported.
The thing I like with that is that it reveals something big about how we see. We're not seeing the actual photon, we're seeing an interpretation of that photon, "projected" if you may on the brains 3-Dimensional "cinema screen". You're normally not aware of the "screen", cause that is how we usually see, but once these lines show up they remind us (or atleast me) that we're seeing a perfect 3-Dimensional movie with awesome physics and we are in it.
So glass is used then? I don't mean lenses but planes (since lenses are notoriously thick and thus add a lot of expensive material).
And yes, I can see the sagging problem. That's what I meant when I talked about adding a weight above.
You might just tilt the primary so secondary would be on adjoining tube. Too bad this would mess up the most sensitive measurements, i guess
There are a number of designs of telescope that do, they are called of-axis designs, the simplest is call a Herschelian style. The problem is that you get something called a geometric aberration. Basically objects don't appear the shape that they ought to because you have moved one of the mirrors. In some cases, this has more advantages than disadvantages, but most of the time it is better to use more traditional designs such as a Cassegrain.
I think it's because that would require the main mirror to be a funky shape, and would require more effort to build to high specs
such telescopes exist, but the mirrors are much harder to make. The allen telescope array looks at radiowaves, it uses such a design, but radiowaves are much more forgiving when it comes to imperfections in the reflectors. For visible light, i guess such a non-symmetrical setup would just be a major pain in the ... when it comes to manufacturing the mirrors.
I think he was mentioning towards that near the end of the video with 'an optical window'
Couldn't they eliminate this problem with the struts all together by using something like the photoreceptors (or whatever they are) in a digital camera instead of mirrors? That way you could have one primary receptor that transfers the information to the computers instead of having the light bounce from one mirror to the next, etc.
Has anyone tried making a telescope like an Offset Gregorian antenna? Then there would be no strut or secondary mirror in the way of the light. Probably a difficult geometry to make the glass-mirror in but someone should try.
Another question, does the segmented primary mirror of for ex Keck produce spikes?
Yes, mirror gaps do produce artifacts, which is probably why the Keck uses a 6-bladed support structure so any artifacts will match in shape and direction. If it used four blades, you'd get fewer spikes, but they'd be much uglier simply by not matching the artifacts the mirrors induce.
Is it not possible to hold the mirror in the same way that Quad electrostatic speakers hold their diaphragm in a kind of levitation using electromagnets?