Starship Asterisk*

by **bystander** » Tue Apr 10, 2012 6:31 pm

neufer wrote:
[b][color=#0000FF][size=165]MOscOW[/size][/color] photo by Shuttle Expedition 30 crew on March 28, 2012.[/b]

Asterisk*

by **neufer** » Tue Apr 10, 2012 2:03 pm

: [b][color=#0000FF][size=165]MOscOW[/size][/color] photo by Shuttle Expedition 30 crew on March 28, 2012.[/b]

bystander wrote:
Chris Peterson wrote:
Beyond wrote:
Now wait a minute! If a five leaf aperture produces ten spikes, why does a six leaf aperture only produce six spikes, instead of twelve??
Each edge produces a pair. A six-leaf iris does produce 12 spikes, but half of them overlap the other half, so you only see six.
Symmetry!

by **Chris Peterson** » Sun Apr 08, 2012 8:24 pm

ta152h0 wrote:Would a pinhole camera with a perfect hole produce spikes?

Diffraction spikes are linear structures, caused by linear elements in the light path (the vanes supporting a central mirror, a polygonal aperture, a leaf iris, etc). Circular structures (the central mirror itself, the aperture, etc) produce circular diffraction artifacts... rings. A pinhole lens does not produce any spikes, but it certainly produces an Airy pattern when imaging a point source, and its circular diffraction artifacts affect the PSF of the system just as those from the circular aperture of a reflective or refractive system do.

by **ta152h0** » Sun Apr 08, 2012 8:18 pm

Would a pinhole camera with a perfect hole produce spikes ? and don't answer the way my father would. Prof Kotenberg would have me build one and find out for myself.

by **Beyond** » Sun Apr 08, 2012 4:39 am

That's why bystander came in a loose second. He used a catch-22 answer--Symmetry.

by **Ann** » Sun Apr 08, 2012 3:35 am

As a language teacher, I'd say Chris' reply contains the relevant information. As for bystander's answer, it helps if you know the answer that Chris gave you to understand his answer!

Ann

by **Beyond** » Sat Apr 07, 2012 8:56 pm

Ah, ok. Thanks for the explanation chris.

Bystander comes in a loose second. Only because there's no third place.

by **bystander** » Sat Apr 07, 2012 7:34 pm

bystander wrote:Symmetry!

by **Chris Peterson** » Sat Apr 07, 2012 7:27 pm

Beyond wrote:Now wait a minute! If a five leaf aperture produces ten spikes, why does a six leaf aperture only produce six spikes, instead of twelve??

Each edge produces a pair. A six-leaf iris does produce 12 spikes, but half of them overlap the other half, so you only see six.

by **bystander** » Sat Apr 07, 2012 6:17 pm

Symmetry!

by **Beyond** » Sat Apr 07, 2012 5:42 pm

Now wait a minute! If a five leaf aperture produces ten spikes, why does a six leaf aperture only produce six spikes, instead of twelve??

by **DavidLeodis** » Sat Apr 07, 2012 5:04 pm

Chris Peterson wrote:
DavidLeodis wrote:What does the "multiple leaves in the aperture of the telephoto lens" mean I know I should try to find out but I'm surely not the only one not to know, so I thought I would ask.

iris.jpg
A standard camera iris, for adjusting the size of the aperture, is made up of multiple leaves. This is a five-leaf iris, which would produce ten diffraction spikes around a bright point. Today's image was made using a six-leaf aperture, which produces just six spikes.

Thanks Chris for your help, which is appreciated.

I did not realise that cameras still used that method to adjust the size of the aperture, though to be honest I'd never given it any thought with my compact digital camera being always on an automatic setting so I don't know what is actually going on when I take a photograph.

by **geckzilla** » Sat Apr 07, 2012 3:17 pm

Of course not. Not a scientist, anyway. For everyday communication between me and my fiance's dad, however... It gets confusing.

by **Chris Peterson** » Sat Apr 07, 2012 2:14 pm

geckzilla wrote:Try large numbers with a Chinese person. Larger sets of numbers are grouped by 10,000 instead of 1,000. So 1,000,000,000 instead ends up as 10,0000,0000 so there's not a simple word for billon. Instead, it's 10 [word for 1,0000,0000]. Something like that. Ten one-hundred millions I guess is the closest translation. (If someone here is Chinese and spotted an error in my post I would be happy to have it fixed.)

There would probably be no confusion at all for a Chinese scientist reading a scientific paper written in English. That was my point. Of course, different numbering systems can cause confusion in other contexts.

by **Chris Peterson** » Sat Apr 07, 2012 2:11 pm

DavidLeodis wrote:What does the "multiple leaves in the aperture of the telephoto lens" mean :?: I know I should try to find out but I'm surely not the only one not to know, so I thought I would ask. :?

: iris.jpg (4.78 KiB) Viewed 2947 times

A standard camera iris, for adjusting the size of the aperture, is made up of multiple leaves. This is a five-leaf iris, which would produce ten diffraction spikes around a bright point. Today's image was made using a six-leaf aperture, which produces just six spikes.

by **DavidLeodis** » Sat Apr 07, 2012 11:01 am

What does the "multiple leaves in the aperture of the telephoto lens" mean

I know I should try to find out but I'm surely not the only one not to know, so I thought I would ask.

by **Ann** » Sat Apr 07, 2012 5:48 am

Chris Peterson wrote:
Ann wrote:I know "billion" is tricky, but I was speaking English. In Swedish, a "biljon" is a trillion - that is, a "biljon" is 1,000,000,000,000. So what is a billion called in Swedish, then? It's a "miljard". A "miljon" is really a million.
A "billion" is pretty unambiguously 10^9 in any scientific discussion. The fact that some countries are still using a different, confusing, obsolete, and illogical set of terminology should not impact a discussion in a technical forum. You're not going to find milliards in scientific papers.

Right. I know, Chris. And an important reason why milliards are missing in scientific papers is that English doesn't use milliards, and English is the lingua franca of the community of science of today.

Carl von Linné, Swedish biologist who classified very many species of the flora and fauna in the 18th century, gave us all Latin designations, because Latin was the language of science back then. And that's why we are called Homo Sapiens and not Intelligent Human. (Or even Klok Människa.)

Ann

by **geckzilla** » Sat Apr 07, 2012 5:33 am

Try large numbers with a Chinese person. Larger sets of numbers are grouped by 10,000 instead of 1,000. So 1,000,000,000 instead ends up as 10,0000,0000 so there's not a simple word for billon. Instead, it's 10 [word for 1,0000,0000]. Something like that. Ten one-hundred millions I guess is the closest translation. (If someone here is Chinese and spotted an error in my post I would be happy to have it fixed.)

by **Chris Peterson** » Sat Apr 07, 2012 4:59 am

Ann wrote:I know "billion" is tricky, but I was speaking English. In Swedish, a "biljon" is a trillion - that is, a "biljon" is 1,000,000,000,000. So what is a billion called in Swedish, then? It's a "miljard". A "miljon" is really a million.

A "billion" is pretty unambiguously 10^9 in any scientific discussion. The fact that some countries are still using a different, confusing, obsolete, and illogical set of terminology should not impact a discussion in a technical forum. You're not going to find milliards in scientific papers.

by **Ann** » Sat Apr 07, 2012 4:44 am

rstevenson wrote:
Ann wrote:Forgive my limited understanding of mathematese. So the new figure would make Venus five billion times fainter than Alcyone, is that right?

Ann
Almost 500 billion. But "billion" is a tricky word to define, having as it does different common usages in different parts of the world. (See Long and short scales.) In the unambiguous scientific notation (using Case's final figure) Alcyone is 4.88 x 10¹¹ times brighter than Venus.

Rob

I know "billion" is tricky, but I was speaking English. In Swedish, a "biljon" is a trillion - that is, a "biljon" is 1,000,000,000,000. So what is a billion called in Swedish, then? It's a "miljard". A "miljon" is really a million.

Ann

by **Chris Peterson** » Sat Apr 07, 2012 1:29 am

Diana wrote:Since we are being picky with numbers, let's be picky with the term "brightness" and the two bodies' relitive brightness.

Can we say that Venus' brightness is a reflective measurement and the Pleiades star's is a emission-type brightness?

Yes, but it doesn't matter in this context. What is being referred to is intensity- the number of photons reaching the observer per unit time. The photons don't know or care if they were emitted directly or reflected from cloud tops.

by **ta152h0** » Fri Apr 06, 2012 11:27 pm

I just googled the Observatory and discovered it is at elevation 4700 feet in southeast Arizona. Amazing clarity on this image for still being imaged with a lot of " air over the camera ".

by **Diana** » Fri Apr 06, 2012 10:31 pm

Since we are being picky with numbers, let's be picky with the term "brightness" and the two bodies' relitive brightness.

Can we say that Venus' brightness is a reflective measurement and the Pleiades star's is a emission-type brightness?

Oh, unless those brightness numbers for the star include the brightness of the surrounding reflecting nebulus/dust clouds.
In that case there is added brightness from the back side of the star reflecting off that surrounding material. hummmm

Diana

by **StarRiderDad** » Fri Apr 06, 2012 8:41 pm

One of the interesting optical artifacts in this picture, beside the diffraction pattern, is the faint ghost image of Venus in the upper left. Ghosts are common in refactive optical systems (e.g. telephoto lenses) and in this particular case the ghost clearly shows the phase of Venus meaning that the ghost is not too far out of focus.

by **ta152h0** » Fri Apr 06, 2012 7:03 pm

not a painting, for sure

Starship Asterisk*

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