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#1
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sensor/system calibration: dark = bias + thermal
I read(*) that a standard practice is to collect a set of "bias"
frames and average them together. You then collect another set of "dark" frames (at some fixed exposure), remove the aforecollected bias, averge them up and you obtain a "thermal" frame which can be scaled by exposure time. My question: is there a need to collect the bias frames at all? If instead you collected a set of "dark" frames at a number of exposure times (say 1s, 2s, 4s, 8s, ..., 32s, 64s, etc), could you not extract the "bias" information from them, as well as obtaining the thermal scaling? If so, then you not only save the time needed to collect the bias frames, store and process them, but you can also measure a goodness-of-fit to the assumed linear model that "dark = bias + thermal*exposure". Or is there some Mystery to this process that makes the extra time and space for separate bias frames desirable? (*) various from www.google.com, as well as Berry/Burnell "Handbook of Astronomical Image Processing", 2000. |
#2
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sensor/system calibration: dark = bias + thermal
Hi,
My take on this is that the bias frame is used to get the noise that is constant (noise caused by the actual readout of the CCD data), and therefore does not change with exposure setting or temperature. I think you should be able to create one master bias frame (a zero second exposure) and use it with all of your exposures. Now, the thermal frame gets the noise that varies with time and temperature, and I guess is assumed to be linear (otherwise you couldn't scale it). I think you should take a set of dark frames for each imaging session so that you get the same temperature as the real exposure, but if you end up using the bias frame, then you don't have to have a dark frame of exactly the same exposure as your real frames. You would subtract the bias frame from the dark frame, and then scale the resulting thermal frame. You then subtract both the bias frame and the scaled thermal frame. I just take a set of dark frames of the same exposure time as my real images, and then just subtract them without any scaling, but then again, I am a rank beginner using a SAC8 (modified security cam). Mark Ritchie wrote in message om... I read(*) that a standard practice is to collect a set of "bias" frames and average them together. You then collect another set of "dark" frames (at some fixed exposure), remove the aforecollected bias, averge them up and you obtain a "thermal" frame which can be scaled by exposure time. My question: is there a need to collect the bias frames at all? If instead you collected a set of "dark" frames at a number of exposure times (say 1s, 2s, 4s, 8s, ..., 32s, 64s, etc), could you not extract the "bias" information from them, as well as obtaining the thermal scaling? If so, then you not only save the time needed to collect the bias frames, store and process them, but you can also measure a goodness-of-fit to the assumed linear model that "dark = bias + thermal*exposure". Or is there some Mystery to this process that makes the extra time and space for separate bias frames desirable? (*) various from www.google.com, as well as Berry/Burnell "Handbook of Astronomical Image Processing", 2000. |
#3
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sensor/system calibration: dark = bias + thermal
"Mark Ritchie" wrote in message news Hi, My take on this is that the bias frame is used to get the noise that is constant (noise caused by the actual readout of the CCD data), and therefore does not change with exposure setting or temperature. I think you should be able to create one master bias frame (a zero second exposure) and use it with all of your exposures. Actually it is the bias that varies with time. In fact bias can vary by a few ADU counts over a few hours. See "The Handbook of Astronomical Image Processing" by Berry and Burnell chapter 4 for more details Now, the thermal frame gets the noise that varies with time and temperature, and I guess is assumed to be linear (otherwise you couldn't scale it). I think you should take a set of dark frames for each imaging session so that you get the same temperature as the real exposure, but if you end up using What you really want to do is to take a set of master darks and master dark biases one day. You median or otherwise combine the biases to create a single master. Ditto for the Dark. Then subtract the Master Bias from the Master Dark and keep the bias subracted Dark as your library dark. When you take images on another day, you simply shoot maybe 10 biases which represent that bias on that particualar day. You combine them together and add that to your Library Dark. Then you use that bias-added Library Dark for your Calibration Dark. Think about it: Taking 10 darks of , say, 30 minutes takes a lot of time. On the other hand 10 biases are much faster: just download time. There's little point in keeping library biases and taking darks each time. The idea behind keeping a library of darks is to save time. I find that so long as I am operating at the same temp, I can use bias-subtracted library darks for months. I just shoot a set of biases each imaging session. |
#4
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sensor/system calibration: dark = bias + thermal
Richard,
I just got "The Handbook of Astronomical Image Processing" book and software for Christmas, and I did read Chapter 4, but obviously I didn't understand it ! I re-read the chapter, and I now see that I had it pretty much backwards. I guess I was reading it with a 'biased' opinion of how the bias frames work. I am taking such short exposures (30 seconds max), that I have never done anything more complicated than just taking darks that match my real images, so I have never tried to actually use a bias frame, which I guess should have made me think twice about responding to the original post in the first place. Anyway, thanks for correcting me on this, I certainly try not to spread bad information. Mark Ritchie "Richard Crisp" wrote in message m... "Mark Ritchie" wrote in message news Hi, My take on this is that the bias frame is used to get the noise that is constant (noise caused by the actual readout of the CCD data), and therefore does not change with exposure setting or temperature. I think you should be able to create one master bias frame (a zero second exposure) and use it with all of your exposures. Actually it is the bias that varies with time. In fact bias can vary by a few ADU counts over a few hours. See "The Handbook of Astronomical Image Processing" by Berry and Burnell chapter 4 for more details Now, the thermal frame gets the noise that varies with time and temperature, and I guess is assumed to be linear (otherwise you couldn't scale it). I think you should take a set of dark frames for each imaging session so that you get the same temperature as the real exposure, but if you end up using What you really want to do is to take a set of master darks and master dark biases one day. You median or otherwise combine the biases to create a single master. Ditto for the Dark. Then subtract the Master Bias from the Master Dark and keep the bias subracted Dark as your library dark. When you take images on another day, you simply shoot maybe 10 biases which represent that bias on that particualar day. You combine them together and add that to your Library Dark. Then you use that bias-added Library Dark for your Calibration Dark. Think about it: Taking 10 darks of , say, 30 minutes takes a lot of time. On the other hand 10 biases are much faster: just download time. There's little point in keeping library biases and taking darks each time. The idea behind keeping a library of darks is to save time. I find that so long as I am operating at the same temp, I can use bias-subtracted library darks for months. I just shoot a set of biases each imaging session. |
#5
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sensor/system calibration: dark = bias + thermal
I am happy I could help Mark.
Tell me have you teamed up with any one named Chretien yet? (yes I know that it needs to be spelled Ritchey, but I couldn't resist). "Mark Ritchie" wrote in message ... Richard, I just got "The Handbook of Astronomical Image Processing" book and software for Christmas, and I did read Chapter 4, but obviously I didn't understand it ! I re-read the chapter, and I now see that I had it pretty much backwards. I guess I was reading it with a 'biased' opinion of how the bias frames work. I am taking such short exposures (30 seconds max), that I have never done anything more complicated than just taking darks that match my real images, so I have never tried to actually use a bias frame, which I guess should have made me think twice about responding to the original post in the first place. Anyway, thanks for correcting me on this, I certainly try not to spread bad information. Mark Ritchie "Richard Crisp" wrote in message m... "Mark Ritchie" wrote in message news Hi, My take on this is that the bias frame is used to get the noise that is constant (noise caused by the actual readout of the CCD data), and therefore does not change with exposure setting or temperature. I think you should be able to create one master bias frame (a zero second exposure) and use it with all of your exposures. Actually it is the bias that varies with time. In fact bias can vary by a few ADU counts over a few hours. See "The Handbook of Astronomical Image Processing" by Berry and Burnell chapter 4 for more details Now, the thermal frame gets the noise that varies with time and temperature, and I guess is assumed to be linear (otherwise you couldn't scale it). I think you should take a set of dark frames for each imaging session so that you get the same temperature as the real exposure, but if you end up using What you really want to do is to take a set of master darks and master dark biases one day. You median or otherwise combine the biases to create a single master. Ditto for the Dark. Then subtract the Master Bias from the Master Dark and keep the bias subracted Dark as your library dark. When you take images on another day, you simply shoot maybe 10 biases which represent that bias on that particualar day. You combine them together and add that to your Library Dark. Then you use that bias-added Library Dark for your Calibration Dark. Think about it: Taking 10 darks of , say, 30 minutes takes a lot of time. On the other hand 10 biases are much faster: just download time. There's little point in keeping library biases and taking darks each time. The idea behind keeping a library of darks is to save time. I find that so long as I am operating at the same temp, I can use bias-subtracted library darks for months. I just shoot a set of biases each imaging session. |
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