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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
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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
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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
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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
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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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