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Sensitivity difference



 
 
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  #1  
Old November 22nd 06, 12:23 AM posted to sci.astro.ccd-imaging
Terry B
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Posts: 64
Default Sensitivity difference

Dear All
I have recently borrowed a ST7 with an KA0401E chip with antiblooming. My
camera is a Genesis CCD with the non antiblooming version of the same chip
that I build many years ago. I have been amazed at the difference in the
sensitivity. Some of this may be related to using different programs to take
the pics. Is it due to the antiblooming or just different design of camera?

Terry B


  #2  
Old November 22nd 06, 12:26 AM posted to sci.astro.ccd-imaging
Chris L Peterson
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Posts: 10,007
Default Sensitivity difference

On Tue, 21 Nov 2006 23:23:22 GMT, "Terry B"
wrote:

I have recently borrowed a ST7 with an KA0401E chip with antiblooming. My
camera is a Genesis CCD with the non antiblooming version of the same chip
that I build many years ago. I have been amazed at the difference in the
sensitivity. Some of this may be related to using different programs to take
the pics. Is it due to the antiblooming or just different design of camera?


With the KAF chips, the antiblooming system works by draining away
excess electrons. By its nature, a certain percentage of all electrons
are lost. In addition, the antiblooming requires a gate structure on top
of each pixel that reduces the collection area (the newer chips with
microlenses largely compensate for this loss). The QE of the NABG sensor
is a good 50% higher than the ABG. In addition, the ABG sensor isn't
linear, so it isn't a very good choice for some scientific work,
especially photometry.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #3  
Old November 22nd 06, 01:54 AM posted to sci.astro.ccd-imaging
Terry B
external usenet poster
 
Posts: 64
Default Sensitivity difference


"Chris L Peterson" wrote in message
...
On Tue, 21 Nov 2006 23:23:22 GMT, "Terry B"
wrote:

I have recently borrowed a ST7 with an KA0401E chip with antiblooming. My
camera is a Genesis CCD with the non antiblooming version of the same

chip
that I build many years ago. I have been amazed at the difference in the
sensitivity. Some of this may be related to using different programs to

take
the pics. Is it due to the antiblooming or just different design of

camera?

With the KAF chips, the antiblooming system works by draining away
excess electrons. By its nature, a certain percentage of all electrons
are lost. In addition, the antiblooming requires a gate structure on top
of each pixel that reduces the collection area (the newer chips with
microlenses largely compensate for this loss). The QE of the NABG sensor
is a good 50% higher than the ABG. In addition, the ABG sensor isn't
linear, so it isn't a very good choice for some scientific work,
especially photometry.

_________________________________________________

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

Thank you Chris.
Another question.
When I image with the genesis version I have an offset of about ~3500 that I
have to subtract out. The SBIG camera seeme to have essentially on offset or
about 100 only. Is this just different electronics or does the SBIG software
remove the offset before displaying the image?
Terry B


  #4  
Old November 22nd 06, 02:13 AM posted to sci.astro.ccd-imaging
Chris L Peterson
external usenet poster
 
Posts: 10,007
Default Sensitivity difference

On Wed, 22 Nov 2006 00:54:21 GMT, "Terry B"
wrote:

Thank you Chris.
Another question.
When I image with the genesis version I have an offset of about ~3500 that I
have to subtract out. The SBIG camera seeme to have essentially on offset or
about 100 only. Is this just different electronics or does the SBIG software
remove the offset before displaying the image?


That's purely a function of the camera electronics or firmware. Cameras
normally have an offset or pedestal added to each pixel to ensure that
there are no negative values. The amount is pretty arbitrary. I'm
surprised that the Genesis uses such a high value, though. It's enough
to seriously bite into the available 16-bit output range, and far more
than should be necessary.

The pedestal shouldn't be removed by processing software from the raw
image. It will be removed automatically during dark subtraction, which
is usually a requirement of processing. For very low noise cameras that
may not require a dark frame, you would still use a bias frame (or
synthesized bias frame) to correct for the pedestal.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #5  
Old November 22nd 06, 03:05 AM posted to sci.astro.ccd-imaging
Terry B
external usenet poster
 
Posts: 64
Default Sensitivity difference


"Chris L Peterson" wrote in message
...
On Wed, 22 Nov 2006 00:54:21 GMT, "Terry B"
wrote:

Thank you Chris.
Another question.
When I image with the genesis version I have an offset of about ~3500

that I
have to subtract out. The SBIG camera seeme to have essentially on offset

or
about 100 only. Is this just different electronics or does the SBIG

software
remove the offset before displaying the image?


That's purely a function of the camera electronics or firmware. Cameras
normally have an offset or pedestal added to each pixel to ensure that
there are no negative values. The amount is pretty arbitrary. I'm
surprised that the Genesis uses such a high value, though. It's enough
to seriously bite into the available 16-bit output range, and far more
than should be necessary.

The pedestal shouldn't be removed by processing software from the raw
image. It will be removed automatically during dark subtraction, which
is usually a requirement of processing. For very low noise cameras that
may not require a dark frame, you would still use a bias frame (or
synthesized bias frame) to correct for the pedestal.

_________________________________________________

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


Thanks Chris

Terry B


 




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