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Main
Date: 16 Nov 2006 12:54:47
From: Klaatu
Subject: Deep Sky Imager II Noise Reduction
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Just noticed that Meade is advertising their Deep Sky Imager II
http://www.meade.com/dsi_ii/index.html as reducing noise without a cooler.
Anyone know how this is done? TIA
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Date: 18 Nov 2006 09:00:56
From:
Subject: Re: Deep Sky Imager II Noise Reduction
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Chris L Peterson wrote:
> On 18 Nov 2006 08:40:11 -0800, info@astrobuysell.com wrote:
> Subtracting dark exposures does not reduce noise. In fact, they
> typically increase noise. Darks are used to reduce certain types of
> fixed pattern problems.
>
Chris - nice web site! You're obviously more knowledgeable about CCD
imaging than me, but what you wrote above surprises me.
If I take an image and don't apply a dark, the image is full of random
bright pixels (noise). After I apply a dark, all the noise is gone.
Why do you say subtracting darks does not reduce noise?
Thanks,
Paul.
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Date: 18 Nov 2006 17:21:31
From: Chris L Peterson
Subject: Re: Deep Sky Imager II Noise Reduction
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On 18 Nov 2006 09:00:56 -0800, info@astrobuysell.com wrote:
>Chris - nice web site! You're obviously more knowledgeable about CCD
>imaging than me, but what you wrote above surprises me.
>If I take an image and don't apply a dark, the image is full of random
>bright pixels (noise). After I apply a dark, all the noise is gone.
>Why do you say subtracting darks does not reduce noise?
Noise is just that- random. As such, it can't be predicted or removed.
Each pixel in your image will have a random error, and each pixel in
your dark will have a random error. When you subtract those, you don't
get rid of any noise at all- the noise is slightly increased.
Sensors aren't perfect. One problem they have is that they accumulate a
false signal as thermal electrons are collected. This is called dark
current, and it adds a bias to each pixel. But like any signal, it has a
noise component (the square root of the signal). When you make a long
exposure, you see these biased pixels interfering with your image. It is
common to refer to this as noise, but in a strict sense it isn't. A dark
frame allows you to subtract off the bias, which generally improves the
appearance of the image. But the noise in the original image and the
noise in the dark frame are still there.
There's nothing magic that the DSI II can do using dark subtraction to
reduce noise. If the camera is less noisy, it is because of a
combination of passive cooling (which can moderately reduce the level of
dark current) and a detector with intrinsically lower dark current than
earlier models.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 18 Nov 2006 09:25:00
From: Brian Tung
Subject: Re: Deep Sky Imager II Noise Reduction
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Paul wrote:
> Chris - nice web site! You're obviously more knowledgeable about CCD
> imaging than me, but what you wrote above surprises me.
> If I take an image and don't apply a dark, the image is full of random
> bright pixels (noise). After I apply a dark, all the noise is gone.
> Why do you say subtracting darks does not reduce noise?
Because any time you add or subtract anything, you're adding noise. As
Chris suggested, I think your bright pixels are not noise--they must be
consistent from image to image, and by subtracting the dark frame, you
are cancelling out those consistent errors. Actual noise--stuff that is
not consistent from image to image--cannot be cancelled out that way.
It can only be counteracted by increasing the amount of signal relative
to the amount of noise. Noise is uncorrelated, so it goes up as the
square root of the exposure--quadruple the exposure, and the noise goes
up as by a factor of two--whereas the signal goes up linearly with the
exposure.
--
Brian Tung <brian@isi.edu >
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.html
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Date: 18 Nov 2006 08:40:11
From:
Subject: Re: Deep Sky Imager II Noise Reduction
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Klaatu wrote:
> Just noticed that Meade is advertising their Deep Sky Imager II
> http://www.meade.com/dsi_ii/index.html as reducing noise without a cooler.
> Anyone know how this is done? TIA
Klaus,
I know a little bit about this - Meade's software matches each light
exposure taken at a certain temperature to a "dark" exposure taken at
the same temperature. This ensures noise is minimized. Also, the CCD
chip in the DSI II has lower dark current noise than a DSI I chip.
Cheers,
Paul.
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Date: 18 Nov 2006 16:50:28
From: Chris L Peterson
Subject: Re: Deep Sky Imager II Noise Reduction
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On 18 Nov 2006 08:40:11 -0800, info@astrobuysell.com wrote:
>Klaus,
>I know a little bit about this - Meade's software matches each light
>exposure taken at a certain temperature to a "dark" exposure taken at
>the same temperature. This ensures noise is minimized. Also, the CCD
>chip in the DSI II has lower dark current noise than a DSI I chip.
Subtracting dark exposures does not reduce noise. In fact, they
typically increase noise. Darks are used to reduce certain types of
fixed pattern problems.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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