Why do stars have a cross shape through them?

I've been following HST advent images the Boston Globe have been running over the last month, and reading the comments along the way.

I was shocked at the amount of nonsense attributing the universe to a supernatural deity of some kind, around half of what was posted was along these lines. I’m tempted to bring out the Flying Spaghetti Monster, and preach to all how the universe was touched by his noodly appendage.

Anyway aside from the nonsense there were several interesting comments, one of which asked:

I was once told that the cross shape (like the above image) that you see surrounding stars when you look up at the sky at night is actually just the result of scratches in our corneas, and that they don't make that '+' sign thing at all.

Yet looking at your beautiful images, most stars do in fact have the cross running through them. Can you explain? I'm most curious because I don't see any reason for that phenomenon to occur.

Stars don't make a cross shape when looked at with the naked eye - they should appear as points of light. If they're making funny shapes you should probably see an optician.

The cross shape in these images comes about because of the type of telescope used. Most people are familiar with the idea of a refracting telescope - a long tube with a lens at the front, and one at the end. These telescopes typically aren't used for astronomy because of their length, usually they're around fifteen times longer than they are wide - the width, or aperture of a telescope (or the lens or mirror of the telescope) is very important for astronomical work as we need to capture as much light as possible.

Astronomers typically use what's called a reflecting telescope. Instead of using a lens at the front of the telescope to gather light, it uses a mirror at the bottom which then reflects the light to a secondary mirror closer to the front of the telescope which sends it either out of the side or back down the tube to an eyepiece or camera.

The advantage with this design is the tube can be much shorter and easier to manage. Seven times longer than the width is a fairly common figure, and some types of reflectors are even shorter still. Reflectors are also cheaper to manufacture for a given aperture. Meaning we can gather more light for less money, when you're trying to get every last photon to reach your eye or camera, the larger the primary mirror or objective lens the better.

There are some drawbacks however. These cross shapes through bright objects are one of them. These are caused by the support struts (3 are shown in the diagram above, but usually there are four, like with the HST) which hold the secondary mirror into place. Light passing close to them diffract slightly, this creates the cross shaped pattern around the brighter stars.

10 comments

Comment from: Tallies [Visitor]
Tallies
5 stars

I've been looking for the answer to this for a while now! Thank you very much, this has been bugging me!

3rd July 2009 @ 11:14
Comment from: Adam [Visitor]
Adam

I don't disagree with the reason for star crosses in photos but you are ignoring the fact that when you squint or look at a light source through glass or a similar medium it breaks into rays. Not perfect cross shaped rays, but still rays. Partially due to eyelashes and involunatary squinting.

17th January 2010 @ 05:12
Comment from: Bill [Visitor]
Bill
5 stars

Thanks very much, this explanation makes a lot of sense.

24th December 2011 @ 23:07
Comment from: Anon [Visitor]
Anon

Astigmatism (eye)

http://en.wikipedia.org/wiki/Astigmatism_%28eye%29

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3688271/Main/3683595

26th January 2012 @ 18:12
Comment from: Willy Roentgen [Visitor]
Willy Roentgen
3 stars

So, if the cross effect is cause by the supports within the telescope, why aren't there these artifacts on all the stars in the photographic field?

21st November 2012 @ 13:55
Comment from: [Member]
Paul

Because the artifacts are faint, and you'd need a longer exposure to capture them.

27th April 2013 @ 12:25
Comment from: Melt [Visitor]
Melt

I was shocked at the amount of nonsense attributing the universe to a supernatural deity of some kind.If you believed in God it will open a whole new starsystem to you.

24th November 2013 @ 11:07
Comment from: Pat [Visitor]  
Pat

I see "crosses" in stars, especially in winter. I am the person in the car who can read the street signs way ahead of anyone else. There's nothing wrong with my night or "far" vision. So why do I see this with the naked eye?

25th December 2013 @ 03:59
Comment from: Randy [Visitor]  
Randy

These reason for the cross in all stars are false.
The telescope lens has nothing to do it.

The crosses are a form of radiation of all stars.
A star is a large photon of radiation or electromagnetic
Radiation.

6th December 2016 @ 05:47
Comment from: Ran [Visitor]  
Ran

The comments about the cross as ignorant as I have ever heard!!!
The true explanation of cross was the one explaining radiation .

31st May 2017 @ 20:40


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