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  #1  
Old June 9th 04, 12:05 AM
LSL
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Default Stupid Question

Hello All,
Not really an amateur - more like a casual observer with an interest -
please forgive the intrusion.

I've been watching the Cassini stuff, and something struck me that
I've never really thought too much about before, so I'm hoping this
isn't the proverbial stupid question!

When I see these images of planets within our solar system, why do I
never see stars in the backdrop (at least that I can remember?)

I've seen images of Pluto at distance, shown as a smudge in a
starfield, so that one's obvious. But when I see pictures of the moon,
for instance, I can't remember ever really seeing stars adjacent.

Do they just crop them out, is the light too great, do they just
happen to choose photographs that have no stars in them for
aesthetics, etc.

Or, if I'm wrong, and just haven't seen the right pictures (I'm
getting your National Geographic and Mars Rovers and Cassini type
images - stuff from NASA and friends), could someone maybe direct me
to a good site?

TIA
Steve
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  #2  
Old June 9th 04, 12:13 AM
Brian Tung
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Default Stupid Question

Steve wrote:
When I see these images of planets within our solar system, why do I
never see stars in the backdrop (at least that I can remember?)


Generally speaking, the planet is too bright for an exposure long enough
to capture any stars in addition.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt
  #4  
Old June 9th 04, 12:25 AM
David Nakamoto
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Default Stupid Question

You don't see the stars because even now CCD cameras don't have the dynamic
range of the eye. Being an avid imager of planets for almost the past year,
I can tell you that recording stars and planets clearly in the same image is
nearly impossible unless the star is of the nearly the same brightness as
the planet.

The only advantage CCDs have over the eye is to save light over longer
periods of time, minutes instead of seconds, but this is not necessary for
the planets due to their high brightness.
--
Sincerely,
--- Dave

----------------------------------------------------------------------
A man is a god in ruins.
--- Duke Ellington
----------------------------------------------------------------------

"LSL" wrote in message
m...
Hello All,
Not really an amateur - more like a casual observer with an interest -
please forgive the intrusion.

I've been watching the Cassini stuff, and something struck me that
I've never really thought too much about before, so I'm hoping this
isn't the proverbial stupid question!

When I see these images of planets within our solar system, why do I
never see stars in the backdrop (at least that I can remember?)

I've seen images of Pluto at distance, shown as a smudge in a
starfield, so that one's obvious. But when I see pictures of the moon,
for instance, I can't remember ever really seeing stars adjacent.

Do they just crop them out, is the light too great, do they just
happen to choose photographs that have no stars in them for
aesthetics, etc.

Or, if I'm wrong, and just haven't seen the right pictures (I'm
getting your National Geographic and Mars Rovers and Cassini type
images - stuff from NASA and friends), could someone maybe direct me
to a good site?

TIA
Steve



  #5  
Old June 9th 04, 12:53 AM
Chris L Peterson
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Posts: n/a
Default Stupid Question

On Tue, 08 Jun 2004 23:25:34 GMT, "David Nakamoto" wrote:

You don't see the stars because even now CCD cameras don't have the dynamic
range of the eye. Being an avid imager of planets for almost the past year,
I can tell you that recording stars and planets clearly in the same image is
nearly impossible unless the star is of the nearly the same brightness as
the planet.


I don't entirely agree. Good CCD cameras have a dynamic range of 1:5000 to
1:10000, which is certainly enough to get a good planetary image showing stars.
For instance, if you expose Jupiter with such a camera, just filling the wells
with the brightest features, you will record stars to magnitude 10. With Saturn,
which isn't as bright, you can go even deeper.

What the many people, including the original poster, might fail to realize is
just how small a field a planet sits in when imaged from Earth, and how few
stars brighter then mag 10 are even in it (typically, none). This is pretty
apparent when you view Jupiter at high magnification with a small scope- there
may be no stars visible to the eye, or at least, very few.



The only advantage CCDs have over the eye is to save light over longer
periods of time, minutes instead of seconds, but this is not necessary for
the planets due to their high brightness.


I'd add the advantage of collecting data which can be manipulated. Thus,
structures with contrast differences too small to be discerned by the eye are
readily stretched into visibility in the digital realm. That's why CCD images of
even bright planets show so much more detail than ever shows up visually.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #6  
Old June 9th 04, 03:46 AM
William Hamblen
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Default Stupid Question

On 2004-06-08, LSL wrote:

Do they just crop them out, is the light too great, do they just
happen to choose photographs that have no stars in them for
aesthetics, etc.


The moon is bright and stars are faint so with the correct exposure
for the moon stars are underexposed.

The photographs of planets you see are at high magnification so you
can see details on the planet. The field of view is small so few
stars would be in the picture. You can take a wide field picture
showing a planet among the stars, but the planet looks like a bright
star.

  #7  
Old June 9th 04, 07:55 AM
John Beaderstadt
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Posts: n/a
Default Stupid Question

While reading in the bathroom on Tue, 08 Jun 2004 21:46:32 -0500, I
saw that William Hamblen had written:

On 2004-06-08, LSL wrote:

Do they just crop them out, is the light too great, do they just
happen to choose photographs that have no stars in them for
aesthetics, etc.


The moon is bright and stars are faint so with the correct exposure
for the moon stars are underexposed.


Liar!

The moon is a fake, otherwise there would be stars. It's all really
set up on a sound stage in the Arizona desert.


--------------
Beady's Corollary to Occam's Razor: "The likeliest explanation of any phenomenon is almost always the most boring one imaginable."


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  #8  
Old June 9th 04, 09:16 AM
Martin Brown
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Posts: n/a
Default Stupid Question

In message , Brian Tung writes
Steve wrote:
When I see these images of planets within our solar system, why do I
never see stars in the backdrop (at least that I can remember?)


Generally speaking, the planet is too bright for an exposure long enough
to capture any stars in addition.


Or you can burn out the planetary disk and see satellites (for Jupiter
and outwards) and any bright field stars. Actually with modern astro-CCD
imaging if you stretch the contrast on a decent image you can usually
see the Jovian satellites and the belt detail on a single exposure.

On the rare occasions when the moon passes between us and a planet you
get to see the difference in brightness and size clearly. The last time
Saturn passed behind a gibbous phase moon was very pretty indeed.

It is quite rare to get a bright star and a planet in close proximity.

Regards,
--
Martin Brown
  #9  
Old June 10th 04, 01:54 AM
Mike Ruskai
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Posts: n/a
Default Stupid Question

On 8 Jun 2004 16:05:47 -0700, LSL wrote:

Hello All,
Not really an amateur - more like a casual observer with an interest -
please forgive the intrusion.

I've been watching the Cassini stuff, and something struck me that
I've never really thought too much about before, so I'm hoping this
isn't the proverbial stupid question!

When I see these images of planets within our solar system, why do I
never see stars in the backdrop (at least that I can remember?)

[snip]

Exposure time. Stars take time to show up on film (and CCD). The light
needs to accumulate.

The exposures of planets and the like are pretty short, especially those
of a full moon. There's not enough time for most stars to begin showing
up.

You can replicate it yourself by taking a picture outside on a clear
night. Make sure the sky is visible in the frame, and use either bright
floodlights or a flash. The exposure will be short, and you won't see
many (or any) stars.

I expect it now seems obvious.


--
- Mike

Remove 'spambegone.net' and reverse to send e-mail.


 




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