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  #1  
Old September 28th 06, 02:47 PM posted to sci.astro.ccd-imaging
murlockc
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Posts: 1
Default Black Sun


Hi everyone!
I am trying to observe the sun using a CMOS chip, but I get the
following problem: when the neutral density located in front of the
sensor is too low, a "black hole" appears at the center of the sun.
Have you already met this kind of problem? Is it a known phenomenon? I
have tried to search for information concerning this type of problem on
the web, but I did not find any clues how to solve it.
It seems that this "black sun" can also appear with CCD chips.

Is there a way to solve it without using post image processing?

Cedric

  #2  
Old September 28th 06, 04:42 PM posted to sci.astro.ccd-imaging
Milton Aupperle
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Posts: 57
Default Black Sun

In article om,
murlockc wrote:

Hi everyone!
I am trying to observe the sun using a CMOS chip, but I get the
following problem: when the neutral density located in front of the
sensor is too low, a "black hole" appears at the center of the sun.
Have you already met this kind of problem? Is it a known phenomenon? I
have tried to search for information concerning this type of problem on
the web, but I did not find any clues how to solve it.
It seems that this "black sun" can also appear with CCD chips.

Is there a way to solve it without using post image processing?

Cedric


It would depend on which CMOS sensor your talking about.

I know that Zoran CMOS sensors had that issue and when we looked at
light bulbs the filament would appear black. Changing the intensity of
the gains helped somewhat but it was an electronics issue and had to be
resolved post.

HTH..

Milton Aupperle
http://www.outcastsoft.com/AstroImages/AstroIndex.html
  #3  
Old September 28th 06, 08:32 PM posted to sci.astro.ccd-imaging
Sjouke Burry
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Posts: 338
Default Black Sun

murlockc wrote:
Hi everyone!
I am trying to observe the sun using a CMOS chip, but I get the
following problem: when the neutral density located in front of the
sensor is too low, a "black hole" appears at the center of the sun.
Have you already met this kind of problem? Is it a known phenomenon? I
have tried to search for information concerning this type of problem on
the web, but I did not find any clues how to solve it.
It seems that this "black sun" can also appear with CCD chips.

Is there a way to solve it without using post image processing?

Cedric

This is what you can expect when you burn in
the ccd chip.
I had it happen with a ccd camera observing/recording
the road in front of a touring car.
Just when the sun came up, the bus had to stop for 2
minutes, with the sun just over the horizon.
And that was the end of the camera, with black
spots all over the place, not actual breaking down,
but almost total loss of sensitivity in spots.
  #4  
Old September 28th 06, 09:15 PM posted to sci.astro.ccd-imaging
Chris L Peterson
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Posts: 10,007
Default Black Sun

On 28 Sep 2006 06:47:09 -0700, "murlockc" wrote:

Hi everyone!
I am trying to observe the sun using a CMOS chip, but I get the
following problem: when the neutral density located in front of the
sensor is too low, a "black hole" appears at the center of the sun.
Have you already met this kind of problem? Is it a known phenomenon? I
have tried to search for information concerning this type of problem on
the web, but I did not find any clues how to solve it.
It seems that this "black sun" can also appear with CCD chips.

Is there a way to solve it without using post image processing?


This problem is usually one with the camera electronics. Each pixel
produces a charge proportional to how much light it collects, and this
charge is converted to a number by an analog-to-digital converter. Some
ADCs can produce invalid codes (for instance, they can wrap around to
small values when their input goes above some level), and some of the
other electronics or software can create similar problems (by redefining
the zero point, or by not properly handling an internal conversion to 8
bits from whatever the internal resolution is).

Regardless of the exact cause, it means you are effectively saturating
those pixels, and the fix doesn't involve processing. You need to reduce
the number of photons, either by adding more optical filtering or by
reducing your exposure time.

With regards to any data you already have, some information may be
recoverable assuming the problem is one of wrap-around. You need to add
your saturation value (for instance, 255 for an 8-bit webcam) to the
artificially dark pixels. Of course, you need to do this in something
larger than an 8-bit workspace.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #5  
Old October 5th 06, 10:08 PM posted to sci.astro.ccd-imaging
[email protected]
external usenet poster
 
Posts: 61
Default Black Sun

There is an effect in regular silver photography that when the grains
are seriously over exposed that a reversal occurs. I think this is
called the Clayden effect.



Chris L Peterson wrote:
On 28 Sep 2006 06:47:09 -0700, "murlockc" wrote:

Hi everyone!
I am trying to observe the sun using a CMOS chip, but I get the
following problem: when the neutral density located in front of the
sensor is too low, a "black hole" appears at the center of the sun.
Have you already met this kind of problem? Is it a known phenomenon? I
have tried to search for information concerning this type of problem on
the web, but I did not find any clues how to solve it.
It seems that this "black sun" can also appear with CCD chips.

Is there a way to solve it without using post image processing?


This problem is usually one with the camera electronics. Each pixel
produces a charge proportional to how much light it collects, and this
charge is converted to a number by an analog-to-digital converter. Some
ADCs can produce invalid codes (for instance, they can wrap around to
small values when their input goes above some level), and some of the
other electronics or software can create similar problems (by redefining
the zero point, or by not properly handling an internal conversion to 8
bits from whatever the internal resolution is).

Regardless of the exact cause, it means you are effectively saturating
those pixels, and the fix doesn't involve processing. You need to reduce
the number of photons, either by adding more optical filtering or by
reducing your exposure time.

With regards to any data you already have, some information may be
recoverable assuming the problem is one of wrap-around. You need to add
your saturation value (for instance, 255 for an 8-bit webcam) to the
artificially dark pixels. Of course, you need to do this in something
larger than an 8-bit workspace.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


 




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