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Hi, everyoneDuring my work with IRAF, I find thatIn Zerocombine or Flatcombine task, there are:
(rdnoise= ) ccdclip: CCD readout noise (electrons)
(gain = ) ccdclip: CCD gain (electrons/DN)But in Apall task, there are
(readnoi= ) Read out noise sigma (photons)
(gain = ) Photon gain (photons/data number)It seems that stand for different meaning. Does that two have any relation? and How can i set these parameters better?Thanks!

Hi,From what I learn about photoelectric effect (modern physics class), one photon = one electron. Unless Iraf has some other interpretation. 8)

The number of [i:8d85d0dde9]incident[/i:8d85d0dde9] photons per electron is the quantum efficiency of the detector and is <100%, just because the detector isn't perfect. However, what matters is what you are actually counting, ie. electrons (or "detected photons"). I'm pretty certain that zerocombine and apall mean the same thing, which is really electrons (== photons registered by the detector).Cheers,James.

Hi Lina and James,Isn't quantum efficiency the number of electrons divided by the number of incident photons? There would be more incident photons than electrons released in the material.By the way, I agree that zerocombine and apall are talking about the same thing, otherwise I will have to redo all my data reduction.As for Lina, probably you can get the readnoise and gain from the CCD specification .cheers,
Siti

[quote:40410feca6="sitij109"]Isn't quantum efficiency the number of electrons divided by the number of incident photons?[/quote:40410feca6]
Yes, sorry!James.

Hi,Thanks very much!!BTW: I am redoing my data reduction now, because during my three times observation, the values of readout noise and gain were set different, but I didn't notice that and set gain=1 before....I must be more careful and patient.

Hi,I apologize for any inconsistency in nomenclature. Yes, in those tasks photons really means the detected photoelectrons. For optical CCDs there is basically a 1:1 correspondence. The main reason to put it this way was to indicate whether one is talking about data numbers or electrons with the gain part removed. In other words [code:1:4c8c50162f]electrons=DN*gain[/code:1:4c8c50162f]. To emphasize, just be careful in knowing whether the gain is treated separately or the task assumes the gain has been removed (data corrected to unit gain). Otherwise treat the words electrons and photons as the same thing.Yours,
Frank Valdes