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cosmologist
 06/11/2011 10:23PM (Read 4004 times)  
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I read the help page of "response" but I am still not sure what it actually does or to be more specific, why we need it.... As far as I understand it normalizes the input flat file (the input file and the normalization file in my case are the same). So I get a result that is a horizontal line equal to 1 (it makes sense, dividing 2 identical images from each other)...
So now when I do the ccdproc for the flat correction I divide by that image (Flatres.fits) which is = 1,,, how is that making any difference (fixing for flat)?
Do you know what I mean?
Why can't one just divide the science images by the average combined flats?

 
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valdes
 06/11/2011 10:23PM  
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This step is optional. The difference is that if you divide directly by the combined flat you will modify the shape of the observed spectrum by the reciprocal of the flat field spectrum. Often times the flat field lamp has a somewhat extreme behavior at the ends. So the purpose of this step is to avoid introducing the flat field lamp spectrum in the data. However, if you later flux calibrate or are only interested in line ratios, etc. it is not necessary.I hope this helps.Frank Valdes

 
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cosmologist
 06/11/2011 10:23PM  
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[quote:f04bf0fbc7="valdes"] Often times the flat field lamp has a somewhat extreme behavior at the ends. So the purpose of this step is to avoid introducing the flat field lamp spectrum in the data.Frank Valdes[/quote:f04bf0fbc7]I am not sure I understand what this part exactly means. Can you explain it more please?Thanks!

 
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valdes
 06/11/2011 10:23PM  
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I will have to explain further later. I was gone on a business trip this week and am gone next week on a vacation.Frank Valdes

 
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cosmologist
 06/11/2011 10:23PM  
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[quote:8b4b4fcc78="valdes"]I will have to explain further later. I was gone on a business trip this week and am gone next week on a vacation.Frank Valdes[/quote:8b4b4fcc78]Looking forward to your explanation whenever you get a chance.
THank you so much!

 
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valdes
 06/11/2011 10:23PM  
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Let me give you a hypothetical example. Suppose you observe a science spectrum that results in a constant value of 1000 between its range of 4000A - 9000A. Then you observer a flat field lamp (an incandescent bulb) which produces a spectrum that is near 1 at 4000A, 1000 at 6000A, and 1 at 9000A. If you simply divide the science spectrum by the flat field spectrum you will get a result that is 1000 at 4000A, 1 at 6000A, and 1000 at 9000A. This is what I mean by "introducing the shape of the flat field lamp" into your data. So if you want to have the science spectrum divided by the flat field to have a similar shape you need to fit the flat field spectrum by some smooth curve which normalizes the flat field spectrum to near 1 at all wavelengths. Then dividing will correct for the pixel-to-pixel sensitivity variations without make drastic changes to the shape of the spectrum.Frank Valdes

 
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cosmologist
 06/11/2011 10:23PM  
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[quote:214973a3a3="valdes"] So if you want to have the science spectrum divided by the flat field to have a similar shape you need to fit the flat field spectrum by some smooth curve which normalizes the flat field spectrum to near 1 at all wavelengths. [/quote:214973a3a3]So I think then that is what "respose" sep does: fit the flat field spectrum by some smooth curve which normalizes the flat field spectrum to near 1 at all wavelengths. Correct?But when I do the fitting with response the fitting function still looks like a curve (a smooth curve but still a curve) so that means that when dividing by it now the spectrum will take the shape of that curve (I am still not understanding how that will be normalized to 1).

 
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valdes
 06/11/2011 10:23PM  
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[quote:064f7fafce]So I think then that is what "respose" sep does: fit the flat field spectrum by some smooth curve which normalizes the flat field spectrum to near 1 at all wavelengths. Correct? [/quote:064f7fafce]Yes that is correct.[quote:064f7fafce]But when I do the fitting with response the fitting function still looks like a curve (a smooth curve but still a curve) so that means that when dividing by it now the spectrum will take the shape of that curve (I am still not understanding how that will be normalized to 1).[/quote:064f7fafce]What you see in the interactive graphics is the shape of the function before it is applied to the data. Note that there are keys that lets you display this in various ways including as a ratio. It is when the output image is produced after the fitting that the output spectrum will have been divided by the smooth function to produce the "normalized" data for applying to the science data.

 
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cosmologist
 06/11/2011 10:23PM  
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Thank you so much! I got it now Big Grin Idea

 
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